E S S E N T I A L
Biology
C A M P B E L L
R E E C E
CHAPTER 7
The Cellular Basis of Reproduction
and Inheritance
You began life as a single cell, but there are now more cells in
your body than stars
in the Milky Way.
Overview: What Cell Reproduction Accomplishes 3
Passing On the Genes from Cell to
Cell
The Reproduction of Organisms
Just in the past second, millions
The Cell Cycle and Mitosis 4
Eukaryotic Chromosomes
The Cell Cycle
Mitosis and Cytokinesis
Cancer Cells: Growing out of Control
divided in two.
Meiosis, the Basis of Sexual
Reproduction 10
Homologous Chromosomes
Gametes and the Life Cycle of a
Sexual Organism
The Process of Meiosis
Review: Comparing Mitosis and
Meiosis
The Origins of Genetic Variation
When Meiosis Goes Amok
of your cells have
The dance
of the chromo-
somes in a dividing cell is so precise that only one error occurs
in 100,000 cell divisions.
Each sperm
or egg
Evolution Link: New Species
from Errors in Cell Division 21
produced in your reproductive organs carries one of over 8
million possible combinations of parental chromosomes.
2
C
onsider the skin on your arm. The surface is a protective layer of
dead cells, but underneath are layers of living cells busy carrying out
the chemical reactions you studied in Unit One. The living cells are also engaged in another vital activity: They are reproducing themselves. The new
cells are moving outward toward the skin’s surface, replacing dead cells that
have rubbed off. This renewal of your skin goes on throughout your life.
And when your skin is injured, additional cell reproduction helps heal the
wound. In this chapter, you’ll learn what actually happens inside reproducing cells and how cell reproduction functions in the perpetuation of all life.
Overview: What Cell
Reproduction Accomplishes
The replacement of lost or damaged cells is just one of the important roles
that cell reproduction—or cell division, as we commonly call it—has
played in your life. Another is growth. All of the trillions of cells in your
body result from repeated cell divisions that began in your mother’s body
with a single fertilized egg cell. (Figure 7.1 shows a human embryo at the
eight-cell stage, after three rounds of cell division.)
Figure 7.1 Human embryo at the eight-cell stage, the
result of three rounds of cell division. The eight cells have
identical sets of genes, as will the cells that they give rise to. As development continues, the ball of cells will transform itself into a baby
and eventually into an adult.
Passing On the Genes from Cell to Cell
When a cell divides, the two “daughter” cells that result are ordinarily genetically identical to each other and to the original “parent” cell. (Biologists
traditionally use the word daughter in this context; it does not imply gender.) Before the parent cell splits into two, it duplicates its chromosomes,
the DNA-containing structures that carry the organism’s genes. Then, during the division process, the two sets of chromosomes are distributed to the
daughter cells. As a rule, the daughter cells receive identical sets of chromosomes, with identical genes.
Some organisms reproduce by simple cell division. Single-celled organisms
such as amoebas reproduce this way, and the offspring are replicas of the
parent (Figure 7.2). Because it does not involve fertilization of an egg by a
sperm, this type of reproduction is called asexual reproLearn more about
duction. Offspring produced by asexual reproduction inasexual and sexual
herit all their chromosomes from a single parent. In addireproduction in
tion to single-celled creatures, many multicellular
Web/CD Activity 7A.
organisms can also reproduce asexually. For example,
some sea stars can divide into two pieces that regrow into two whole new
individuals. And if you’ve ever grown an African violet from a clipping,
you’ve observed asexual reproduction in plants. In asexual reproduction,
there is one simple principle of inheritance: The parent and each of its offspring have identical genes.
Sexual reproduction, which requires fertilization of an egg by a sperm,
is different. The production of egg and sperm cells involves a special type of
cell division, called meiosis, that occurs only in reproductive organs (such
as testes and ovaries). As we’ll discuss later, a sperm or egg cell has only half
as many chromosomes as the parent cell that gave rise to it. So two kinds of
LM 300
The Reproduction of Organisms
Figure 7.2 Asexual reproduction of an amoeba. This
single-celled organism is reproducing by dividing in half. Its chromosomes have been duplicated, and the two identical sets of chromosomes have been allocated to opposite sides of the parent. When
division is complete, the two daughter amoebas will be genetically
identical to each other and to their parent.
CHAPTER 7
The Cellular Basis of Reproduction and Inheritance
3
CheckPoint
LM 600
cell division—meiosis and ordinary cell division—are involved in the lives
of sexually reproducing organisms.
The remainder of the chapter is divided into two main sections. The
first section deals with the cell cycle and mitosis, the type of cell division responsible for asexual reproduction and for the growth and maintenance of
multicellular organisms. The second section focuses on meiosis, the special
type of cell division that produces sperm and eggs for sexual reproduction.
Ordinary cell division produces two daughter cells that are genetically
identical. Give three functions of this type of cell division.
Answer: Cell replacement, growth of an organism, asexual reproduction of an organism
Figure 7.3 A plant cell just before division. The colors
result from staining. The purple threads are the chromosomes. (The
thinner red threads in the surrounding cytoplasm are the cytoskeleton.)
TEM 29,000×
The Cell Cycle and Mitosis
Almost all the genes of a eukaryotic cell—around 75,000 genes in humans
—are located on chromosomes in the cell nucleus. (The main exceptions
are genes on small DNA molecules found in mitochondria and chloroplasts.) As leading players in cell division, the chromosomes deserve a little
more of our attention before we broaden our focus to the cell as a whole.
Centromere
Figure 7.4 Electron micrograph of
a duplicated chromosome. The two
identical copies of the chromosome are
called sister chromatids. The constricted
region is the centromere. The fuzzy appearance comes from the intricate twists
and folds of the chromatin fibers.
Sister chromatids
Chromosome
duplication
Sister
chromatids
Figure 7.5 Chromosome dupliChromosome
distribution
to
daughter
cells
4
UNIT TWO
cation and distribution. In the
course of cell reproduction, the cell
duplicates each chromosome and
distributes the two copies to the daughter cells. This diagram focuses on a
single chromosome.
Genetics
Eukaryotic Chromosomes
Chromosomes get their name (Greek chroma, colored, and soma, body)
from their attraction for certain stains used in microscopy. In the micrograph of a plant cell in Figure 7.3, each dark purple thread is an individual
chromosome. Chromosomes are clearly visible under the light microscope
as structures like these only when a cell is in the process of dividing. The
rest of the time, the chromosomes exist as a diffuse mass of very long fibers
that are too thin to distinguish in a light micrograph. The chromosomal
material, called chromatin, is a combination of DNA and protein molecules. As a cell prepares to divide, its chromatin fibers coil up, forming compact, distinct chromosomes.
Each eukaryotic chromosome contains one long DNA molecule, typically bearing thousands of genes. The attached protein molecules help organize the chromatin and help control the activity of its genes. The number
of chromosomes in a eukaryotic cell, like the number of genes, depends on
the species. For example, human body cells generally have 46 chromosomes.
Well before a cell begins the division process, it duplicates all of its chromosomes. The DNA molecule of each chromosome is replicated, and new
protein molecules attach as needed. The result is that each chromosome
now consists of two copies called sister chromatids, which contain identical genes. Figure 7.4 is an electron micrograph of a human chromosome
that has been duplicated. The two chromatids are joined together especially
tightly at a region called the centromere.
When the cell divides, the sister chromatids of a duplicated chromosome separate from each other, as shown in the simple diagram in Figure
7.5. Once separated from its sister, each chromatid is considered a fullfledged chromosome, and it is identical to the chromosome we started with.
One of the new chromosomes goes to one daughter cell, and the other goes
to the other daughter cell. In this way, each daughter cell receives a complete
and identical set of chromosomes. A dividing human skin cell, for example,
has 46 duplicated chromosomes, and each of the two daughter cells that results from it has 46 single chromosomes.
Let’s now summarize how cell division fits into the life of an organism.
Cell division is the basis of reproduction for every organism, and it enables
a multicellular organism to grow from a single cell. It also replaces wornout or damaged cells, keeping the total cell number in a mature individual
relatively constant. In your own body, for example, millions of cells must
divide every second to maintain the total number of about 60 trillion cells.
G1
The Cell Cycle
How do chromosome duplication and cell division fit into
the life of a cell? How often a cell divides depends on the role
of the cell in the organism’s body. Some cells divide once a
day, others less often, and highly specialized cells, such as mature muscle cells, not at all. Eukaryotic cells that do divide un- Cytokinesis
dergo a cell cycle, an orderly sequence of events that extends
from the time a cell first arises from cell division until it itself
divides.
As Figure 7.6 shows, most of the cell cycle is spent in interphase. This is a time when a cell metabolizes and performs its
various other functions within the organism. For example, a cell in Mitosis
your stomach lining might be making and releasing enzyme molecules that aid in digesting the food you eat. During interphase, a cell
roughly doubles everything in its cytoplasm. It increases its supply of proteins, increases the number of many of its organelles (such as mitochondria
and ribosomes), and grows in size. Typically, interphase lasts for at least
90% of the cell cycle.
From the cell reproduction standpoint, the most important event of interphase is chromosome duplication, when the DNA in the nucleus is precisely doubled. This occurs approximately in the middle of interphase, and
the period when it is occurring is called the S phase (for DNA synthesis).
The interphase periods before and after the S phase are called the G1 and G2
phases, respectively (G stands for gap). During G2, each chromosome in the
cell consists of two identical sister chromatids, and the cell is preparing to
divide.
The part of the cell cycle when the cell is actually dividing is called the
mitotic phase (M phase). It includes two overlapping processes, mitosis and
cytokinesis. In mitosis, the nucleus and its contents, notably the duplicated
chromosomes, divide and are evenly distributed to form
Test your knowledge
two daughter nuclei. In cytokinesis, the cytoplasm is diof the cell cycle with
vided in two. Cytokinesis usually begins before mitosis is
Web/CD Activity 7B .
completed. The combination of mitosis and cytokinesis
produces two genetically identical daughter cells, each with a single nucleus,
surrounding cytoplasm, and plasma membrane.
Mitosis is a remarkably accurate mechanism for allocating identical
copies of a large amount of genetic material to two daughter cells. Experiments with yeast cells, for example, indicate that an error in chromosome
distribution occurs only once in about 100,000 cell divisions. Mitosis is
unique to eukaryotes. Prokaryotes have only a single small chromosome
(see Chapter 9) and use a simpler mechanism for allocating DNA to daughter cells.
Mitotic
phase (M)
Interphase
S
(DNA synthesis—
chromosome
duplication)
G2
Figure 7.6 The eukaryotic cell cycle. The cell cycle extends from the “birth” of a cell as a result of cell reproduction to the
time the cell itself divides in two. The cell spends most of the cycle
in interphase. A key event of interphase is the duplication of the
chromosomes; the period during which this occurs is called the S
phase (for DNA synthesis). Before S, the cell is said to be in the G1
phase; after S, the cell is in G2. The cell metabolizes and grows
throughout interphase. The actual division process occurs during
the mitotic phase (M phase), which includes mitosis (the division of
the cell’s nucleus) and cytokinesis (the division of the cytoplasm).
During interphase, the chromosomes are diffuse masses of thin
fibers; they do not actually appear in the rodlike form you see here.
CHAPTER 7
The Cellular Basis of Reproduction and Inheritance
5
Mitosis and Cytokinesis
Figure 7.7 Cell reproduction: A dance of the chromosomes. After the chromatin doubles during interphase, the
elaborately choreographed stages of mitosis—prophase,
metaphase, anaphase, and telophase—distribute the duplicate sets
of chromosomes to two separate nuclei. Cytokinesis then divides
the cytoplasm, yielding two genetically identical daughter cells. The
micrographs here show cells from a fish. The drawings include details not visible in the micrographs. For simplicity, only four
chromosomes appear in the drawings.
The light micrographs in Figure 7.7 show the cell cycle for an animal cell,
with most of the figure devoted to the mitotic phase. With the onset of mitosis, striking changes are visible in the nucleus and other
See mitosis animacellular structures. The text under the figure describes the
tions and videos in
events occurring at each stage. Mitosis is a continuum,
Web/CD Activities 7C
but biologists distinguish four main stages: prophase,
and 7D.
metaphase, anaphase, and telophase.
The chromosomes are the stars of the mitotic drama, and their movements depend on the mitotic spindle, a football-shaped structure of microtubules that guides the separation of the two sets of daughter chromosomes. The spindle microtubules grow from two centrosomes, clouds of
Prophase
Interphase
Centrosomes
(with
centriole
pairs)
Chromatin
Early mitotic Centrosome Centromere
spindle
Fragments of
nuclear envelope
Nucleolus
Nuclear
envelope
Plasma
membrane
Interphase
Interphase is the period of cell growth,
when the cell makes new molecules and
organelles. At the point shown here, late
interphase (G2), the cytoplasm contains
two centrosomes. Within the nucleus, the
chromosomes are duplicated, but they
cannot be distinguished individually
because they are still in the form of loosely
packed chromatin fibers. The prominent
nucleolus is an indication that the cell is
making ribosomes.
6
UNIT TWO
Genetics
Chromosome, consisting
of two sister chromatids
Spindle
microtubules
Prophase
During prophase, changes occur in both
nucleus and cytoplasm. In the nucleus, the
chromatin fibers coil, so that the
chromosomes become thick enough to be
seen with the light microscope. The nucleoli
disappear. Each chromosome appears as
two identical sister chromatids joined
together, with a narrow “waist” at the
centromere. In the cytoplasm, the mitotic
spindle begins to form as microtubules
grow out from the centrosomes, which are
moving away from each other.
Late in prophase, the nuclear envelope
breaks up. The spindle microtubules can
now reach the chromosomes, which are
thick and have a protein structure (black
dot) at their centromeres. Some of the
spindle microtubules capture chromosomes
by attaching to these structures, throwing
the chromosomes into agitated motion.
Other microtubules make contact with
microtubules coming from the opposite
spindle pole. The spindle moves the
chromosomes toward the center of the cell.
cytoplasmic material that in animal cells contain centrioles. (Centrioles are
can-shaped structures made of microtubules; microtubules and centrioles
were introduced in Chapter 3.) The role of centrioles in cell division is a
mystery; destroying them experimentally does not interfere with normal
spindle formation, and plant cells lack them entirely.
Cytokinesis, the actual division of the cytoplasm into two cells, typically
occurs during telophase. In animal cells, the cytokinesis process is known as
cleavage. The first sign of cleavage is the appearance of a cleavage furrow,
an indentation at the equator of the cell (Figure 7.8a). A ring of microfilaments in the cytoplasm just under the plasma membrane is responsible for
the cleavage furrow. The ring contracts like the pulling of a drawstring,
deepening the furrow and pinching the parent cell in two. Microfilaments
are made of actin, a protein that also enables muscle cells to contract.
Metaphase
Anaphase
Telophase and Cytokinesis
Nucleolus
forming
Cleavage
furrow
Nuclear
envelope
forming
Daughter
chromosomes
Spindle
Metaphase
Anaphase
Telophase and Cytokinesis
The mitotic spindle is now fully formed. The
chromosomes convene on an imaginary
plate equidistant from the two poles of the
spindle. The centromeres of all the
chromosomes are lined up at this plate.
For each chromosome, the spindle
microtubules attached to the two sister
chromatids pull toward opposite poles.
This tug of war keeps the chromosomes in
the middle of the cell.
Anaphase begins suddenly, when the
sister chromatids of each chromosome
separate. Each is now considered a fullfledged (daughter) chromosome. Motor
proteins at the centromeres “walk” the
daughter chromosomes along their
microtubules toward opposite poles of the
cell (see motor proteins in Figure 5.8).
Meanwhile, these microtubules shorten.
However, the microtubules not attached to
chromosomes lengthen, pushing the poles
farther apart and elongating the cell.
Telophase begins when the two
groups of chromosomes have reached the
cell poles. Telophase is the reverse of
prophase: Nuclear envelopes form, the
chromosomes uncoil, nucleoli reappear,
and the spindle disappears. Mitosis, the
division of one nucleus into two genetically
identical daughter nuclei, is now finished.
Cytokinesis, the division of the
cytoplasm, usually occurs with telophase.
In animals, a cleavage furrow pinches the
cell in two, producing two daughter cells.
CHAPTER 7
The Cellular Basis of Reproduction and Inheritance
7
SEM 100×
Cancer Cells: Growing out of Control
Cleavage
furrow
For a plant or animal to grow and develop normally and maintain its tissues
once full grown, it must be able to control the timing of cell division in different parts of its body. The sequential events of the cell cycle are directed
by a cell cycle control system that consists of special proteins within the cell.
When this control system malfunctions, cells may reproduce at the wrong
time or in the wrong place. This can result in a benign tumor, an abnormal
mass of essentially normal cells. Benign tumors can cause problems if they
grow in certain organs, such as the brain, but usually they can be completely
removed by surgery. They always remain at their original site in the body.
Cleavage furrow
Contracting ring of
microfilaments
What Is Cancer? Cancer, which currently claims the lives of one out of
Daughter cells
(a) Animal cell
Cell plate
forming
Daughter
nucleus
LM 850×
Wall of
parent cell
Cell wall
Vesicles containing
cell wall material
Cytokinesis in a plant cell occurs differently (Figure 7.8b). A cell plate, a
membranous disk containing cell-wall material, forms inside the cell and
grows outward. Eventually the new piece of wall divides the cell in two.
every five people in the United States and other developed nations, is a serious disease of the cell cycle. Unlike normal cells of the body, cancer cells
have a severely deranged cell cycle control system; not only do they divide
excessively, but they also exhibit other kinds of bizarre behavior. A lump resulting from the reproduction of a cancer cell is called a malignant tumor.
The most dangerous attribute of cancer cells is their ability to spread
into neighboring tissues and often to other parts of the body. Like a benign
tumor, a malignant tumor displaces normal tissue as it grows (Figure 7.9).
But if a malignant tumor is not killed or removed, it can spread into surrounding tissues. More alarming still, cells may split off from the tumor, invade the circulatory system (lymph vessels and blood vessels), and travel to
new locations, where they can form new tumors. The spread of cancer cells
beyond their original site is called metastasis.
Cancers are named according to where they originate. They are grouped
into four categories. Carcinomas are cancers that originate in the external
or internal coverings of the body, such as the skin or the lining of the intestine. Sarcomas arise in tissues that support the body, such as bone and
muscle. Cancers of blood-forming tissues, such as bone marrow and lymph
nodes, are called leukemias and lymphomas.
Cancer Treatment In addition to removing malignant tumors with surgery, physicians combat cancers in two other ways: radiation therapy and
chemotherapy. Both types of cancer treatment attempt to stop cancer cells
from dividing. In radiation therapy, parts of the body that have cancerous
Cell plate
New cell wall
Figure 7.8 Cytokinesis in animal and plant cells. (a) In a dividing animal
Daughter cells
(b) Plant cell
8
UNIT TWO
Genetics
cell, a contracting ring of microfilaments pinches the cell in two. (b) In a plant cell, the
development of a new piece of cell wall brings about cytokinesis. First, membrane-enclosed vesicles containing cell wall material collect at the middle of the cell. The
vesicles gradually fuse, forming a membranous disk called the cell plate. The cell plate
grows outward, accumulating more cell wall material as more vesicles join it. Eventually, the membrane of the cell plate fuses with the plasma membrane, and the cell plate’s
contents join the parental cell wall. The result is two daughter cells, each bounded by its
own continuous plasma membrane and a cell wall.
CheckPoint
tumors are exposed to high-energy radiation,
which disrupts cell division. Because cancer
cells divide more often than most normal
cells, they are more likely to be dividing at any
given time. So radiation can often destroy
cancer cells without seriously injuring the
Tumor
normal cells of the body. However, there is
sometimes enough damage to normal body
cells to produce bad side effects. For example,
Glandular
damage to cells of the ovaries or testes can
tissue
cause sterility.
Chemotherapy generally uses the same
A tumor grows from a
strategy as radiation; in this case, drugs that
single cancer cell.
disrupt cell division are administered to the
patient. These drugs work in a variety of
ways. Some, called antimitotic drugs, prevent cell division by interfering
with the mitotic spindle. One antimitotic drug, vinblastine, prevents the
spindle from forming in the first place; another, Taxol, freezes the spindle
after it forms, keeping it from functioning.
Vinblastine was first obtained from the periwinkle, a flowering plant native to tropical rain forests in Madagascar. Taxol is made from a chemical
found in the bark of the Pacific yew, a tree found mainly in the northwestern United States. Taxol has fewer side effects than many anticancer drugs
and seems to be effective against some hard-to-treat cancers of the ovary
and breast.
In the laboratory, researchers can grow cancer cells in culture. The cells
are placed in a glass container, and nutrients are provided by an artificial
liquid medium (Figure 7.10). Normal mammalian cells grow and divide in
culture for only about 50 cell generations. But cancer cells are “immortal”
—they can go on dividing indefinitely, as long as they have a supply of nutrients. It is by studying cancer cells in culture that researchers are learning
about the molecular changes that make a cell cancerous. We will return to
the topic of cancer in Chapter 10, after learning more about genes.
1. When in the cell cycle does each chromosome consist of two chromatids?
2. An organism called a plasmodial slime mold is one huge cytoplasmic
mass with many nuclei. Explain how this “monster cell” could arise.
Lymph
vessels
Metastasis
Cancer cells invade
neighboring tissue.
Cancer cells spread through lymph and
blood vessels to other parts of the body.
Figure 7.9 Growth and metastasis of a malignant
(cancerous) tumor of the breast.
Figure 7.10 Growing cancer cells in the lab. This
researcher is working under a fume hood to help prevent contamination of the cells by microbes from the air.
3. In what sense are the two daughter cells produced by mitosis identical?
4. When a cancer patient is treated with vinblastine, which prevents the
mitotic spindle from forming, in what stage of mitosis are dividing
cells trapped?
Answers: 1. During G2, the last part of interphase, and during prophase and metaphase of the next
mitosis 2. Mitosis occurs repeatedly without cytokinesis. 3. They have identical genes (DNA).
4. Prophase
CHAPTER 7
The Cellular Basis of Reproduction and Inheritance
9
Meiosis,the Basis of Sexual Reproduction
Figure 7.11 The varied products of sexual reproduction. Eddie Murphy, his wife Nicole, and their children pose for a
family snapshot. Each child has inherited a unique combination of
genes from the parents and displays a unique combination of traits.
Pair of homologous
chromosomes
Centromere
LM 3600
Sister chromatids
Only maple trees produce more maple trees, only goldfish make more goldfish, and only people make more people. These simple facts of life have
been recognized for thousands of years and are reflected in the age-old saying “Like begets like.” But in a strict sense, “Like begets like” applies only to
asexual reproduction, such as the reproduction of the amoeba in Figure 7.2.
In that case, because offspring inherit all their DNA from a single parent,
they are exact genetic replicas of that one parent and of each other, and
their appearances are very similar.
The family photo in Figure 7.11 makes the point that in a sexually reproducing species, like does not exactly beget like. You probably resemble
your parents more closely than you resemble a stranger, but you do not
look exactly like your parents or your siblings. Each offspring of sexual reproduction inherits a unique combination of genes from its two parents,
and this combined set of genes programs a unique combination of traits. As
a result, sexual reproduction can produce great variation among offspring.
Notice in the photograph that despite the family resemblances, each Murphy child has a unique appearance. You’ll find the same sort of similarities
and differences in pictures of your own relatives.
Long before anyone knew about genes or chromosomes, people recognized that individuals of sexually reproducing species are highly varied.
What’s more, they learned to develop domestic breeds of plants and animals by controlling sexual reproduction. A domestic breed displays particular traits from among the great variety of traits found in the species as a
whole. All dachshunds, for instance, have sausage-shaped bodies and short
legs, whereas Saint Bernards are much taller and bulkier. The ancestry of
dog breeds can be traced back for many generations, during which breeders
reduced variability in the breed by mating only those dogs with specific
traits. In a sense, selective breeding is an attempt to make like beget like
more than it does in nature.
Sexual reproduction depends on the cellular processes of meiosis and
fertilization. But before discussing these processes, we return to chromosomes and their role in the life cycles of sexually reproducing organisms.
Homologous Chromosomes
Figure 7.12 Pairs of homologous chromosomes. To
make this karyotype (chromosome display) of a man, a scientist
broke open a cell in metaphase of mitosis, stained the released
chromosomes with special dyes, made a micrograph, and then
arranged the chromosome images in matching pairs. The result: 22
well-matched pairs (autosomes) and a twenty-third pair that consists of an X chromosome and a Y chromosome (sex chromosomes).
Each chromosome consists of two sister chromatids closely attached all along their lengths. Notice that with the exception of X
and Y, the homologous chromosomes of each pair match in size,
centromere position, and staining pattern.
10
UNIT TWO
Genetics
If we examine a number of cells from any individual organism, we discover
that virtually all of them have the same number and types of chromosomes.
Likewise, if we examine cells from different individuals of a single species—
sticking to one gender, for now—we find that they have the same number
and types of chromosomes. Viewed with a microscope, your chromosomes
would look just like those of Queen Elizabeth (if you’re a woman) or
Michael Jordan (if you’re a man).
A typical body cell, called a somatic cell, has 46 chromosomes in humans. If we break open a human cell in metaphase of mitosis, make a micrograph of the chromosomes, and arrange the chromosome images in an
orderly array, we produce a display called a karyotype (Figure 7.12). Every
(or almost every) duplicated chromosome has a twin that resembles it in
size and shape. The two chromosomes of such a matching pair, called homologous chromosomes, carry the same sequence of genes controlling the
same inherited characteristics. For example, if a gene influencing eye color
is located at a particular place on one chromosome—for example, within
the yellow band in the Figure 7.12 inset—then the homologous chromosome has a similar gene for eye color there. However, the two genes may be
slightly different versions, unlike the ones on sister chromatids, which are
identical. Altogether, we humans have 23 homologous pairs of chromosomes. Other species have different numbers of chromosomes, but these,
too, usually match in pairs.
For a human female, the 46 chromosomes fall neatly into 23 homologous pairs, with the members of each pair essentially identical in appearance. For a male, however, one pair of chromosomes do not look alike (see
Figure 7.12). The nonmatching pair, called the sex chromosomes, determines the person’s gender. Like all mammals, human males have one X
chromosome and one Y chromosome. Only small parts of the X and Y are
homologous; most of the genes carried on the X chromosome do not have
counterparts on the tiny Y, and the Y has genes lacking on the X. Females
have two X chromosomes. The remaining chromosomes, found in both
males and females, are called autosomes. For both autosomes and sex chromosomes, we inherit one chromosome of each pair from our mother and
the other from our father.
Haploid gametes (n = 23)
n
Egg cell
n
Sperm cell
Fertilization
Meiosis
Diploid
zygote
(2n = 46)
Multicellular
diploid adults
(2n = 46)
2n
Gametes and the Life Cycle of a Sexual Organism
Having two sets of chromosomes, one inherited from each parent, is a key
Mitosis and
factor in the human life cycle, outlined in Figure 7.13, and in the life cycles
development
of all other species that reproduce sexually. The life cycle of a multicellular
organism is the sequence of stages leading from the adults of one generaFigure 7.13 The human life cycle. In each generation, the
tion to the adults of the next. Let’s follow the human chromosomes through
doubling of chromosome number that results from fertilization is offthe human life cycle.
set by the halving of chromosome number that occurs in meiosis.
Humans are said to be diploid organisms because almost all our cells are
For humans, the number of chromosomes in a haploid cell (sperm or
diploid: They contain two homologous sets of chromosomes. The total
egg) is 23 (that is, n 23). The number of chromosomes in the
number of chromosomes, 46 in humans, is the diploid
diploid zygote and all somatic cells arising from it is 46 (2n 46).
Review sexual life
number (abbreviated 2n). The exceptions are the egg and
cycles in Web/CD
sperm cells, known as gametes. Made by meiosis in an
Activity 7E.
ovary or testis, each gamete has a single set of chromosomes: 22 autosomes plus a single sex chromosome, X or Y. A cell with a single chromosome set is called a haploid cell; it has only one member of each
homologous pair. For humans, the haploid number (abbreviated n) is 23.
In the human life cycle, sexual intercourse allows a haploid sperm cell
from the father to reach and fuse with a haploid egg cell of the mother in
the process known as fertilization. The resulting fertilized egg, called a zygote, is diploid. It has two homologous sets of
2 Homologous
chromosomes, one set from each par1 Chromosomes
chromosomes
duplicate
separate
ent. The life cycle is completed as a
sexually mature adult
3 Sister
chromatids
develops from the zyseparate
gote. Mitotic cell division ensures that all
Homologous
Homologous
Sister
somatic cells of the
pair of
pair of
chromatids
human body receive
chromosomes
duplicated
in diploid
chromosomes
copies of all of the zyparent cell
gote’s 46 chromosomes.
All sexual life cycles
Interphase before meiosis
Meiosis I
Meiosis II
involve an alternation
of diploid and haploid
homologous pair, packaging them in separate (haploid) daughFigure 7.14 How meiosis halves chromosome
stages. Producing haploid number. This simplified diagram tracks just one pair of hoter cells. But each chromosome is still doubled. 3 Meiosis II
gametes by meiosis keeps mologous chromosomes. 1 Each of the chromosomes is
separates the sister chromatids. Each of the four daughter
the chromosome number duplicated during the preceding interphase. 2 The first divicells is haploid and contains only one single chromosome from
from doubling in every
the homologous pair.
sion, meiosis I, segregates the two chromosomes of the
generation (Figure 7.14).
CHAPTER 7
The Cellular Basis of Reproduction and Inheritance
11
The Process of Meiosis
Meiosis, the process that produces haploid gametes in diploid organisms, resembles mitosis, but with two special features. The first is the halving of the
number of chromosomes. In meiosis, a cell that has duplicated its chromosomes undergoes two consecutive divisions, called meiosis I and meiosis II. Because the two divisions of meiosis are preceded by only one
Watch an animation
duplication of the chromosomes, each of the four daughter
of meiosis in Web/CD
cells resulting from meiosis has only half as many chromoActivity 7F.
somes as the starting cell—a haploid set of chromosomes.
The chromosome number is actually haploid by the end of meiosis I, although there are still two sister chromatids per chromosome at that point.
Figure 7.15 The stages of meiosis. The drawings here show the two cell divisions of meiosis, starting with a diploid animal cell containing four chromosomes. Each
homologous pair consists of a red chromosome and a blue chromosome of the same
size. The colors remind us that the members of a homologous pair were inherited from
different parents and carry different versions of some genes.
Meiosis I: Homologous chromosomes separate
Interphase
Centrosomes
(with
centriole
pairs)
Nuclear
envelope
Chromatin
Like mitosis, meiosis is preceded
by an interphase during which
the chromosomes duplicate.
Each chromosome then consists
of two identical sister chromatids.
Genetics
Microtubules
attached to
chromosome
Anaphase I
Sister chromatids
remain attached
Spindle
Interphase
UNIT TWO
Metaphase I
Sites of crossing over
Chromosomes duplicate
12
Prophase I
Sister
chromatids
Tetrad
Homologous chromosomes pair
and exchange segments
Centromere
Tetrads line up
Pairs of homologous
chromosomes split up
Meiosis I
Prophase I Prophase I is the most complicated
stage of meiosis. As the chromatin condenses,
special proteins cause the homologous chromosomes to stick together in pairs. The resulting
structure has four chromatids and is called a
tetrad. Within each tetrad, chromatids of the
homologous chromosomes exchange corresponding segments—they “cross over.” Because the
versions of the genes on a chromosome (or one of
its chromatids) may be different from those on its
homologue, crossing over rearranges genetic
information.
As prophase I continues, the chromosomes
condense further, a spindle forms, and the tetrads
are moved toward the center of the cell.
Metaphase I At metaphase I, the tetrads are
aligned in the middle of the cell. The sister chromatids of each chromosome are still attached at their
centromeres, where they are anchored to spindle
microtubules. Notice that for each tetrad, the
spindle microtubules attached to one homologous
chromosome come from one pole of the cell, and
the microtubules attached to the other chromosome come from the opposite pole. With this
arrangement, the homologous chromosomes of
each tetrad are poised to move toward opposite
poles of the cell.
Anaphase I As in anaphase of mitosis, chromosomes now migrate toward the poles of the cell.
But in contrast to mitosis, the sister chromatids
migrate as a pair instead of splitting up. They are
separated not from each other, but from their
homologous partners. So in the drawing, you see
two still-doubled chromosomes moving toward
each pole.
The second special feature of meiosis is an exchange of genetic material
—pieces of chromosomes—between homologous chromosomes. This exchange, called crossing over, occurs during the first prophase of meiosis.
We’ll look more closely at crossing over later. For now, study Figure 7.15
and the text below it, which describe the stages of meiosis in detail.
As you go through Figure 7.15, keep in mind the difference between homologous chromosomes and sister chromatids: The two chromosomes of a
homologous pair are individual chromosomes that were inherited from different parents. Homologues appear alike in the microscope, but they have
different versions of some of their genes (for example, a gene for freckles on
one chromosome and a gene for the absence of freckles at the same place on
the homologue). The homologues in Figure 7.15 (and later figures) are colored red and blue to remind you that they differ in this way. In the interphase just before meiosis, each homologue replicates to form sister chromatids that remain together until anaphase of meiosis II. Before crossing
over occurs, sister chromatids are identical and carry the same versions of
all their genes.
Meiosis II: Sister chromatids separate
Telophase I
and Cytokinesis
Prophase II
Metaphase II
Telophase II
and Cytokinesis
Anaphase II
Cleavage
furrow
Sister chromatids
separate
Two haploid cells form;
chromosomes are still double
Telophase I and Cytokinesis
In telophase I, the chromosomes arrive at the poles of
the cell. When they finish
their journey, each pole has a
haploid chromosome set, although each chromosome is
still in duplicate form. Usually,
cytokinesis occurs along with
telophase I, and two haploid
daughter cells are formed.
Depending on the
species, the nuclei may or
may not return to an interphase state. But in either
case, there is no further
chromosome duplication.
Haploid daughter cells
forming
During another round of cell division, the sister chromatids finally separate; four haploid daughter cells
result, containing single chromosomes
Meiosis II
Meiosis II is essentially the same as mitosis. The
important difference is that meiosis II starts with a
haploid cell.
During prophase II, a spindle forms and moves
the chromosomes toward the middle of the cell.
During metaphase II, the chromosomes are aligned
as they are in mitosis, with the microtubules attached
to the sister chromatids of each chromosome
coming from opposite poles. In anaphase II, the
centromeres of sister chromatids finally separate,
and the sister chromatids of each pair, now individual
daughter chromosomes, move toward opposite poles
of the cell. In telophase II, nuclei form at the cell
poles, and cytokinesis occurs at the same time.
There are now four daughter cells, each with the
haploid number of single chromosomes.
CHAPTER 7
The Cellular Basis of Reproduction and Inheritance
13
Mitosis
Meiosis
Parent cell
(before chromosome duplication)
Duplicated chromosome
(two sister
chromatids)
Chromosome
duplication
Chromosome
duplication
Site of crossing
over
Meiosis I
Tetrad formed by
pairing of homologous
chromosomes
Pairing of
homologous
chromosomes
Prophase
Prophase I
2n = 4
Chromosomes
align at the
middle of
the cell
Metaphase
Anaphase
Telophase
Sister chromatids
separate during
anaphase
2n
Daughter cells
of mitosis
2n
Tetrads
align at the
middle of
the cell
Metaphase I
Homologous
chromosomes
separate during
anaphase I;
sister
chromatids
remain
together
Anaphase I
Telophase I
Daughter
cells of meiosis I
Haploid
n=2
No further
chromosomal
duplication; sister
chromatids
separate during
anaphase II
Meiosis II
n
n
n
Daughter cells of meiosis II
Figure 7.16 Comparing mitosis and
meiosis. The events unique to meiosis occur during meiosis I: In prophase I, duplicated homologous
chromosomes pair to form tetrads, and crossing
over occurs between homologous (nonsister) chromatids. In metaphase I, tetrads (rather than
14
UNIT TWO
Genetics
individual chromosomes) are aligned at the center
of the cell. During anaphase I, sister chromatids of
each chromosome stay together and go to the same
pole of the cell as homologous chromosomes separate. At the end of meiosis I, there are two haploid
cells, but each chromosome still has two sister
n
chromatids.Meiosis II is virtually identical to mitosis
and separates sister chromatids. But unlike mitosis,
meiosis II yields daughter cells with a haploid set of
chromosomes.
Review: Comparing Mitosis and Meiosis
We have now described the two ways that cells of eukaryotic organisms divide. Mitosis, which provides for growth, tissue repair, and asexual reproduction, produces daughter cells genetically identical to the parent cell.
Meiosis, needed for sexual reproduction, yields haploid daughter cells—
cells with only one member of each homologous chromosome pair.
For both mitosis and meiosis, the chromosomes duplicate only once, in
the preceding interphase. Mitosis involves one division of the nucleus, and
it is usually accompanied by cytokinesis, producing two diploid cells. Meiosis entails two nuclear and cytoplasmic divisions, yielding four haploid cells.
Figure 7.16 (at the left) compares mitosis and meiosis, tracing these two
processes for a diploid parent cell with four chromosomes. As before, homologous chromosomes are those matching in size. Notice that all the
events unique to meiosis occur during meiosis I.
The Origins of Genetic Variation
As we discussed earlier, offspring that result from sexual reproduction
are genetically different from their parents and from one another.
When we discuss natural selection and evolution in Unit Three, we
will see that this genetic variety in offspring is the raw material for
natural selection. For now, let’s take another look at meiosis and fertilization to see how genetic variety arises.
Possibility 1
Possibility 2
Metaphase of
meiosis I
Independent Assortment of Chromosomes Figure 7.17 illustrates
one way in which meiosis contributes to genetic variety. The figure
shows how the arrangement of homologous chromosome
pairs at metaphase of meiosis I affects the resulting gametes.
Once again, our example is from an organism with a diploid
chromosome number of 4, with red and blue used to differentiate homologous chromosomes. These colors highlight the fact
that homologous chromosomes differ genetically, although the
two look alike under a microscope. (For example, the chromosome 3 you inherited from your mother undoubtedly
carries many genes that are slightly different from those on
the homologous chromosome 3 you received from your father.)
Combination 1
The orientation of the homologous pairs of chromosomes
(tetrads) at metaphase I is a matter of chance, like the flip of a coin. In this
example, there are two possible ways that the two tetrads can align during
metaphase I. In possibility 1, the tetrads are oriented with both red chromosomes on the same side. In this case, each of the gametes produced at the
end of meiosis II has only red or only blue chromosomes (combinations 1
and 2). In possibility 2, the tetrads are oriented differently. This arrangement produces gametes that each have one red and one blue chromosome.
Furthermore, half the gametes have a big blue chromosome and a small red
one (combination 3), and half have a big red one and a small blue one
(combination 4).
So we see that for this example, a total of four chromosome combinations
is possible in the gametes, and the organism will in fact produce gametes of
all four types. This variety in gametes arises because each homologous pair
Metaphase of
meiosis II
Gametes
Combination 2
Combination 3
Combination 4
Figure 7.17 Results of alternative arrangements of
chromosomes at metaphase of meiosis I. In this figure, we
consider the consequences of meiosis in a diploid organism with
four chromosomes (two homologous pairs). The positioning of each
homologous pair of chromosomes (tetrad) at metaphase of meiosis I
is random; the two red chromosomes can be on the same side (possibility 1) or on opposite sides (possibility 2). The arrangement of
chromosomes at metaphase I determines which chromosomes will
be packaged together in the haploid gametes. Because possibilities
1 and 2 are equally likely, the four possible types of gametes will be
made in approximately equal numbers.
CHAPTER 7
The Cellular Basis of Reproduction and Inheritance
15
Tetrad
Prophase I
of meiosis
Chiasma,
site of
crossing
over
Metaphase I
Spindle
microtubules
Metaphase II
Gametes
of chromosomes orients itself at metaphase I independently of the other
pair. For a species with more than two pairs of chromosomes, such as the
human, every chromosome pair orients independently of all the others at
metaphase I. (Chromosomes X and Y behave as a homologous pair in
meiosis.)
For any species, the total number of chromosome combinations that
can appear in gametes is 2n, where n is the haploid number. For the organism in this figure, n 2, so the number of chromosome combinations is 22,
or 4. For a human (n 23), there are 223, or about 8 million, possible chromosome combinations! This means that every gamete a human produces
contains one of about 8 million possible combinations of maternal and paternal chromosomes.
Random Fertilization How many possibilities are there when a gamete
from one individual unites with a gamete from another individual during
fertilization? A human egg cell, representing one of about 8 million possibilities, is fertilized at random by one sperm cell, representing one of about
8 million other possibilities. By multiplying 8 million by 8 million, we find
that a man and a woman can produce a diploid zygote with any of 64 trillion combinations of chromosomes! So we see that the random nature of
fertilization adds a huge amount of potential variability to the offspring of
sexual reproduction.
These large numbers suggest that independent orientation of chromosomes at metaphase I and random fertilization could account for all the variety we see among people. Actually, these two events are only part of the
picture, as we see next.
Crossing Over So far, we have focused on genetic variability in gametes and
Recombinant
chromosomes
Figure 7.18 The results of crossing over during meiosis. This diagram focuses on a single pair of homologous chromosomes (a tetrad). Early in prophase I of meiosis, homologous
(nonsister) chromatids exchange corresponding segments, remaining attached at the crossover points. Sister chromatids are joined at
their centromeres. Following these chromosomes through the rest
of meiosis, we see that crossing over gives rise to recombinant
chromosomes—individual chromosomes that combine genetic information originally derived from different parents. With multiple
pairs of homologous chromosomes, the result is a huge variety of
gametes.
16
UNIT TWO
Genetics
zygotes at the whole-chromosome level. We have ignored crossing over, the
exchange of corresponding segments between two homologous chromosomes, which occurs during prophase I of meiosis. Figure 7.18 shows
crossing over between two homologous chromosomes and the results in the
gametes. At the time that crossing over begins, homologous chromosomes
are closely paired all along their lengths, with a precise gene-by-gene alignment. The sites of crossing over appear as X-shaped regions; each is called a
chiasma (Greek for cross; plural, chiasmata.) The homologous chromatids
remain attached to each other at chiasmata until anaphase I.
The exchange of segments by homologous chromatids adds to the genetic variety that results from the independent orientation of chromosome
pairs at metaphase I. In Figure 7.18, if there were no crossing over, meiosis
could produce only two types of gametes. These would be the ones ending
up with the “parental” types of chromosomes, either all blue or all red (as in
Figure 7.17). With crossing over, gametes arise that have chromosomes that
are part red and part blue. These chromosomes are called “recombinant”
because they result from genetic recombination, the production of gene
combinations different from those carried by the parental chromosomes.
Because most chromosomes contain thousands of genes, a single
crossover event can affect many genes. When we also consider that multiple
crossovers can occur in each tetrad, it’s no wonder that gametes and the offspring that result from them can be so varied. In fact, it’s surprising that
even siblings resemble one another as much as they do.
We have now examined three sources of genetic variability in sexually
reproducing organisms: crossing over during prophase I of meiosis, independent orientation of chromosome pairs at metaphase I, and random fer-
tilization. When we take
up molecular genetics
in Chapter 9, we will
see yet another source
of variation—mutations, which are rare changes in the DNA
of genes. The different versions of genes
found on homologous chromosomes
originally arose from mutations, and it is
mutations that are ultimately responsible
for the genetic diversity in living organisms.
To help you understand the origins of
genetic variability, go
to Web/CD Activity 7G.
When Meiosis Goes Amok
Down Syndrome: An Extra Chromosome 21 Figure 7.12 showed a normal
human complement of 23 pairs of chromosomes. Figure 7.19 is different;
besides having two X chromosomes (because it’s from a female), it has three
number 21 chromosomes. This condition is called trisomy 21.
In most cases, a human embryo with an abnormal number of chromosomes is spontaneously aborted (miscarried) long before birth. However,
some aberrations in chromosome number, including trisomy 21, seem to
upset the genetic balance less drastically, and individuals carrying them survive. These people usually have a characteristic set of symptoms, called a
syndrome. A person with trisomy 21, for instance, is said to have Down
syndrome (named after John Langdon Down, who described it in 1866).
Trisomy 21 is the most common chromosome number abnormality. Affecting about one out of every 700 children born, it is the most common serious birth defect in the United States. Chromosome 21 is one of our smallest chromosomes, but an extra copy produces a number of effects. Down
syndrome includes characteristic facial features—frequently a fold of skin at
the inner corner of the eye (epicanthic fold), a round face, flattened nose
bridge, and small, irregular teeth—as well as short stature, heart defects, and
susceptibility to respiratory infection, leukemia, and Alzheimer’s disease.
People with Down syndrome usually have a life span shorter than normal. They also exhibit varying degrees of mental retardation. However, individuals with the syndrome may live to middle age or beyond, and many
are socially adept and able to hold a job. A few women with Down syndrome have had children, though most people with the syndrome are sexually underdeveloped and sterile. Half the eggs produced by a woman with
Down syndrome will have an extra chromosome 21, so there is a 50%
chance that she will transmit the syndrome to her child.
As indicated in Figure 7.20, the incidence of Down syndrome in the offspring of normal parents increases markedly with the age of the mother.
Down syndrome strikes less than 0.05% of children (fewer than one in
2000) born to women under age 30. The risk climbs to 1% for mothers in
their late 30s and is even higher for older mothers. Because of this relatively
high risk, pregnant women over 35 are candidates for fetal testing for trisomy 21 and other chromosomal abnormalities (see Chapter 8).
What causes trisomy 21? We address that question next.
Figure 7.19 Trisomy 21 and Down syndrome. The karyotype (left) shows trisomy 21; notice the three copies of chromosome
21. The child displays the characteristic facial features of Down
syndrome.
Infants with Down syndrome
(per 1000 births)
So far, our discussion of meiosis has focused on the process as it normally and
correctly occurs. But what happens when
an error occurs in the process?
90
80
70
60
50
40
30
20
10
0
20
25
30
35
40
Age of mother
45
50
Figure 7.20 Maternal age and Down syndrome. The
chance of having a baby with Down syndrome rises with the age of
the mother.
CHAPTER 7
The Cellular Basis of Reproduction and Inheritance
17
(a)
(b)
Meiosis I
How Accidents During Meiosis Can Alter Chromosome Number Meiosis occurs repeatedly in
our lifetime as our testes or ovaries produce gametes. Almost always, the meiotic spindle
distributes chromosomes to daughter cells withNondisjunction
out error. But occasionally there is an accident,
called a nondisjunction, in which the members of
Meiosis II
a chromosome pair fail to separate at anaphase.
Nondisjunction can occur in meiosis I or II (Figure 7.21). In either case, gametes with abnormal
numbers of chromosomes result.
Nondisjunction
Figure 7.22 shows what can happen when an
abnormal
gamete produced by nondisjunction
Gametes
unites with a normal gamete in fertilization.
When a normal sperm fertilizes an egg cell with
an extra chromosome, the result is a zygote with a
total of 2n 1 chromosomes. Mitosis then transn+1
n+1
n–1
n–1
n+1
n–1
n
n
mits the abnormality to all embryonic cells. If the
Number of chromosomes
organism survives, it will have an abnormal karyFigure 7.21 Two types of nondisjunction. In both parts of
otype and probably a syndrome of disorders
the figure, the cell at the top is diploid (2n), with two pairs of homolocaused by the abnormal number of genes.
gous chromosomes. (a) A pair of homologous chromosomes fails to
Nondisjunction can lead to an abnormal chromosome number in either
separate during anaphase of meiosis I, even though the rest of
sex of any sexually reproducing, diploid organism, including humans. If, for
meiosis occurs normally. In this case, all the resulting gametes end
example, there is nondisjunction affecting human chromosome 21 during
up with abnormal numbers of chromosomes. (b) Meiosis I is normal,
meiosis I, half the resulting gametes will carry an extra chromosome 21.
but a pair of sister chromatids fail to move apart in one of the cells
Then if one of these gametes unites with a normal gamete in fertilization,
during anaphase of meiosis II. In this case, two gametes have the
trisomy 21 will result.
normal complement of two chromosomes each, but the other two
Nondisjunction explains how abnormal chromosome numbers come
gametes are abnormal.
about, but what causes nondisjunction in the first place? We do not know
the answer, nor do we fully understand why offspring with trisomy 21 are
more likely to be born as a woman ages. We do know, however, that meiosis
begins in a woman’s ovaries before she is born but is not completed until
years later, at the time of an ovulation. Because only one egg cell usually
matures each month, a cell might remain arrested in the middle of meiosis
Egg
for decades. Perhaps damage to the cell during this time leads to meiotic ercell
rors. It seems that the longer the time lag, the greater the chance that there
will be errors such as nondisjunction when meiosis is completed.
n+1
Sperm
cell
n (normal)
Zygote
2n + 1
Figure 7.22 Fertilization after nondisjunction in the
mother. Assuming that the organism has a diploid number of 4
(2n 4), the sperm is a normal haploid cell (n 2). The egg cell,
however, contains an extra copy of the larger chromosome as a result of nondisjunction in meiosis; it has a total of n 1 3
chromosomes. When the sperm and egg fuse during fertilization, the
result is an abnormal zygote with an extra chromosome; it has
2n 1 5 chromosomes.
18
UNIT TWO
Genetics
Abnormal Numbers of Sex Chromosomes Nondisjunction in meiosis
does not affect just autosomes, such as chromosome 21. It can also lead to
abnormal numbers of sex chromosomes (X and Y). Unusual numbers of
sex chromosomes seem to upset the genetic balance less than unusual numbers of autosomes. This may be because the Y chromosome is very small
and carries fewer genes than other chromosomes. Also, most of the genes
on the Y chromosome affect maleness but not functions that are essential to
the person’s survival. A peculiarity of X chromosomes in humans and other
mammals also helps an individual tolerate unusual numbers of X chromosomes: In mammals, the cells usually operate with only one functioning X
chromosome because extra copies of the chromosome become inactivated
in each cell (see Chapter 10).
Table 7.1 lists the most common sex chromosome abnormalities. An extra X chromosome in a male, making him XXY, occurs approximately once
Table 7.1
Sex
Chromosomes
Abnormalities of Sex
Chromosome Number in Humans
Syndrome
Origins of
Nondisjunction
Frequency in
Population
XXY
Klinefelter
syndrome
(male)
Meiosis in egg or
sperm formation
1
2000
XYY
Normal male
Meiosis in sperm
formation
1
2000
XXX
Metafemale
Meiosis in egg or
sperm formation
1
1000
XO
Turner
syndrome
(female)
Meiosis in egg or
sperm formation
1
5000
in every 2000 live births (once in every 1000 male births). This disorder is
called Klinefelter syndrome. Men with this disorder have male sex organs,
but the testes are abnormally small and the individual is sterile. The syndrome often includes breast enlargement and other feminine body contours
(Figure 7.23, left). The person is usually of normal intelligence. Klinefelter
syndrome is also found in individuals with more than one additional sex
chromosome, such as XXYY, XXXY, or XXXXY. These abnormal numbers of
sex chromosomes probably result from multiple nondisjunctions. Such
men are more likely to be mentally retarded than XY or XXY individuals.
Human males with a single extra Y chromosome (XYY) do not have any
well-defined syndrome, although they tend to be taller than average. Females with an extra X chromosome (XXX) are called metafemales. They
have limited fertility but are otherwise apparently normal.
Females who are lacking an X chromosome are designated XO; the O
simply indicates the absence of a second sex chromosome. These women
have Turner syndrome. They have a characteristic appearance, including
short stature and often a web of skin extending between the neck and
shoulders (Figure 7.23, right). Women with Turner syndrome are sterile because their sex organs do not fully mature at adolescence, and they have
poor development of breasts and other secondary sex characteristics. However, they are usually of normal intelligence. The XO condition occurs in
about one in 5000 babies born (about one in 2500 female births).
The sex chromosome abnormalities described here illustrate the crucial
role of the Y chromosome in determining a person’s sex. In general, a single
Y chromosome is enough to produce “maleness,” even when it is combined
with several X chromosomes. The absence of a Y chromosome results in
“femaleness.”
Breast
development
Poor beard
growth
Web of
skin
Constriction
of aorta
Poor
breast
development
Underdeveloped
testes
A man with Klinefelter
syndrome (XXY)
Underdeveloped
ovaries
A woman with Turner
syndrome (XO)
Figure 7.23 Syndromes associated with unusual numbers of sex chromosomes.
Alterations of Chromosome Structure Even if all chromosomes are pres-
ent in normal numbers, abnormalities in chromosome structure may cause
disorders. Breakage of a chromosome can lead to a variety of rearrangements
CHAPTER 7
The Cellular Basis of Reproduction and Inheritance
19
Deletion
(a)
Duplication
Homologous chromosomes
(b)
Inversion
(c)
Reciprocal
translocation
Nonhomologous chromosomes
(d)
Figure 7.24 Alterations in chromosome structure. (a)
A deletion is the removal of a chromosome segment. (b) A duplication is the repetition of a segment. When a duplication results from
the movement of a chromosomal segment to a homologous chromosome, as shown here, the “duplicates” may be slightly different.
(c) An inversion is the reversal of a segment within a chromosome.
(d) A translocation is the movement of a segment to a nonhomologous chromosome. In the case shown here, the two nonhomologous
chromosomes trade segments, producing a reciprocal translocation.
affecting the genes of that chromosome. Figure 7.24 shows four types of rearrangement. If a fragment of a chromosome is lost, the remaining
chromosome has a deletion. If a fragment from one chromosome is inserted into a homologous chromosome, it produces a duplication there. If a
fragment reattaches to the original chromosome but in the reverse direction, an inversion results.
Inversions are less likely than deletions or duplications to produce
harmful effects, because in inversions, all genes are still present in their normal number. Deletions, especially large ones, tend to have the most serious
effects. One example in humans is a specific deletion in chromosome 5 that
causes the cri du chat (“cat-cry”) syndrome. A child born with this syndrome
is mentally retarded and has a small head and a cry like the mewing of a cat.
Death usually occurs in infancy or early childhood.
Another type of chromosome change is chromosomal translocation,
the attachment of a chromosome fragment to a nonhomologous chromosome. Figure 7.24d shows a translocation that is reciprocal; that is, two nonhomologous chromosomes exchange segments. Like inversions, translocations may or may not be harmful. Some people with Down syndrome have
only part of a third chromosome 21; as the result of a translocation, it is attached to another (nonhomologous) chromosome.
Whereas chromosome alterations in sperm or egg can cause congenital
disorders, such changes in a somatic cell may contribute to the development
of cancer. For example, a chromosomal translocation in somatic cells in the
bone marrow is associated with chronic myelogenous leukemia (CML). CML
is the most common of the leukemias, the cancers affecting cells that give
rise to white blood cells (leukocytes). In the cancerous cells of most CML
patients, a part of chromosome 22 has switched places with a small fragment from chromosome 9 (Figure 7.25). This reciprocal translocation activates a gene that leads to leukemia. The chromosome ending up with the
activated cancer-causing gene is called the “Philadelphia chromosome,” after the city where it was discovered.
Chromosome 9
Reciprocal
translocation
Chromosome 9
Activated
cancer-causing
gene
Chromosome 22
“Philadelphia chromosome”
Figure 7.25 The translocation associated with a type of leukemia. This
is a reciprocal translocation in which one end of chromosome 9 trades places with
about half of chromosome 22. One of the genes moved from chromosome 9 is activated
to become a cancer-causing gene in its new home, called a “Philadelphia chromosome.” When a Philadelphia chromosome arises in a bone marrow cell that is destined
to develop into a white blood cell (leukocyte), the result is a type of leukemia called
chronic myelogenous leukemia (CML).
20
UNIT TWO
Genetics
CheckPoint
1.
is to somatic cells as haploid is to
.
2. If a diploid cell with 18 chromosomes undergoes meiosis, the resulting
gametes will each have
chromosomes.
3. Explain how mitosis conserves chromosome number while meiosis reduces the number in half.
4. In what important way is anaphase of meiosis II like anaphase of
mitosis?
5. Name two events during meiosis that contribute to genetic variety
among gametes.
6. How does the karyotype of a human female differ from that of a male?
7. What is the chromosomal basis of Down syndrome?
8. Explain how nondisjunction in meiosis could result in a diploid
gamete.
9. How is reciprocal translocation different from normal crossing over?
Answers: 1. Diploid; gametes. 2. 9 3. In mitosis, a single replication of the chromosomes is followed by one division of the cell. In meiosis, a single replication of the chromosomes is followed by
two cell divisions. 4. Sister chromatids separate. 5. Crossing over between homologous chromosomes during prophase I and independent orientation of tetrads at metaphase I 6. A female has two
X chromosomes; a male has an X and a Y. 7. Three copies of chromosome 21 (trisomy 21) 8. A
diploid gamete would result if there were nondisjunction of all the chromosomes during meiosis I or
II. 9. Normal crossing over is an exchange of segments between homologous chromosomes; reciprocal translocation occurs between nonhomologous chromosomes.
Evolution Link: New Species
from Errors in Cell Division
Errors in meiosis or mitosis do not always lead to problems. In fact, biologists believe that such errors have been instrumental in the evolution of
many species. Numerous plant species, in particular, seem to have originated from accidents during cell division that resulted in extra sets of chromosomes. The new species is polyploid, meaning that it has more than two
sets of homologous chromosomes in each somatic cell. At least half of all
species of flowering plants are polyploid, including such useful ones as
wheat, potatoes, apples, and cotton.
Let’s consider one scenario by which a diploid (2n) plant species might
generate a tetraploid (4n) plant. Imagine that, like many plants, our diploid
plant produces both sperm and egg cells and can selfLearn more about
fertilize. If meiosis fails to occur in the plant’s reproductive
new species resulting
organs and gametes are instead produced by mitosis, the
from errors in cell digametes will be diploid. The union of a diploid (2n) sperm
vision in the Web/CD
with a diploid (2n) egg in self-fertilization will produce a
Evolution Link and
The Process of Science.
tetraploid (4n) zygote, which may develop into a mature
tetraploid plant that can itself reproduce by self-fertilization.
The tetraploid plants will constitute a new species, one that has evolved in
just one generation.
Although polyploid animal species are less common than polyploid
plants, they are known to occur among the fishes and amphibians. Recently,
researchers in Chile have identified the first candidate for polyploidy among
CHAPTER 7
The Cellular Basis of Reproduction and Inheritance
21
the mammals, a rat whose cells seem to be
tetraploid (Figure 7.26). Tetraploid organisms
are sometimes strikingly different from their recent diploid ancestors—larger, for example. Scientists don’t yet understand exactly how polyploidy brings about such differences.
You’ll learn more about the evolution of
polyploid species in Chapter 13. In Chapter 8,
we continue our study of genetic principles by
looking at the rules governing the inheritance of
biological traits and the connection between
these traits and the organism’s chromosomes.
Figure 7.26 Chock full of chromosomes+a tetraploid mammal? The somatic cells
of this red viscacha rat from Argentina have about twice as many chromosomes as those of closely
related species. (Interestingly, the heads of its sperm are unusually large, presumably a necessity
for holding all that genetic material.) Scientists think that this rat is a tetraploid species that arose
when an ancestor somehow doubled its chromosome number, probably by errors in mitosis or meiosis within the animal’s reproductive organs. Researchers are studying the rat’s chromosomes to
verify that it actually has four homologous sets.
Chapter Review
Summary of Key Concepts
Overview: What Cell Reproduction Accomplishes
Cell reproduction, usually called cell division, enables a multicellular organism
to grow and develop and to replace damaged or lost cells.
Passing On the Genes from Cell to Cell Most cell division involves a duplication of all the chromosomes, followed by the distribution of the two identical sets of chromosomes to two “daughter” cells when the cell divides in two.
The daughter cells are genetically identical.
●
The Reproduction of Organisms Some organisms use ordinary cell division to reproduce. Their offspring are therefore genetically identical to the one
parent and to each other. Organisms that reproduce sexually, by the union of a
sperm with an egg cell, carry out another type of cell division in their reproductive organs. This process, meiosis, yields sperm and egg cells with only half
as many chromosomes as ordinary body cells.
Web/CD Activity 7A Asexual and Sexual Reproduction
●
●
The Cell Cycle and Mitosis
●
Eukaryotic Chromosomes The many genes of a eukaryotic cell are
grouped into multiple chromosomes in the nucleus. Each chromosome contains a very long DNA molecule with thousands of genes. Individual chromosomes are visible only when the cell is in the process of dividing; otherwise,
they are in the form of thin, loosely packed chromatin fibers. Before a cell
starts dividing, the chromosomes duplicate, producing sister chromatids (containing identical DNA) joined together at the centromere. Cell division involves the separation of sister chromatids and results in two daughter cells,
each containing a complete and identical set of chromosomes.
●
The Cell Cycle Cell division is only one phase, called the mitotic phase, of
the eukaryotic cell cycle. Most of the cycle is spent in interphase, when meta-
22
UNIT TWO
Genetics
bolic activity is high, chromosomes duplicate, many cell parts are made, and
the cell grows in size. Eukaryotic cell division consists of two processes: mitosis
and cytokinesis. Mitosis is the process that distributes the duplicated chromosomes to daughter nuclei; cytokinesis is the division of the cytoplasm to create
two daughter cells.
Web/CD Activity 7B The Cell Cycle
●
Mitosis and Cytokinesis At the start of mitosis, the chromosomes coil up,
becoming thick enough to be visible with a light microscope. The nuclear envelope breaks down, and a mitotic spindle made of microtubules moves the
chromosomes to the middle of the cell. The sister chromatids then separate
and are moved to opposite poles of the cell, where two new nuclei form. Cytokinesis overlaps the end of mitosis. Mitosis and cytokinesis produce genetically identical cells. In animals, cytokinesis occurs by cleavage, which pinches
the cell apart. In plants, a membranous cell plate splits the cell in two.
Web/CD Activity 7C Mitosis and Cytokinesis Animation
Web/CD Activity 7D Mitosis and Cytokinesis Video
●
●
●
●
Cancer Cells: Growing out of Control When the cell cycle control system
malfunctions, a cell may divide excessively and form a tumor. Cancer cells
have highly abnormal cell cycles. They can grow to form malignant tumors,
invade other tissues (metastasize), and even kill the organism. Radiation and
chemotherapy are effective as treatments because they interfere with cell division.
Meiosis, the Basis of Sexual Reproduction
Homologous Chromosomes The somatic cells (body cells) of each species
contain a specific number of chromosomes; for example, human cells have 46,
making up 23 pairs (two sets) of homologous chromosomes. The chromosomes of a homologous pair carry genes for the same characteristics at the
same places. In mammalian males, one pair of chromosomes are only partially
homologous: the sex chromosomes X and Y. Females have two X chromosomes.
●
Gametes and the Life Cycle of a Sexual Organism Cells with two sets of
homologous chromosomes are said to be diploid. Gametes—eggs and sperm
—are haploid cells. Each gamete contains a single set of chromosomes. At fertilization, a sperm fuses with an egg, forming a diploid zygote. Repeated mitotic cell divisions lead to a multicellular adult made of diploid cells. The
diploid adult produces haploid gametes by meiosis, the kind of cell division
that reduces the chromosome number by half. Although sexual life cycles differ with the species, they all involve the alternation of haploid and diploid
stages.
Web/CD Activity 7E Sexual Life Cycles
●
●
The Process of Meiosis Meiosis, like mitosis, is preceded by chromosome
duplication. But in meiosis, the cell divides twice to form four daughter cells.
The first division, meiosis I, starts with the pairing of homologous chromosomes. In crossing over, homologous chromosomes exchange corresponding
segments. Meiosis I separates the members of the homologous pairs and produces two daughter cells, each with one set of (duplicated) chromosomes.
Meiosis II is essentially the same as mitosis; in each of the cells, the sister chromatids of each chromosome separate.
Web/CD Activity 7F Meiosis Animation
●
●
Review: Comparing Mitosis and Meiosis See Figure 7.16 for a review and
comparison of the two processes.
●
The Origins of Genetic Variation Because the chromosomes of a homologous pair come from different parents, they carry different versions of many
of their genes. The large number of possible arrangements of chromosome
pairs at metaphase of meiosis I leads to many different combinations of chromosomes in eggs and sperm. This is one source of the variation in offspring
that results from sexual reproduction. Random fertilization of eggs by sperm
greatly increases the variation. Crossing over during prophase of meiosis I increases variation still further.
Web/CD Activity 7G The Origins of Genetic Variation
●
●
●
When Meiosis Goes Amok Sometimes a person has an abnormal number
of chromosomes, which causes problems. Down syndrome is caused by an extra copy of chromosome 21. The abnormal chromosome count is a product of
nondisjunction, the failure of a homologous pair of chromosomes to separate
during meiosis I or of sister chromatids to separate during meiosis II . Nondisjunction can also produce gametes with extra or missing sex chromosomes,
which lead to varying degrees of malfunction in humans but do not usually
affect survival. Chromosome breakage can lead to rearrangements—deletions,
duplications, inversions, and translocations—that can produce genetic disorders or, if the changes occur in somatic cells, cancer.
Evolution Link: New Species from Errors in Cell Division
New species can arise very quickly when errors in meiosis or mitosis create
polyploid cells, which have more than two sets of homologous chromosomes.
Many plants are polyploid, as well as certain animals.
Web/CD Evolution Link New Species from Errors in Cell Division
Web/CD The Process of Science Polyploid Plants
●
●
Self-Quiz
1. If an intestinal cell in a grasshopper contains 24 chromosomes, a
grasshopper sperm cell would contain
chromosomes.
a. 3
d. 24
b. 6
e. 48
c. 12
2. Which of the following phases of mitosis is essentially the opposite of
prophase in terms of nuclear changes?
a. telophase
d. interphase
b. metaphase
e. anaphase
c. S phase
3. A biochemist measures the amount of DNA in cells growing in the laboratory and finds that the quantity of DNA in a cell has doubled
a. between prophase and anaphase of mitosis.
b. between the G1 and G2 phases of the cell cycle.
c. during the M phase of the cell cycle.
d. between prophase I and prophase II of meiosis.
e. between anaphase and telophase of mitosis.
4. Which of the following is not a function of mitosis in humans?
a. repair of wounds
b. growth
c. production of gametes from diploid cells
d. replacement of lost or damaged cells
e. multiplication of somatic cells
5. A micrograph of a dividing cell from a mouse shows 19 chromosomes,
each consisting of two sister chromatids. During which of the following stages
of cell division could this picture have been taken? (Explain your answer.)
a. prophase of mitosis
b. telophase II of meiosis
c. prophase I of meiosis
d. anaphase of mitosis
e. prophase II of meiosis
6. Cytochalasin B is a chemical that disrupts microfilament formation. This
chemical would interfere with
a. DNA replication.
b. formation of the mitotic spindle.
c. cleavage.
d. formation of the cell plate.
e. crossing over.
7. It is difficult to observe individual chromosomes during interphase because
a. the DNA has not been replicated yet.
b. they are in the form of very long, thin strands.
c. they leave the nucleus and are dispersed to other parts of the cell.
d. homologous chromosomes do not pair up until division starts.
e. the spindle must move them to the center of the cell before they become visible.
8. A fruit fly somatic cell contains eight chromosomes. This means that
different combinations of chromosomes are possible in its gametes.
a. 4
d. 32
b. 8
e. 64
c. 16
CHAPTER 7
The Cellular Basis of Reproduction and Inheritance
23
9. If a fragment of a chromosome breaks off and then reattaches to the original chromosome but in the reverse direction, the resulting chromosome abnormality is called
a. a deletion.
d. a nondisjunction.
b. an inversion.
e. a reciprocal translocation.
c. a translocation.
10. Why are individuals with an extra chromosome 21, which causes Down
syndrome, more numerous than individuals with an extra chromosome 3 or
chromosome 16?
a. There are probably more genes on chromosome 21 than on the others.
b. Chromosome 21 is a sex chromosome and 3 and 16 are not.
c. Down syndrome is not more common, just more serious.
d. Extra copies of chromosomes 3 or 16 are probably fatal.
e. Nondisjunction of chromosome 21 probably occurs much more frequently.
LM 600×
11. The following light micrograph shows dividing cells near the tip of an
onion root. Identify the stage of mitosis for each of the outlined cells, a–d.
(b)
(a)
(c)
(d)
●
Go to the website or CD-ROM for more self-quiz questions.
The Process of Science
1. A mule is the offspring of a horse and a donkey. A donkey sperm contains
31 chromosomes and a horse egg 32 chromosomes, so the zygote contains a
total of 63 chromosomes. The zygote develops normally. The combined set of
chromosomes is not a problem in mitosis, and the mule combines some of the
best characteristics of horses and donkeys. However, a mule is sterile; meiosis
cannot occur normally in its testes or ovaries. Explain why mitosis is normal
in cells containing both horse and donkey chromosomes but the mixed set of
chromosomes interferes with meiosis.
2. Explore how new species can result from errors in cell division in The
Process of Science activity available on the website and CD-ROM.
Biology and Society
Every year about a million Americans are diagnosed as having cancer. This
means that about 75 million Americans now living will eventually have cancer,
and one in five will die of the disease. There are many kinds of cancers and
many causes of the disease. For example, smoking causes most lung cancers.
Overexposure to ultraviolet rays in sunlight causes most skin cancers. There is
evidence that a high-fat, low-fiber diet is a factor in breast, colon, and prostate
cancers. And agents in the workplace such as asbestos and vinyl chloride are
also implicated as causes of cancer. Hundreds of millions of dollars are spent
each year in the search for effective treatments for cancer; far less money is
spent on preventing cancer. Why might this be the case? What kinds of
lifestyle changes could we make to help prevent cancer? What kinds of prevention programs could be initiated or strengthened to encourage these changes?
What factors might impede such changes and programs? Should we devote
more of our resources to treating cancer or preventing it? Why?
Go to www.essentialbiology.com to explore
the media for Essential Biology.
Sample chapter from Essential Biology by Neil
A. Campbell and Jane B. Reece
© 2001 by Benjamin Cummings, an imprint of
Addison Wesley, Inc.
24
UNIT TWO
Genetics