Chapter 8 – The Cellular Basis of Reproduction and Inheritance
8.1 Cell division plays many important roles in the lives of organisms
• Ability of organisms to reproduce their own kind is the one trait that distinguishes living from nonliving.
In the sense of “like begets like,” it is true that humans make more humans and maple trees make more
maple trees. However, in terms of genetics, the only ones who beget identical copies are those that
reproduce asexually.
• In asexual reproduction, the cell produces an identical copy of its DNA and then allocates the
chromosomes to the developing daughter cells. The offspring produced is identical to that of the parent.
• Sexually reproduced offspring do resemble the parents more than they do any other member of the
population, but they have their own unique genetic code.
• In unicellular organisms, cell division produces an entire new organism. In some multicellular organisms,
it allows asexual reproduction, such as grafting of a plant. In other multicellular organisms, cell division is
the basis of sperm and egg formation. It also allows a sexually reproducing organism to develop from a
single cell, the zygote, into the adult organism. During adulthood, cell division continues to function in
renewal and repair, replacing cells that die or get damaged.
8.2 Prokaryotes reproduce by binary fission
Prokaryotes reproduce by a type of cell division termed binary fission (“dividing in half”).
1. As the chromosome duplicates, one copy moves towards the opposite end of the cell
2. The cell elongates
3. When duplication has completed and the cell has about doubled in size, the membrane grows inward
and divides the parent cell into two daughter cells Prokaryotic genes are usually carried on a circular
DNA molecule with its associated proteins.
8.3 The large, complex chromosomes of eukaryotes duplicate with each cell division
• Most of the time, chromosomes are a diffuse mass of long, thin fibers called chromatin, which are a
combination of DNA and protein. Eukaryotic chromosomes are more complex than prokaryotic
chromosomes and include more protein to help maintain structure and control the activity of the genes.
• Before division, the chromosomes are duplicated. The DNA of each chromosome is copied and new
proteins are added as needed. The copies now exist as two chromosomes called sister chromatids, which
contain identical copies of the DNA material. They join together in a region called the centromere.
• When the cell divides, the sister chromatids separate from each other and become a chromosome. The
new chromosomes divide and enter the daughter cells, giving each daughter cell complete and identical
sets of chromosomes.
8.4 The cell cycle multiplies cells
• The cell cycle is the sequence of events that extend from the time a cell is formed from its parent cell until
it divides itself or dies. Cell cycle is made of two stages: interphase (growing) and mitosis (dividing).
• Most of the cell cycle is spent in interphase. During this time, the cell has high metabolic activity and is
performing its various functions within the organism. It will create more organelles, increase the supply
of proteins, and it grows in size. Chromosomes will also duplicate during this period.
Interphase can be divided into three sub phases:
• G1 – first gap: the cell grows
• S phase – Synthesis: the chromosomes double and produce the sister chromatids.
• G2 – second gap: cell grows more and completes preparations for cell division.
Mitosis Phase (M phase)
When the cell actually divides accounts for only 10% of the total time of the cell cycle. It is divided into two
phases: the mitotic phase and cytokinesis. During mitosis, the nucleus and its contents divide and are
distributed to form two daughter nuclei. During cytokinesis, the cytoplasm divides in two. The end result of
these processes is two genetically identical daughter cells.
8.5 Cell division is a continuum of dynamic changes
Mitosis is divided in five main stages: prophase, prometaphase, metaphase, anaphase, and telophase. The
chromosomes are directed by the mitotic spindle, a structure of microtubules designed to guide the
separation of the two sets of chromosomes.
Prophase: Changes occur in the nucleus and cytoplasm. The chromatin coils up into discrete chromosomes.
The nucleoli disappear. Each chromosome appears as two identical sister chromatids joined at the
centromere. The spindle forms as microtubules and rapidly spread away from each other.
Prometaphase: The nuclear envelope breaks and disappears. Microtubules of the spindle reach the
centromeres and attach.
Metaphase: The spindle is fully formed, and the poles are on opposite ends of the cell. The centromeres are
lined up on the metaphase plate in the center of the cell. The microtubules connected to a particular
chromosome all come from one pole of the spindle, while those attached to its sister come from the opposite
pole.
Anaphase: The two centromeres of the sister chromatids separate, separating the sister chromatids and
pulling them toward the poles. While this happens, the poles themselves are pulling farther apart. Anaphase
is complete when the chromosomes have reached the two poles of the cell.
Telophase: Cell elongation that started in anaphase continues. Daughter nuclei appear at the two poles of the
cell as nuclear envelopes around the chromosomes. The chromatin uncoils. The equal division of one nucleus
into two genetically identical nuclei is finished.
Cytokinesis: The division of the cytoplasm with two daughter cells completely separating involves a cleavage
furrow that pinches the cell in two.
8.6 Cytokinesis differs for plant animal cells
• Cytokinesis typically begins during telophase, but may begin in late anaphase. A first sign of this is the
formation of the cleavage furrow, which is a shallow groove in the cells surface. At the site of the furrow,
a ring of microfilaments interacts to contract around the cell. The furrow deepens and pinches the parent
cell in two separate cells with their own nucleus and cytoplasm.
• In plant cells, which have cell walls, vesicles containing cell wall material collect at the center of the cell.
The vesicles fuse forming a cell plate. The plate grows outward and accumulates more cell wall material.
• The edges of the cell plate fuse with plasma membrane and the cell plate’s contents join the parental cell
wall. This results in two daughter cells bounded by their own plasma membrane and cell wall.
8.7 Anchorage, cell density, and chemical growth factors affect cell division
For a plant or animal to grow and develop normally and to maintain its tissues once full grown, it must be
able to control the timing of its cell division in different parts of its body. Many factors include of cell division
have been discovered:
• Growth factor – protein secreted by certain body cells to stimulate other cells to divide.
• Density-dependent inhibition – crowded cells stop dividing. If some cells are removed, then the
surrounding cells will divide until the space is filled.
• Anchorage dependence – most animal cells must be in contact with a solid surface in order to divide.
8.8 Growth factor signal the cell cycle control system
The sequential events of the cell cycle are directed by a distinct cell cycle control system. It is a set of
molecules in the cell that triggers and coordinates the phases of the cell cycle. Inside the cell cycle are
checkpoints that regulate the cycle. Three major checkpoints exist: one during G1, one during G2, and one
during the M phase.
8.9 Growing out of control, cancer cells produce malignant tumors
 Cancer claims the lives of 1 in 5 people in the US. Cancer cells ignore the normal signals that would stop cell
division and divide excessively. In some cases, they invade other tissues, and if left untreated, will lead to death
of the organism.
• Cancer begins when a normal cell changes and is converted to a cancer cell. This is usually following a
mutation in one or more genes that control the cell cycle. The immune system usually detects and
destroys the cell, but if it doesn’t, then a tumor can develop.
•
•
A tumor is an abnormally growing mass of body cells. If the cells stay in the original site, the tumor is
considered benign. If the cells migrate into neighboring tissues or other body parts, then the tumor is
considered malignant. Spread of the cancer cells is called metastasis.
• Carcinoma is a cancer originating in the outer covering of the body such as the skin or a lining
tissue
• Sarcomas arise in tissues that support the body, such as bone or muscle
• Lymphomas or leukemias are cancers of blood forming tissue
Many cancers are treatable. Chemotherapy is used. It administers chemicals that disrupt certain phases of
the cell cycle.
8.10 Review: Mitosis provides for growth, cell replacement and asexual reproduction
8.11 Chromosomes are matched in homologous pairs
• A typical body cell (somatic cell) has 46 chromosomes. They are put into 23 pairs, with the two
chromosomes composing each pair called homologous chromosomes (homologs) because they carry the
genes controlling the same inherited characteristics. Genes located in a particular place are termed locus.
• Two distinct chromosomes are the exception to the general pattern of homologous chromosomes. Human
females have a homologous set in pair 23 of XX. However, males have an XY pair in the 23 pair. These are
considered sex chromosomes, while the other 22 pairs are termed autosomes.
8.12 Gametes have a single set of chromosomes
• Humans are considered to be diploid because the cells of the body have all 46 chromosomes. The
exception is the presence of egg and sperm cells, the gametes. Each gamete has a single set of
chromosomes, 22 autosomes and a single sex chromosome – either X or Y. A cell with half the
chromosome number is haploid. Therefore, the human haploid number is 23.
• During reproduction the haploid sperm of the male and haploid egg of the female fuse during fertilization.
The resulting fertilized egg, the zygote, is diploid. It has one set of chromosomes from the mother and one
from the father. The zygote will undergo mitotic divisions to ensure that all of the body cells will receive
46 chromosomes.
8.13 Meiosis reduces the chromosome number from diploid to haploid
• Meiosis produces haploid gametes in diploid organisms. Many stages of meiosis resemble the stages of
mitosis. Meiosis is preceded by the duplication of chromosomes and then occurs in two consecutive
divisions called meiosis I and II.
Interphase: Chromosomes duplicate. Sister chromatids attach.
Prophase I: Chromatin coils up; chromosomes become visible. Homologs come together in pairs. The 4
attached chromatids are termed a tetrad. During this time, chromatids of the homologs exchange segments.
Metaphase I: Tetrads align on the metaphasal plate. Spindle microtubules attach to the centromeres.
Anaphase I: Chromosomes migrate toward two poles of the cell. Sister chromatids remain attached, but the
tetrad splits apart.
Telophase I and Cytokinesis: Chromosomes arrive at the poles. Usually cytokinesis occurs along with
telophase I. Chromosomes uncoil and the nuclear envelope reforms. No chromosome duplication occurs
Meiosis II: Chromosomes condense again, and the nuclear envelope breaks down. The chromosomes begin to
move, line up at the metaphasal plate, split apart, and move towards the poles again. Four daughter cells are
made, each with a single set of chromosomes.
8.14 Mitosis and Meiosis have important similarities and differences
For both mitosis and meiosis, chromosomes duplicate only once during interphase. Mitosis involves one
division of the nucleus and then one division of the cytoplasm, yielding two diploid cells. Meiosis includes two
nuclear and cytoplasmic divisions, yielding four haploid cells.
8.15 Independent orientation of chromosomes in meiosis and random fertilization lead to varied
offspring
• The orientation of the homologous pairs of chromosomes (tetrads) at metaphase I, whether the maternal
or paternal chromosome is closer to a given pole is random. There is a 50-50 chance of which
chromosomes will be received.
• For a species with multiple chromosomes, all the chromosome pairs orient independently during
metaphase I (chromosomes X and Y will behave as a pair during meiosis). The total number of
combinations is extremely high (around 8 million different possibilities exist for humans). When taking
into account the fusion of two gametes, 64 million possibilities exist.
8.16 Homologous chromosomes can carry different versions of genes
Homologous chromosomes can bear two different kinds of genetic information for the same characteristic.
8.17 Crossing over further increases genetic variability
Crossing over is an exchange of corresponding segments between two homologous chromosomes. The
chromosomes are a tetrad (four chromatids attached at a centromere). The crossing over appear as an X
shaped region called the chiasma; in this area, the two homologous (nonsister) chromosomes attach and
exchange genes to create new combinations of genes.
1. The DNA of two nonsister chromatids (one maternal and one paternal) break at the same place.
2. The two broken chromatids join together in a new way by trading places and producing hybrids.
3. When the homologous chromosomes separate in anaphase I, each contains a new segment
originating from its homolog.
4. In meiosis II, the sister chromatids separate and go into different gametes
Genetic recombination results from chromosomes crossing over.
8.18 A karyotype is a photographic inventory of an individual’s chromosomes
• A karyotype is a display of magnified images of an individual’s chromosomes arranged in pairs, starting
with the longest. A karyotype shows the condensed and doubled pairs of chromosomes as they appear in
metaphase.
• To prepare a karyotype, lymphocytes (a type of white blood cells) are used. The sample is treated with a
chemical that stimulates mitosis. After growing several days, they are treated with another chemical that
stops mitosis at metaphase, when the chromosomes are most condensed.
8.19 An extra copy of chromosome 21 causes Down Syndrome
In most cases, the presence of an extra chromosome will cause a spontaneous miscarriage. In the case of
trisomy 21 (Down), the genetic balance is less upset and the individuals carrying it can survive. Trisomy 21 is
the most common chromosomal abnormality.
8.20 Accidents during meiosis can alter chromosome number
Nondisjunction occurs when a chromosome pair fails to separate. If an abnormal gamete produced by
nondisjunction is fertilized, then a zygote with an abnormal number of chromosomes is the result. Mitosis of
the zygote’s cells will pass the anomaly to all of the body cells.
8.21 Abnormal numbers of sex chromosomes do not usually affect survival
Nondisjunction can result in abnormal numbers of sex chromosomes. Abnormalities in the sex chromosomes
do not seem to upset the genetic balance. This could be due to a few things:
 The Y chromosome is small and carries few genes, and mammal cells usually operate with only one
functioning X chromosome.
In males: Klinefelter Syndrome (XXY) may have a normal life, but they may be sterile and have small testes.
Males with XYY may not notice any differences, except a possibility of being taller than average
In females: Females with XXX may not notice any differences
• Turner Syndrome – females only having one X chromosome (designated XO) may be sterile and
exhibit lack of development in the secondary sex traits.
8.22 New species can arise from errors in cell division
Scientists suggest that errors in mitosis or meiosis may have led to evolution in many species. Some species
may be polyploid, meaning there are more than two sets of chromosomes in a somatic cell
8.23 Alterations of chromosome structure can cause birth defects and cancer
Even if all chromosomes are present, abnormalities in their structure can cause problems.
Duplication – if a fragment from the chromosome gets repeated
Deletion – when a fragment of a chromosome is lost
Translocation – attachment of a portion of one chromosome to a nonhomologous chromosome
Inversion – when a fragment is reversed within the chromosome