CHAPTER
8: The Cellular Basis of Reproduction and Inheritance
Cell Division and Reproduction
8.1 Cell division plays many important roles in the lives of organisms
1. Organisms reproduce their own kind, a key characteristic of life.
2. Cell division
a. is reproduction at the cellular level, requires the duplication of chromosomes, and
b. sorts new sets of chromosomes into the resulting pair of daughter cells.
3. Cell division is used
a. for reproduction of single-celled organisms, growth of multicellular organisms from a fertilized
egg into an adult, repair and replacement of cells, and sperm and egg production.
4. Living organisms reproduce by two methods.
a. Asexual reproduction
i. produces offspring that are identical to the original cell or organism and
ii. involves inheritance of all genes from one parent.
b. Sexual reproduction
i. produces offspring that are similar to the parents, but show variations in traits and
ii. involves inheritance of unique sets of genes from two parents.
8.2 Prokaryotes reproduce by binary fission
1. Prokaryotes (bacteria and archaea) reproduce by binary fission (“dividing in half”).
2. The chromosome of a prokaryote is
a. a singular circular DNA molecule associated with proteins and much smaller than those of eukaryotes.
3. Binary fission of a prokaryote occurs in three stages.
a. duplication of the chromosome and separation of the copies,
b. continued elongation of the cell and movement of the copies, and division into two daughter
cells.
The Eukaryotic Cell Cycle and Mitosis
8.3 The large, complex chromosomes of eukaryotes duplicate with each cell division
1
1. Eukaryotic cells
a. are more complex and larger than prokaryotic cells,
b. have more genes, and store most of their genes on multiple chromosomes within the nucleus.
2. Eukaryotic chromosomes are composed of chromatin consisting of
a. one long DNA molecule and
b. proteins that help maintain the chromosome structure and control the activity of its genes.
3. To prepare for division, the chromatin becomes
a. highly compact and visible with a microscope.
4. Before a eukaryotic cell begins to divide, it duplicates all of its chromosomes,
resulting in
a. two copies called sister chromatids
b. joined together by a narrowed “waist” called the centromere.
5. When a cell divides, the sister chromatids
a. separate from each other, now called chromosomes, and sort into separate daughter cells.
8.4 The cell cycle multiplies cells
1. The cell cycle is an ordered sequence of events that extends
a. from the time a cell is first formed from a dividing parent cell until its own division.
2. The cell cycle consists of two stages, characterized as follows:
a. Interphase: duplication of cell contents
i. G1—growth, increase in cytoplasm
ii. S—duplication of chromosomes
iii. G2—growth, preparation for division
b. Mitotic phase: division
i. Mitosis—division of the nucleus
ii. Cytokinesis—division of cytoplasm
2
8.5 Cell division is a continuum of dynamic changes
1. Mitosis progresses through a series of stages.
a. prophase, prometaphase, metaphase, anaphase, and telophase.
2. Cytokinesis often overlaps telophase.
3. A mitotic spindle is
a. required to divide the chromosomes, composed of microtubules, and
b. produced by centrosomes, structures in the cytoplasm that
i. organize microtubule arrangement and contain a pair of centrioles in animal cells.
4. Interphase
a. The cytoplasmic contents double, two centrosomes form,
b. chromosomes duplicate in the nucleus during the S phase, and
c. nucleoli, sites of ribosome assembly, are visible.
5. Prophase
a. In the cytoplasm microtubules begin to emerge from centrosomes, forming the spindle.
b. In the nucleus
i. chromosomes coil and become compact, and nucleoli disappear.
6. Prometaphase
a. Spindle microtubules reach chromosomes, where they
i. attach at kinetochores on the centromeres of sister chromatids and
ii. move chromosomes to the center of the cell through associated protein
“motors.”
b. Other microtubules meet those from the opposite poles. The nuclear envelope disappears.
7. Metaphase
a. The mitotic spindle is fully formed. Chromosomes align at the cell equator.
b. Kinetochores of sister chromatids are facing the opposite poles of the spindle.
8. Anaphase
a. Sister chromatids separate at the centromeres.
b. Daughter chromosomes are moved to opposite poles of the cell as
i. motor proteins move the chromosomes along the spindle microtubules, and
ii. kinetochore microtubules shorten.
c. The cell elongates due to lengthening of nonkinetochore microtubules.
9. Telophase
3
a. The cell continues to elongate.
b. The nuclear envelope forms around chromosomes at each pole, establishing daughter nuclei.
c. Chromatin uncoils and nucleoli reappear. The spindle disappears.
10. During cytokinesis, the cytoplasm is divided into separate cells.
11. The process of cytokinesis differs in animal and plant cells.
8.6 Cytokinesis differs for plant and animal cells
1. In animal cells, cytokinesis occurs as
a. a cleavage furrow forms from a contracting ring of microfilaments, interacting with myosin,
and the cleavage furrow deepens to separate the contents into two cells.
2. In plant cells, cytokinesis occurs as
a. a cell plate forms in the middle, from vesicles containing cell wall material.
b. The cell plate grows outward to reach the edges, dividing the contents into two cells.
c. Each cell now possesses a plasma membrane and cell wall.
4
Cleavage Furrow
Cell Plate
8.7 Anchorage, cell density, and chemical growth factors affect cell division
1. The cells within an organism’s body divide and develop at different rates.
2. Cell division is controlled by
a. the presence of essential nutrients, growth factors, proteins that stimulate division,
b. density-dependent inhibition, in which crowded cells stop dividing, and
c. anchorage dependence, the need for cells to be in contact with a solid surface to divide.
8.8 Growth factors signal the cell cycle control system
1. The cell cycle control system is a cycling set of molecules in the cell that
a. triggers and coordinates key events in the cell cycle.
2. Checkpoints in the cell cycle can
a. stop an event or signal an event to proceed.
3. There are three major checkpoints in the cell cycle.
a. G1 checkpoint
i. allows entry into the S phase or causes the cell to leave the cycle, entering a nondividing G0
phase.
b. G2 checkpoint, and M checkpoint.
4. Research on the control of the cell cycle is one of the hottest areas in biology today.
8.9 Growing out of control, cancer cells produce malignant tumors
1. Cancer currently claims the lives of 20% of the people in the United States and other industrialized nations.
2. Cancer cells escape controls on the cell cycle.
3. Cancer cells
a. divide rapidly, often in the absence of growth factors, spread to other tissues through the circulatory system, and
b. grow without being inhibited by other cells.
4. A tumor is an abnormally growing mass of body cells.
a. Benign tumors remain at the original site.
b. Malignant tumors spread to other locations, called metastasis.
5. Cancer treatments
a. Localized tumors can be
i. removed surgically and/or treated with concentrated beams of high-energy radiation.
b. Chemotherapy is used for metastatic tumors.
8.10 Review: Mitosis provides for growth, cell replacement, and asexual reproduction
1. When the cell cycle operates normally, mitosis produces genetically identical cells for
a. growth, replacement of damaged and lost cells, and asexual reproduction.
5
Meiosis and Crossing Over
8.11 Chromosomes are matched in homologous pairs
1. In humans, somatic cells have
a. 23 pairs of homologous chromosomes and one member of each pair from each parent.
2. The human sex chromosomes X and Y differ in size and genetic composition.
3. The other 22 pairs of chromosomes are autosomes with the same size and genetic
composition.
4. Homologous chromosomes are matched in
a. length, centromere position, and gene locations.
5. A locus (plural, loci) is the position of a gene.
6. Different versions of a gene may be found at the same locus on maternal and paternal chromosomes.
8.12 Gametes have a single set of chromosomes
1. An organism’s life cycle is the sequence of stages leading
a. from the adults of one generation to the adults of the next.
2. Humans and many animals and plants are diploid, with body cells that have
a. two sets of chromosomes, one from each parent.
3. Meiosis is a process that converts diploid nuclei to haploid nuclei.
a. Diploid cells have two homologous sets of chromosomes.
b. Haploid cells have one set of chromosomes.
c. Meiosis occurs in the sex organs, producing gametes—sperm and eggs.
4. Fertilization is the union of sperm and egg.
5. The zygote has a diploid chromosome number, one set from each parent.
6. All sexual life cycles include an alternation between
a. a diploid stage and a haploid stage.
7. Producing haploid gametes prevents the chromosome number from doubling in every generation
8.13 Meiosis reduces the chromosome number from diploid to haploid
1. Meiosis is a type of cell division that produces haploid gametes in diploid organisms.
2. Two haploid gametes combine in fertilization to restore the diploid state in the zygote.
3. Meiosis and mitosis are preceded by the duplication of chromosomes. However,
a. meiosis is followed by two consecutive cell divisions, and
b. mitosis is followed by only one cell division.
4. Because in meiosis, one duplication of chromosomes is followed by two divisions, each of the
four daughter cells produced has a haploid set of chromosomes.
5. Meiosis I – Prophase I – events occurring in the nucleus.
a. Chromosomes coil and become compact.
b. Homologous chromosomes come together as pairs by synapsis.
c. Each pair, with four chromatids, is called a tetrad.
d. Nonsister chromatids exchange genetic material by crossing over.
6. Meiosis I – Metaphase I – Tetrads align at the cell equator.
7. Meiosis I – Anaphase I – Homologous pairs separate and move toward opposite poles of the cell.
8. Meiosis I – Telophase I
a. Duplicated chromosomes have reached the poles.
b. A nuclear envelope re-forms around chromosomes in some species.
c. Each nucleus has the haploid number of chromosomes.
9. Meiosis II follows meiosis I without chromosome duplication.
10. Each of the two haploid products enters meiosis II.
11. Meiosis II – Prophase II
6
a. Chromosomes coil and become compact (if uncoiled after telophase I).
b. Nuclear envelope, if re-formed, breaks up again.
12. Meiosis II – Metaphase II – Duplicated chromosomes align at the cell equator.
13. Meiosis II – Anaphase II
a. Sister chromatids separate, and chromosomes move toward opposite poles.
14. Meiosis II – Telophase II
a. Chromosomes have reached the poles of the cell.
b. A nuclear envelope forms around each set of chromosomes.
c. With cytokinesis, four haploid cells are produced.
8.14 Mitosis and meiosis have important similarities and differences
1. Mitosis and meiosis both begin with diploid parent cells that
a. have chromosomes duplicated during the previous interphase.
2. However, the end products differ.
7
a. Mitosis produces two genetically identical diploid somatic daughter cells.
b. Meiosis produces four genetically unique haploid gametes.
8.15 Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring
1. Genetic variation in gametes results from
a. independent orientation at metaphase I and random fertilization.
2. Independent orientation at metaphase I
a. Each pair of chromosomes independently aligns at the cell equator.
b. There is an equal probability of the maternal or paternal chromosome facing a
given pole.
c. The number of combinations for chromosomes packaged into gametes is 2n where
n = haploid number of chromosomes.
3. Random fertilization – The combination of each unique sperm with each unique egg increases genetic variability.
8.16 Homologous chromosomes may carry different versions of genes
1. Separation of homologous chromosomes during meiosis can lead to genetic differences between
gametes.
a. Homologous chromosomes may have different versions of a gene at the same locus.
b. One version was inherited from the maternal parent and the other came from the
paternal parent.
c. Since homologues move to opposite poles during anaphase I, gametes will receive either the
maternal or paternal version of the gene.
8.17 Crossing over further increases genetic variability
1. Genetic recombination is the production of new combinations of genes due to crossing over.
2. Crossing over is an exchange of corresponding segments between separate (non-sister) chromatids on homologous chromosomes.
a. Nonsister chromatids join at a chiasma (plural, chiasmata), the site of attachment and crossing
over.
b. Corresponding amounts of genetic material are exchanged between maternal and paternal (nonsister) chromatids.
8
Alterations of Chromosome Number and Structure
8.18 A karyotype is a photographic inventory of an individual’s chromosomes
1. A karyotype is an ordered display of magnified images of an individual’s chromosomes arranged
in pairs.
2. Karyotypes
a. are often produced from dividing cells arrested at metaphase of mitosis and
b. allow for the observation of homologous chromosome pairs,
c. chromosome number, and chromosome structure.
8.19 CONNECTION: An extra copy of chromosome 21 causes Down syndrome
1. Trisomy 21
a. involves the inheritance of three copies of chromosome 21 and
b. is the most common human chromosome abnormality.
2. Trisomy 21, called Down syndrome, produces a characteristic set of symptoms, which include:
a. mental retardation, characteristic facial features, short stature,
b. heart defects, susceptibility to respiratory infections, leukemia, and Alzheimer’s disease, and
c. shortened life span.
3. The incidence increases with the age of the mother.
8.20 Accidents during meiosis can alter chromosome number
9
1. Nondisjunction is the failure of chromosomes or chromatids to separate normally during meiosis.
This can happen during
a. meiosis I, if both members of a homologous pair go to one pole or
b. meiosis II if both sister chromatids go to one pole.
2. Fertilization after nondisjunction yields zygotes with altered numbers of
chromosomes.
8.21 CONNECTION: Abnormal numbers of sex chromosomes do not usually affect
survival
1. Sex chromosome abnormalities tend to be less severe, perhaps because of
a. the small size of the Y chromosome or X-chromosome inactivation.
2. Table 8.21 in the textbook lists the most common human sex chromosome abnormalities. In general,
a. a single Y chromosome is enough to produce “maleness,” even in combination with several X
chromosomes, and
b. the absence of a Y chromosome yields “femaleness.”
8.23 CONNECTION: Alterations of chromosome structure can cause birth defects and cancer
1. Chromosome breakage can lead to rearrangements that can produce
a. genetic disorders or, if changes occur in somatic cells, cancer.
2. These rearrangements may include
a. a deletion, the loss of a chromosome segment,
b. a duplication, the repeat of a chromosome segment,
c. an inversion, the reversal of a chromosome segment, or
d. a translocation, the attachment of a segment to a nonhomologous chromosome that can be reciprocal.
10
Chapter 8: the Cellular Basis of Reproduction and Inheritance
Word Parts
a- = not or without
cyto- = cell;
ana-= up, throughout, again
-kinet = move
auto- = self
di- = two
bi- = two
fertil- = fruitful
centro- = the center;
gamet- = a wife or husband
-mere = a part;
gen- = produce
-soma = body
haplo- = single
chiasm- = marked crosswise
homo- = like
chroma- = colored;
inter- = between
-soma = body
karyo- = nucleus
mal- = bad or evil
meio- = less
meta- = between
mito- = a thread
non- = not;
dis- = separate
pro- = before
soma- = body
telos- = an end
trans- = across
tri- = three;
soma- = body
Vocabulary
1. cell division- The reproduction of a cell through duplication of the genome and division of the
cytoplasm.
2. chromosome- A threadlike, gene-carrying structure found in the nucleus of a eukaryotic cell and
most visible during mitosis and meiosis. It consists of one very long piece of chromatin.
3. asexual reproduction- The creation of genetically identical offspring by a single parent, without
the participation of sperm and egg.
4. sexual reproduction- The creation of genetically unique offspring by the fusion of two haploid
sex cells (gametes), forming a diploid zygote.
5. binary fission- A means of asexual reproduction in which a parent organism, often a single cell,
divides into two genetically identical individuals of about equal size.
6. chromatin- The combination of DNA and proteins that constitutes eukaryotic chromosomes.
7. sister chromatids- Two identical parts of a duplicated chromosome in a eukaryotic cell. Prior to
mitosis, these remain attached to each other at the centromere.
11
8. centromere- The region of a duplicated chromosome where two sister chromatids are joined and
where spindle microtubules attach during mitosis and meiosis.
9. cell cycle- An ordered sequence of events that extends from the time a eukaryotic cell is first
formed from a dividing parent cell until its own division into two cells.
10. interphase- The period in the eukaryotic cell cycle when the cell is not actually dividing. It constitutes the majority of the time spent in the cell cycle.
11. mitotic phase (M phase)- The part of the cell cycle when the nucleus divides, its chromosomes
are distributed to the daughter nuclei, and the cytoplasm divides, producing two other cells.
12. mitosis- The division of a single nucleus into two genetically identical nuclei. This division and
cytokinesis make up the mitotic (M) phase of the cell cycle.
13. cytokinesis- the division of the cytoplasm to form two separate daughter cells. This division and
mitosis make up the mitotic (M) phase of the cell cycle.
14. prophase- The first stage of mitosis, during which the chromatin condenses to form structures
visible with a light microscope and the mitotic spindle begins to form, but the nucleus is still intact.
15. prometaphase- The second stage of mitosis, during which the nuclear envelope fragments and
the spindle microtubules attach to the kinetochores of the sister chromatids.
16. metaphase- The third stage of mitosis, during which all the cell's duplicated chromosomes are
lined up at an imaginary plane equidistant between the poles of the mitotic spindle.
17. anaphase- The fourth stage of mitosis, beginning when sister chromatids separate from each
other and ending when a complete set of daughter chromosomes arrives at each of two poles of the
cell.
18. telophase- The fourth stage of mitosis, during which daughter nuclei form at the two poles of a
cell. Telophase usually occurs together with cytokinesis.
19. mitotic spindle- A football-shaped structure formed of microtubules and associated proteins that
is involved in the movement of chromosomes during mitosis and meiosis.
20. centrosome- Material in the cytoplasm of a eukaryotic cell that gives rise to microtubules.
21. cleavage furrow- The first sign of cytokinesis during cell division in an animal cell, a shallow
groove in the cell surface near the old metaphase plate.
22. cell plate- A double membrane across the midline of a dividing plant cell, between which the
new cell wall forms during cytokinesis.
23. growth factor- a protein secreted by certain body cells that stimulates other cells to divide.
12
24. density-dependent inhibition- The ceasing of cell division that occurs when cells touch one another.
25. anchorage dependence- The requirement that to divide, a cell must be attached to a solid surface.
26. cell cycle control system- A cyclically operating set of proteins that triggers and coordinates
events in the eukaryotic cell cycle.
27. tumor- An abnormal mass of rapidly growing cells that forms within otherwise normal tissue.
28. benign tumor- An abnormal mass of cells that remains at its original site in the body.
29. malignant tumor- An abnormal tissue mass that can spread into neighboring tissue and to other
parts of the body; a cancerous tumor.
30. metastasis- The spread of cancer cells beyond their original site.
31. carcinomas- Cancer that originates in the coverings of the body, such as skin or the lining of the
intestinal tract.
32. sarcomas- Cancer of the supportive tissues, such as bone, cartilage, and muscle.
33. leukemias- A type of cancer of the blood-forming tissues, characterized by an excessive production of white blood cells and an abnormally high number of them in the blood; cancer of the bone
marrow cells that produce leukocytes.
34. lymphomas- Cancer of the tissues that form white blood cells.
35. somatic cell- Any cell in a multicellular organism except a sperm or egg cell or a cell that develops into a sperm or egg.
36. homologous chromosomes- The two chromosomes that make up a matched pair in a diploid
cell. They are of the same length, centromere position, and staining pattern and possess genes for
the same characteristics at corresponding loci.
37. locus- The particular site where a gene is found on a chromosome. Homologous chromosomes
have corresponding gene loci.
38. sex chromosomes- A chromosome that determines whether an individual is male or female.
39. autosomes- A chromosome not directly involved in determining the sex of an organism; in
mammals for example, any chromosome other than X or Y.
40. life cycle- The entire sequence of stages in the life of an organism, from the adults of one generation to the adults of the next.
41. diploid- In an organism that reproduces sexually, a cell containing two homologous sets of
chromosomes, one set inherited from each parent; a 2n cell.
13
42. gametes- A sex cell; a haploid egg or sperm. The union of these two of oppsoite sex produces a
zygote.
43. haploid- In the life cycle of an organism that reproduces sexually, a cell containing a single set
of chromosomes; an n cell.
44. fertilization- The union of the nucleus of a sperm cell with the nucleus of an egg cell, producing
a zygote.
45. zygote- The diploid fertilized egg, which results from union of a sperm cell nucleus and an egg
cell nucleus.
46. meiosis- In a sexually reproducing organism, the division of a single diploid nucleus into four
haploid daughter nuclei. Meiosis and cytokinesis produce haploid gametes from diploid cells in the
reproductive organs of the parents.
47. tetrads- A paired set of homologous chromosomes, each composed of two sister chromatids.
These form during prophase I of meiosis, when crossing over may occur.
48. crossing over- The exchange of segments between chromatids of homologous chromosomes
during the synapsis in prophase I of meiosis; also the exchange of segments between DNA molecules in prokaryotes.
49. chiasma- The microscopically visible site where crossing over has occurred between chromatids
of homologous chromosomes during prophase I of meiosis.
50. genetic recombination- The production, by crossing over and/or independent assortment of
chromosomes during meiosis, of offspring with allele combinations different from those in the parents.
51. karyotypes- A display of micrographs of the metaphase chromosomes of a cell, arranged by size
and centromere position. These may be used to identify certain chromosomal abnormalities.
52. trisomy 21- Down syndrome.
53. Down syndrome- A human genetic disorder resulting from the presence of an extra chromosome 21; characterized by ear and respiratory defects and varying degrees of mental retardation.
54. nondisjunction- An accident of meiosis or mitosis in which a pair of homologous chromosomes
or a pair of sister chromatids fail to separate at anaphase.
55. deletion- The loss of one or more nucleotides from a gene by mutation; the loss of a fragment of
a chromosome.
56. duplication- Repetition of part of a chromosome resulting from fusion with a fragment from a
homologous chromosome; can result from an error in meiosis or from mutagenesis.
57. translocation- The attachment of a chromosomal fragment to a nonhomologous chromosome.
14