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Chapter Two: Chromosomes and Cellular Reproduction
1. What are some genetic differences between prokaryotic and eukaryotic cells?
Prokaryotic cell Eukaryotic cell
No paired chromosomes (haploid)
Typically single circular chromosome
containing a single origin of replication
Single chromosome is replicated with each
copy moving to opposite sides of the cell
No histone proteins bound to DNA
Paired chromosomes common (diploid)
Typically multiple linear chromosomes
containing centromeres, telomeres, and
multiple origins of replication
Chromosomes are replicated and segregate
during mitosis or meiosis to the proper
Histone proteins are bound to DNA
2. Why are viruses often used in the study of genetics?
The close relationship between a virus and its cell host, along with the simpler structure
of the viral particle, makes it useful in studying the genetics of mammals. The viral
genome will have a similar genetic structure to its cell host, but because it has fewer
genes, it will be easier to decipher the interactions and regulation of the viral genes.
3. List three fundamental events that must take place in cell reproduction.
(1) A cell’s genetic information must be copied.
(2) The copies of the genetic information must be separated from one another.
(3) The cell must divide into two daughter cells.
4. Outline the process by which prokaryotic cells reproduce.
(1) Replication of the circular chromosome takes place.
(2) The two replicated chromosomal copies attach to the plasma membrane.
Chapter Two: Chromosomes and Cellular Reproduction 15
(3) The plasma membrane grows, which results in the separation of the two
(4) A new cell wall is formed between the two chromosomes, producing two cells, each
with its own chromosome.
5. Name three essential structural elements of a functional eukaryotic chromosome and
describe their functions.
(1) Centromere: serves as the point of attachment for the kinetochore to which spindle
fibers (microtubules) attach
(2) Telomeres, or the natural ends of the linear eukaryotic chromosome: serve to
stabilize the ends of the chromosome; may have a role in limiting cell division
(3) Origins of replication: serve as the starting place for DNA synthesis
6. Sketch and identify four different types of chromosomes based on the position of the
metacentric submetacentric acrocentric telocentric
7. List the stages of interphase and the major events that take place in each stage.
Three predominant stages are found in interphase of cells active in the cell cycle.
(1) G1 (Gap 1): In this phase, the cell grows and synthesizes proteins necessary for cell
division. During G1, the G1/S checkpoint takes place. Once the cell has passed this
checkpoint, it is committed to divide.
(2) S phase: During S phase, DNA replication takes place.
(3) G2 (Gap 2): In G2, additional biochemical reactions take place that prepare the cell
for mitosis. A major checkpoint in G2 is the G2/M checkpoint. Once the cell has
passed this checkpoint, it enters into mitosis.
A fourth stage is frequently found in cells prior to the G1/S checkpoint. Cells may exit
the active cell cycle and enter into a nondividing stage called G0.
8. What are checkpoints? List some of the important checkpoints in the cell cycle.
16 Chapter Two: Chromosomes and Cellular Reproduction
Checkpoints function to ensure that all the cellular components, such as important
proteins and chromosomes, are present and functioning before the cell moves to the next
stage of the cell cycle. If components are missing or not functioning, the checkpoint will
prevent the cell from moving to the next stage. The checkpoints prevent defective cells
from replicating and malfunctioning.
These checkpoints occur throughout the various stages of the cell cycle. Important
checkpoints include the G1/S checkpoint, which occurs during G1 prior to the S phase;
the G2/M checkpoint, which occurs in G2 prior to mitosis; and the spindle-assembly
checkpoint, which occurs during mitosis.
9. List the stages of mitosis and the major events that take place in each stage.
(1) Prophase: The chromosomes condense and become visible, the centrosomes move
apart, and microtubule fibers form from the centrosomes.
(2) Prometaphase: The nucleoli disappear and the nuclear envelope begins to
disintegrate, allowing for the cytoplasm and nucleoplasm to join. The sister
chromatids of each chromosome are attached to microtubules from the opposite
(3) Metaphase: The spindle microtubules are clearly visible and the chromosomes
arrange themselves on the equatorial plane of the cell.
(4) Anaphase: The sister chromatids separate at the centromeres after the breakdown of
cohesin protein, and the newly formed daughter chromosomes move to the opposite
poles of the cell.
(5) Telophase: The nuclear envelope reforms around each set of daughter
chromosomes. Nucleoli reappear. Spindle microtubules disintegrate.
10. Briefly describe how the chromosomes move toward the spindle poles during anaphase.
Due to the actions of the microtubule subunits attached to the kinetochores of the
chromosome and motor proteins (e.g., the protein kinesin is a motor protein), the
chromosomes are pulled toward the spindle poles during anaphase. The spindle fibers
are composed of tubulin protein subunits. As the tubulin subunits are removed from the
“–” end of the microtubule, the chromosome is pulled (or “reeled in”) toward the spindle
pole as the microtubule is shortened. While at the “+” end, the kinetochore is removing
tubulin subunits of the microtubule attached to the kinetochore with the net effect being
the movement of the chromosome closer to the spindle pole. Molecular motor proteins,
such as kinesin, are responsible for removing the subunits at the “+” and “–” ends of the
microtubules and thus generate the force needed to move the chromosomes.
11. What are the genetically important results of the cell cycle and mitosis?
Chapter Two: Chromosomes and Cellular Reproduction 17
In the mitotic cell cycle, the genetic material is precisely copied and mitosis ensures that
the identical copies of the genetic material are separated accurately into the new
daughter cells, resulting in two cells containing the same genetic information. In other
words, the cells have genomes identical to each other and to the mother cell.
12. Why are the two cells produced by the cell cycle genetically identical?
The two cells are genetically identical because during S phase an exact copy of each
DNA molecule was created. These exact copies give rise to the two identical sister
chromatids. Mitosis ensures that each new cell receives one of the two identical sister
chromatids. Thus, the newly formed cells will contain identical daughter chromosomes.
13. What are the stages of meiosis and what major events take place in each stage?
Meiosis I: Separation of homologous chromosomes
Prophase I: The chromosomes condense and homologous pairs of
chromosomes undergo synapsis. While the chromosomes are synapsed,
crossing over occurs. The nuclear membrane disintegrates and the meiotic
spindle begins to form.
Metaphase I: The homologous pairs of chromosomes line up on the
equatorial plane of the metaphase plate.
Anaphase I: Homologous chromosomes separate and move to opposite
poles of the cell. Each chromosome possesses two sister chromatids.
Telophase I: The separated homologous chromosomes reach the spindle
poles and are at opposite ends of the cell.
Meiosis I is followed by cytokinesis, resulting in the division of the
cytoplasm and the production of two haploid cells. These cells may skip
directly into meiosis II or enter interkinesis, where the nuclear envelope
reforms and the spindle fibers break down.
Meiosis II: Separation of sister chromatids
Prophase II: Chromosomes condense, the nuclear envelope breaks down,
and the spindle fibers form.
Metaphase II: Chromosomes line up at the equatorial plane of the
18 Chapter Two: Chromosomes and Cellular Reproduction
Anaphase II: The centromeres split, which results in the separation of sister
Telophase II: The daughter chromosomes arrive at the poles of the spindle.
The nuclear envelope reforms, and the spindle fibers break down.
Following meiosis II, cytokinesis takes place.
14. What are the major results of meiosis?
Meiosis involves two cell divisions, thus producing four new cells (in many species).
The chromosome number of a haploid cell produced by meiosis I (haploid) is half the
chromosome number of the original diploid cell. Finally, the cells produced by meiosis
are genetically different from the original cell and genetically different from each other.
15. What two processes unique to meiosis are responsible for genetic variation? At what
point in meiosis do these processes take place?
(1) Crossing over, which begins during the zygotene stage of prophase I and is
completed near the end of prophase I.
(2) The random distribution of separated members of the homologous chromosomes (the
maternal and paternal chromosomes) to daughter cells, which takes place in
anaphase I of meiosis. The arrangement for separation is determined by the random
alignment of homologs in metaphase I.
16. How does anaphase I of meiosis differ from anaphase of mitosis?
In anaphase I of meiosis, homologous chromosomes separate whereas in anaphase of
mitosis the sister chromatids separate.
17. Briefly explain why sister chromatids remain together in anaphase I but separate in anaphase
II of meiosis.
In meiosis, a similar process to mitosis occurs. Meiosis-specific cohesin complexes (different
from cohesion proteins in mitosis) form at the centromeres of the sister chromatids during the
S phase. At the beginning of meiosis, cohesin molecules are also found along the entire
length of the chromosome arms assisting in the formation of the synaptonemal complex and
holding together the two homologs. During anaphase I of meiosis, the cohesin molecules
along the arms are cleaved by activated separase allowing the homologs to separate.
However, the cohesin complexes at the centromeres of the sister chromatids are protected
from the action of separase by the protein shugoshin and are unaffected. The result is that
sister chromatids remained attached during anaphase I. At the end of metaphase II, the
Chapter Two: Chromosomes and Cellular Reproduction 19
protection of the cohesin molecules at the centromeres is lost, and the separase proteins can
now cleave the cohesin complex, which allows the sister chromatids to separate.
18. Outline the processes of spermatogenesis and oogenesis in animals.
In animals, spermatogenesis occurs in the testes. Primordial diploid germ cells divide
mitotically to produce diploid spermatogonia that can either divide repeatedly by mitosis
or enter meiosis. A spermatogonium that has entered prophase I of meiosis is called a
primary spermatocyte and is diploid. Upon completion of meiosis I, two haploid cells,
called secondary spermatocytes, are produced. Upon completing meiosis II, the
secondary spermatocytes produce a total of four haploid spermatids.
Female animals produce eggs through the process of oogenesis. Similar to what takes
place in spermatogenesis, primordial diploid cells divide mitotically to produce diploid
oogonia that can divide repeatedly by mitosis, or enter meiosis. An oogonium that has
entered prophase I is called a primary oocyte and is diploid. Upon completion of meiosis
I, the cell divides, but unequally. One of the newly produced haploid cells receives most
of the cytoplasm and is called the secondary oocyte. The other haploid cell receives only
a small portion of the cytoplasm and is called the first polar body. Ultimately, the
secondary oocyte will complete meiosis II and produce two haploid cells. One cell, the
ovum, will receive most of the cytoplasm from the secondary oocyte. The smaller
haploid cell is called the second polar body. Typically, the polar bodies disintegrate, and
only the ovum is capable of being fertilized.
19. Outline the processes of male gamete formation and female gamete formation in plants.
Plants alternate between a multicellular haploid stage called the gametophyte and a
multicellular diploid stage called the sporophyte. Meiosis in the diploid sporophyte stage
of plants produces haploid spores that develop into the gametophyte. The gametophyte
produces gametes by mitosis.
In flowering plants, the microsporocytes found in the stamen of the flower undergo
meiosis to produce four haploid microspores. Each microspore divides by mitosis to
produce the pollen grain, or the microgametophyte. Within the pollen grain are two
haploid nuclei. One of the haploid nuclei divides by mitosis to produce two sperm cells.
The other haploid nucleus directs the formation of the pollen tube.
Female gamete production in flowering plants takes place within the megagametophyte.
Megasporocytes found within the ovary of a flower divide by meiosis to produce four
megaspores. Three of the megaspores disintegrate, while the remaining megaspore
divides mitotically to produce eight nuclei that form the embryo sac (or female
gametophyte). Of the eight nuclei, one will become the egg.
20 Chapter Two: Chromosomes and Cellular Reproduction
APPLICATION QUESTIONS AND PROBLEMS
*20. Answer the following questions about the blind men’s riddle, presented in the
introduction to this chapter.
a. What do the two socks of a pair represent in the cell cycle?
The two chromatids of a chromosome
b. In the riddle, each blind man buys his own pairs of socks, but the clerk places all
the pairs in one bag. Thus, there are two pairs of socks of each color in the bag
(two black pairs, two blue pairs, two gray pairs, etc.). What do the two pairs
(four socks in all) of each color represent?
The two chromosomes of a homologous pair
c. What is the thread that connects the two socks of a pair?
d. What is the molecular knife that cuts the thread holding the two socks in a pair
The enzyme separase
e. What in the riddle performs the same function as spindle microtubles?
The hands of the two blind men
f. What would happen if one man failed to grasp his sock of a particular pair? How does
that outcome relate to events in the cell cycle?
If one man failed to grasp his sock, it would be difficult for the knife to cut the string
holding them together. The two socks of a pair would not be separated and both would
end up in one man’s bag. Similarly, if each chromatid is not attached to spindle fibers
and pulled in opposite directions, the two chromatids will not separate and both would
migrate to the same cell. This cell would have two copies of one chromosome.
Chapter Two: Chromosomes and Cellular Reproduction 21
21. A cell has a circular chromosome and no nuclear membrane. Its DNA is complexed with
some histone proteins. Does this cell belong to a eubacterium, an archaean, or a
eukaryote? Explain your reasoning.
This cell is most likely an archaea. The cell is not eukaryotic because it lacks a nuclear
membrane and has a single circular chromosome. The cell is not a eubacterium because
it has histone proteins, which are present in archaea and eukaryotes but lacking in
22. A certain species has three pairs of chromosomes: an acrocentric pair, a metacentric pair,
and a submetacentric pair. Draw a cell of this species as it would appear in metaphase of
23. Examine Figure 2.6a. What type of chromosome (metacentric, submetacentric,
acrocentric, or telocentric) is chromosome 1? What about chromosome 4?
The centromere in chromosome 1 is centrally located, so it is metacentric. The
centromere of chromosome 4 is located between the center and the end of the
chromosome, so it is submetacentric.
22 Chapter Two: Chromosomes and Cellular Reproduction
*24. A biologist examines a series of cells and counts 160 cells in interphase, 20 cells in
prophase, six cells in prometaphase, two cells in metaphase, seven cells in anaphase, and
five cells in telophase. If the complete cell cycle requires 24 hours, what is the average
duration of the M phase in these cells? Of metaphase?
To determine the average duration of M phase in these cells, the proportion of cells in
interphase, or in each stage of M phase, should be calculated by dividing the number of
cells in each stage by the total number of cells counted. To calculate the time required
for a given phase, multiply 24 hours by the proportion of cells at that stage. This will
give the average duration of each stage in hours.
The average duration of M phase can be determined by adding up the hours spent in
each stage of mitosis. In these cells, M phase lasts 4.8 hours. The table shows that
metaphase requires 0.24 hour, or 14.4 minutes.
25. In what stage of mitosis is the cell illustrated in the chapter-opening figure (p. 17)?
In the chapter-opening figure, the sister chromatids within the cell have already
separated and have moved apart. Likely the cell is either in late anaphase or in
26. A certain species has three pairs of chromosomes: one acrocentric pair and two
metacentric pairs. Draw a cell of this species as it would appear in the following stages
a. Metaphase I
b. Anaphase I
at each stage
Interphase 160 0.80 19.2
Prophase 20 0.10 2.4
Prometaphase 6 0.03 0.72
Metaphase 2 0.01 0.24
Anaphase 7 0.035 0.84
Telophase 5 0.025 0.6
Totals 200 1.0 24
Chapter Two: Chromosomes and Cellular Reproduction 23
c. Metaphase II
d. Anaphase II
a.Metaphase I b. Anaphase I c. Anaphase d. Anaphase II
27. Construct a table similar to that in Figure 2.12 for the different stages of meiosis, giving
the number of chromosomes per cell and the number of DNA molecules per cell for a
cell that begins with four chromosomes (two homologous pairs) in G1. Include the
following stages in your table: G1, S, G2, prophase I, metaphase I, anaphase I, telophase I
(after cytokinesis), prophase II, metaphase II, anaphase II, and telophase II (after
G1 S G2 MI A1 T1 PII MII AII TII
Number of Chromosomes per cell 4 4 4 4 4 2 2 2 4 2
Number of DNA Molecules per cell 4 4 to 8 8 8 8 4 4 4 4 2
*28. A cell in G1 of interphase has 12 chromosomes. How many chromosomes and DNA
molecules will be found per cell when this original cell progresses to the following
The number of chromosomes and DNA molecules depends on the stage of the cell cycle.
Each chromosome contains only one centromere, but after the completion of S phase,
and prior to anaphase of mitosis or anaphase II of meiosis, each chromosome will consist
of two DNA molecules.
a. G2 of interphase
G2 of interphase occurs after S phase, when the DNA molecules are replicated. Each
chromosome now consists of two DNA molecules. So a cell in G2 will contain 12
chromosomes and 24 DNA molecules.
b. Metaphase I of meiosis
24 Chapter Two: Chromosomes and Cellular Reproduction
Neither homologous chromosomes nor sister chromatids have separated by
metaphase I of meiosis. Therefore, the chromosome number is 12, and the number of
DNA molecules is 24.
c. Prophase of mitosis
This cell will contain 12 chromosomes and 24 DNA molecules.
d. Anaphase I of meiosis
During anaphase I of meiosis, homologous chromosomes separate and begin moving
to opposite ends of the cell. However, sister chromatids will not separate until
anaphase II of meiosis. The number of chromosomes is still 12, and the number of
DNA molecules is 24.
e. Anaphase II of meiosis
Homologous chromosomes were separated and migrated to different daughter cells at
the completion of meiosis I. However, in anaphase II of meiosis, sister chromatids
separate, resulting in a temporary doubling of the chromosome number in the now
haploid daughter cell. The number of chromosomes and the number of DNA
molecules present will both be 12.
f. Prophase II of meiosis
The daughter cells in prophase II of meiosis are haploid. The haploid cells will
contain six chromosomes and 12 DNA molecules.
g. After cytokinesis following mitosis
After cytokinesis following mitosis the daughter cells will enter G1. Each cell will
contain 12 chromosomes and 12 DNA molecules.
h. After cytokinesis following meiosis II
After cytokinesis following meiosis II, the haploid daughter cells will contain six
chromosomes and six DNA molecules.
Chapter Two: Chromosomes and Cellular Reproduction 25
29. How are the events that take place in spermatogenesis and oogenesis similar? How are
Both spermatogenesis and oogenesis begin similarly in that the diploid primordial cells
(spermatogonia and oogonia) can undergo multiple rounds of mitosis to produce more
primordial cells, or both types of cells can enter into meiotic division. In
spermatogenesis, cytokinesis is equal, resulting in haploid cells of similar sizes. Upon
completion of meiosis II, four haploid spermatids have been produced for each
spermatogonium that began meiosis. In oogenesis, cytokinesis is unequal. At the
completion of meiosis I in oogenesis, a secondary oocyte is produced, which is much
larger and contains more cytoplasm than the other haploid cell produced, called the first
polar body. At the completion of meiosis II, the secondary oocyte divides, producing the
ovum and the second polar body. Again, the division of the cytoplasm in cytokinesis is
unequal, with the ovum receiving most of the cytoplasmic material. Usually, the polar
bodies disintegrate, leaving the ovum as the only product of meiosis.
*30. All of the following cells, shown in various stages of mitosis and meiosis, come from the
same rare species of plant.
1. 2. 3.
a. What is the diploid number of chromosomes in this plant?
To determine the diploid chromosome number in this plant, the number of
centromeres present within a cell that contains homologous pairs of chromosomes
must be determined. Remember, each chromosome possesses a single centromere.
The location and presence of a centromere are determined by the attachment of the
spindle fibers to the chromosome, which occurs at the centromere in the above
diagram. Only the cell in stage (a) clearly has homologous pairs of chromosomes. So
the diploid chromosome number for cells of this species of plant is six.
b. Give the names of each stage of mitosis or meiosis shown.
26 Chapter Two: Chromosomes and Cellular Reproduction
Cell 1 is undergoing anaphase of meiosis I, as indicated by the separation of the
homologous pairs of chromosomes. Cell 2 in the diagram contains six chromosomes,
the diploid chromosome number for this species. Also in this cell, sister chromatids
have separated, resulting in a doubling of the chromosome number within the cell
from six to 12. Based on the number of chromosomes, the separation of sister
chromatids in this cell must be occurring during anaphase of mitosis. In cell 3 again,
sister chromatids are being separated, but the number of chromosomes present in the
cell is only six. This indicates that no homologs are present within the cell, so in this
cell the separation of sister chromatids is occurring in anaphase II of meiosis.
c. Give the number of chromosomes and number of DNA molecules per cell present at
Cell 1, which is in anaphase I of meiosis contains six chromosomes and 12 DNA
molecules (or sister chromatids). Cell 2 has 12 chromosomes and 12 DNA molecules
in anaphase of mitosis. Cell 3, which is in anaphase II of meiosis has six
chromosomes and six DNA molecules.
*31. The amount of DNA per cell of a particular species is measured in cells found at
various stages of meiosis, and the following amounts are obtained:
Amount of DNA per cell in pictograms (pg)
3.7 pg 7.3 pg 14.6 pg
Match the amounts of DNA above with the corresponding stages of meosis (a
through f). You may use more than one stage for each amount of DNA.
Stage of meiosis
G1 occurs prior to S phase and the doubling of the amount of DNA and prior to the
completion of the meiosis II and cytokinesis, which will result in a haploid cell
containing one-half the amount of DNA that was contained in the cell in G1.
b. Prophase I
During prophase I of meiosis, the amount of DNA in the cell is two times the amount
in G1. The homologous chromosomes are still located within a single cell, and there
are two sister chromatids per chromosome.
Chapter Two: Chromosomes and Cellular Reproduction 27
G2 takes place directly after the completion of S phase, so the amount of DNA is two
times the amount prior to the S phase.
d. Following telophase II and cytokinesis
Following cytokinesis associated with meiosis II, each daughter cell will contain only
one-half the amount of DNA of a mother cell found in G1 of interphase. By the
completion of cytokinesis associated with meiosis II, both homologous pairs of
chromosomes and sister chromatids have been separated into different daughter cells.
Therefore, each daughter cell will contain only one-half the amount of DNA of the
original cell in G1.
e. Anaphase I
During anaphase I of meiosis, the amount of DNA in the cell is two times the amount
in G1. The homologous chromosomes are still located within a single cell, and there
are two sister chromatids per chromosome.
f. Metaphase II
Metaphase II takes place after the cytokinesis associated with meiosis I and results in
the daughter cells receiving only one-half the DNA found in their mother cell. In
metaphase II of meiosis, the amount of DNA in each cell is the same as G1 because
each chromosome still consists of two DNA molecules (two sister chromatids per
The amount of DNA in the cell will be doubled after the completion of S phase in the
cell cycle and prior to cytokinesis in either mitosis or meiosis I. At the completion of
cytokinesis following meiosis II, the amount of DNA will be halved.
*32. How would each of the following events affect the outcome of mitosis or meiosis?
a. Mitotic cohesin fails to form early in mitosis.
Cohesin is necessary to hold the sister chromatids together until anaphase of mitosis.
If cohesin fails to form early in mitosis, the sister chromatids could separate prior to
28 Chapter Two: Chromosomes and Cellular Reproduction
anaphase. The result would be improper segregation of chromosomes to daughter
b. Shugoshin is absent during meiosis.
Shugoshin protects cohesin proteins from degradation at the centromere during
meiosis I. Cohesin at the arms of the homologous chromosomes is not protected by
shugoshin and is broken in anaphase I, allowing for the two homologs to separate. If
shugosin is absent during meiosis, then the cohesin at the centromere may be broken,
allowing for the separation of sister chromatids along with the homologs during
anaphase I, leading to improper segregation of chromosomes to daughter cells.
c. Shugoshin does not break down after anaphase I of meiosis.
If shugoshin is not broken down, then the cohesins at the centromere will remain
protected from degradation. The intact cohesins will prevent the sister chromatids
from separating during anaphase II of meiosis, resulting in an improper separation of
sister chromatids and daughter cells with too many or too few chromosomes.
d. Separase is defective.
Homologous chromosomes and sister chromatids would not separate in meiosis and
mitosis, resulting in some cells that have too few chromosomes and some cells that
have too many chromosomes.
*33. A cell in prophase II of meiosis contains 12 chromosomes. How many chromosomes
would be present in a cell from the same organism if it were in prophase of mitosis?
Prophase I of meiosis?
A cell in prophase II of meiosis will contain the haploid number of chromosomes. For
this organism, 12 chromosomes represent the haploid chromosome number of a cell, or
one complete set of chromosomes.
A cell from the same organism that is undergoing prophase of mitosis would contain a
diploid number of chromosomes, or two complete sets of chromosomes, which means
that homologous pairs of chromosomes are present. So a cell in this stage should contain
Homologous pairs of chromosomes have not been separated by prophase I of meiosis.
During this stage, a cell of this organism will contain 24 chromosomes.
Chapter Two: Chromosomes and Cellular Reproduction 29
34. A cell has eight chromosomes in G1 of interphase. Draw a picture of this cell with its
chromosomes at the following stages. Indicate how many DNA molecules are present at
a. Metaphase of mitosis b. Anaphase of mitosis
8 chromosomes 16 Chromosomes
16 DNA molecules 16 DNA molecules
c. Anaphase II of meiosis d. Diplotene of meiosis I
8 chromosomes 8 Chromosomes
8 DNA molecules 16 DNA molecules
*35. The fruit fly Drosophila melanogaster has four pairs of chromosomes, whereas the
house fly Musca domestica has six pairs of chromosomes. In which species would you
expect to see more genetic variation among the progeny of a cross? Explain your answer.
30 Chapter Two: Chromosomes and Cellular Reproduction
The progeny of an organism whose cells contain more homologous pairs of
chromosomes should be expected to exhibit more variation. The number of different
combinations of chromosomes that are possible in the gametes is 2n
, where n is equal to
the number of homologous pairs of chromosomes. For the fruit fly with four pairs of
chromosomes, the number of possible combinations is 24
= 16. For Musca domestica
with six pairs of chromosomes, the number of possible combinations is 26
*36. A cell has two pairs of submetacentric chromosomes, which we will call chromosomes
Ia, Ib, IIa, and IIb (chromosomes Ia and Ib are homologs, and chromosomes IIa and IIb are
homologs). Allele M is located on the long arm of chromosome Ia, and allele m is located
at the same position on chromosome Ib. Allele P is located on the short arm of
chromosome Ia, and allele p is located at the same position on chromosome Ib. Allele R is
located on chromosome IIa and allele r is located at the same position on chromosome
a. Draw these chromosomes, identifying genes M, m, P, p, R, and r, as they might
appear in metaphase I of meiosis. Assume that there is no crossing over.
b. Taking into consideration the random separation of chromosomes in anaphase I, draw
the chromosomes (with genes identified) present in all possible types of gametes that
might result from this cell’s undergoing meiosis. Assume that there is no crossing
M M m m
P P p p
R R r r
Chapter Two: Chromosomes and Cellular Reproduction 31
37. A horse has 64 chromosomes and a donkey has 62 chromosomes. A cross between a
female horse and a male donkey produces a mule, which is usually sterile. How many
chromosomes does a mule have? Can you think of any reasons for the fact that most
mules are sterile?
The haploid egg produced by the female horse contains 32 chromosomes. The haploid
sperm produced by the male donkey contains 31 chromosomes. The union of the horse
and donkey gametes will produce a zygote containing 63 chromosomes. From the
zygote, the adult mule will develop and will contain cells with a chromosome number of
63. Because an odd number of chromosomes in the mule’s cells are present, at least one
chromosome will not have a homolog. During the production of gametes by meiosis
when pairing and separation of homologous chromosomes occurs, the odd chromosome
will be unable to pair up. Furthermore, the mule’s chromosomes, which are contributed
by the horse and donkey, are from two different species. Not all of the mule’s
chromosomes may be able to find a suitable homolog during meiosis I and thus may not
synapse properly during prophase I of meiosis. If improper synapsis or no synapsis
occurs during prophase I, this will result in faulty segregation of chromosomes to the
daughter cells produced at the conclusion of meiosis I. This leads to gametes that have
abnormal numbers of chromosomes. When these abnormal gametes unite, the resulting
zygote has an abnormal number of chromosomes and will be nonviable.
*38. Normal somatic cells of horses have 64 chromosomes (2n = 64). How many
chromosomes and DNA molecules will be present in the following types of horse cells?
Cell type Number of chromosomes Number of DNA molecules
a. Spermatogonium 64 64
Assuming the spermatogonium is in G1 prior to the production of sister chromatids in
S phase, the chromosome number will be the diploid number of chromosomes.
r R r
P P p p
M M m m
32 Chapter Two: Chromosomes and Cellular Reproduction
b. First polar body 32 64
The first polar body is the product of meiosis I, so it will be haploid; but the sister
chromatids have not separated, so each chromosome will consist of two sister
c. Primary oocyte 64 128
The primary oocyte has stopped in prophase I of meiosis. So the homologs have not
yet separated, and each chromosome consists of two sister chromatids.
d. Secondary spermatocyte 32 64
The secondary spermatocyte is a product of meiosis I and has yet to enter meiosis II.
So the secondary spermatocyte will be haploid because the homologous pairs were
separated in meiosis I; but each chromosome is still composed of two sister
39. Indicate whether each of the following cells is haploid or diploid.
Cell Type Haploid or Diploid?
Primary spermatocyte diploid
First polar body haploid
Secondary oocyte haploid
*40. A primary oocyte divides to give rise to a secondary oocyte and a first polar body. The
secondary oocyte then divides to give rise to an ovum and a second polar body.
a. Is the genetic information found in the first polar body identical with that found in the
secondary oocyte? Explain your answer.
No, the information is not identical with that found in the secondary oocyte. The first
polar body and the secondary oocyte are the result of meiosis I. In meiosis I,
homologous chromosomes segregate and thus both the first polar body and secondary
oocyte will contain only one member of each original chromosome pair, and these
will have different alleles of some of the genes. Also the recombination that took
place in prophase I will have generated new and different arrangements of genetic
material for each member of the pair.
b. Is the genetic information found in the second polar body identical with that in the
ovum? Explain your answer.
Chapter Two: Chromosomes and Cellular Reproduction 33
No, the information is not identical. The second polar body and the ovum will contain
the same members of the homologous pairs of chromosomes that were separated
during meiosis I and produced by the separation of sister chromatids during anaphase
II. However, the sister chromatids are no longer identical. The sister chromatids have
undergone recombination during prophase I and thus contain genetic information that
is not identical to the other sister chromatids.
41. From 80% to 90% of the most common human chromosome abnormalities arise because
the chromosomes fail to divide properly in oogenesis. Can you think of a reason why
failure of chromosome division might be more common in female gametogenesis than
Male gametogenesis, or spermatogenesis in human males, occurs regularly. Once the
spermatogonium begins meiosis, the process quickly goes to completion, resulting in the
formation of four spermatids that can mature into sperm cells. Female gametogenesis, or
oogenesis in human females, is more complicated. Each oogonium enters meiosis I but
stops at prophase I, generating a primary oocyte. This primary oocyte remains frozen in
prophase I until ovulation begins and continues through meiosis I. Only if the egg is
fertilized will meiosis II be completed. Because the primary oocyte is present at birth,
the completion of meiosis I by a primary oocyte may not occur for many years (35 to 40
years or more). The length of time could lead to degradation or damaging of the meiotic
machinery (such as the meiotic spindle fibers or cohesin complex). The damaged meiotic
machinery could result in an improper separation of homologous pairs or of sister
chromatids during the meiotic process. The spermatogenesis process does not have this
time delay, which may protect the process from age-induced damage to the meiotic
42. On average, what proportion of the genome in the following pairs of humans would be
exactly the same if no crossing over took place? (For the purposes of this question only,
we will ignore the special case of the X and Y sex chromosomes and assume that all
genes are located on nonsex chromosomes.)
a. Father and child
The father will donate one-half of his chromosomes to his child. Therefore, the father
and child will have one-half of their genomes that are similar.
b. Mother and child
34 Chapter Two: Chromosomes and Cellular Reproduction
The mother will donate one-half of her chromosomes to her child. Therefore, the
mother and child will have one-half of their genomes that are similar.
c. Two full siblings (offspring that have the same two biological parents)
The parents can contribute only one-half of their genome to each offspring. So it is
likely that the siblings share one-fourth of their genes from one parent. Because each
sibling would share one-fourth of their genes from each parent, their total relatedness
is one-half (or 1⁄4 + 1⁄4).
d. Half siblings (offspring that have only one biological parent in common)
Half siblings share only one-fourth of their genomes with each other because they
have only one parent in common.
e. Uncle and niece
An uncle would share one-half of his genomes with his sibling, who would share one-
half of his or her genome with his or her child. So, an uncle and niece would share
one-fourth of their genomes (1⁄2 × 1⁄2).
f. Grandparent and grandchild
The grandparent and grandchild would share one-fourth of their genomes because the
grandchild would share one-half of her genome with her parent and the parent would
share one-half of her genome with the child’s grandparent.
*43. Female bees are diploid, and male bees are haploid. The haploid males produce sperm
and can successfully mate with diploid females. Fertilized eggs develop into females
and unfertilized eggs develop into males. How do you think the process of sperm
production in male bees differs from sperm production in other animals?
Most male animals produce sperm by meiosis. In haploid male bees, meiosis will not
occur since meiosis can only occur in diploid cells. Male bees can still produce sperm,
but only through mitosis. Haploid cells that divide mitotically produce more haploid