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90
BRS Genetics
One level of genetic variability
General recombination
Another level of genetic variability
Random distribution
2
23
possible combinations
B
A
A schematic diagram of chromosome 18 shown in its “single chromosome” state and “duplicated
chromosome” state that is formed by DNA replication during meiosis I. It is important to understand that both the “single
chromosome” state and “duplicated chromosome” state will be counted as one chromosome 18. As long as the additional
DNA in the “duplicated chromosome” is bound at the centromere, the structure will be counted as one chromosome 18
even though it has twice the amount of DNA. The “duplicated chromosome” is often referred to as consisting of two sis-
ter chromatids (chromatid 1 and chromatid 2). (B) Schematic representation of meiosis I and meiosis II, emphasizing the
changes in chromosome number and amount of DNA that occur during gametogenesis. Only one pair of homologous
chromosomes (i.e., chromosome 18) is shown (white
maternal origin and black paternal origin) for simplicity sake.
The point at which DNA crosses over is called the chiasma. Segments of DNA are exchanged thereby introducing genetic
variability to the gametes. In addition, various cell types along with their appropriate designation of number of chromo-
somes and amount of DNA is shown.
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Chapter 9
Mitosis, Meiosis, and Gametogenesis
91
t a b l e
9-1
Comparison of Meiosis and Mitosis
Meiosis
Mitosis
Occurs only in the testis and ovary
Occurs in a wide variety of tissues and organs
Produces haploid (23,1N) gametes (sperm and
Produces diploid (46, 2N) somatic daughter cells
secondary oocyte)
Involves two cell divisions and one round of DNA replication
Involves one cell division and one round of DNA replication
Stages of Meiosis
Stages of Mitosis
Meiosis I
Interphase
Prophase
G
0
Phase
Leptotene (long, thin DNA strands)
G
1
Phase
Zygotene (synapsis occurs; synaptonemal complex)
d
d
G
1
Checkpoint
Pachytene (crossover occurs; short, thick DNA strands)
S Phase
Diplotene (chromosomes separate except at centromere)
G
2
Phase
Prometaphase
d
d
G
2
Checkpoint
Metaphase
Anaphase
Mitosis Phase
Telophase
Prophase
Prometaphase
Meiosis II
(essentially identical to mitosis)
Metaphase
Prophase
Anaphase
Prometaphase
Telophase
Metaphase
Anaphase
Telophase
Male:
Prophase of meiosis I lasts
22 days and completes
Interphase lasts
15 hours
meiosis II in a few hours
M phase lasts
1 hour
Female:
Prophase of meiosis I lasts
12 (puberty) – 50 years
(menopause) and completes meiosis II when
fertilization occurs
Pairing of homologous chromosomes occurs
No pairing of homologous chromosomes
Genetic recombination occurs (exchange of large segments
Genetic recombination does not occur
of maternal and paternal DNA via crossover
during meiosis I)
Maternal and paternal homologous chromosomes are
Maternal and paternal homologous chromosomes are
randomly distributed among the gametes to ensure
faithfully distributed among the daughter cells to ensure
genetic variability
genetic similarity
Gametes are genetically different
Daughter cells are genetically identical
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92
1.
The X and Y chromosomes pair in meiosis
at the pseudoautosomal regions. A nondis-
junction of the X and Y chromosomes in a
male during meiosis I would produce which
of the following combinations of gametes?
(A)
one sperm with two X’s, and three sperm
with Y’s
(B)
two sperm with two X’s and two sperm
with two Y’s
(C)
one sperm with no X’s, and three sperm
with an X and a Y
(D)
a sperm with two X’s, a sperm with two
Y’s, and two sperm with no sex chromo-
somes
2.
Which of the following describes the main
difference between meiosis and mitosis?
(A)
homologous chromosomes pair during
meiosis
(B)
the number of chromosomes is reduced
by half during mitosis
(C)
after meiosis is complete, there are 46
chromosomes in each cell
(D)
after mitosis is complete there are 23
chromosomes in each cell.
3.
Crossing over and random segregation
produce much of the genetic variation in
human populations. These events occur dur-
ing which of the following?
(A)
mitosis
(B)
meiosis
(C)
fertilization
(D)
transcription
4.
Tetraploid cells are the result of the failure
of which one of the following processes?
(A)
anaphase of mitosis
(B)
S (synthesis) phase of the cell cycle
(C)
cytokinesis of mitosis
(D)
G
1
phase of the cell cycle
5.
The “reduction division” in which the
number of chromosomes in a germ cell is
reduced from 46 to 23 chromosomes occurs
during which of the following?
(A)
mitosis
(B)
meiosis I
(C)
meiosis II
(D)
synapsis
6.
At the completion of oogenesis, how
mature oocytes are formed from each pri-
mary oocyte?
(A)
one
(B)
two
(C)
three
(D)
four
7.
Which one of the following has a haploid
number of chromosomes?
(A)
primary spermatocyte
(B)
secondary spermatocyte
(C)
spermatogonia
(D)
oogonia
8.
Karyotype analysis can be conducted on
cells that have entered which one of the fol-
lowing stages of cell division?
(A)
meiosis I
(B)
meiosis II
(C)
metaphase of mitosis
(D)
anaphase of mitosis
9.
One of the two places in the cell cycle
where a response to DNA damage occurs is
which one of the following?
(A)
G
0
phase
(B)
metaphase
(C)
m (synthesis) phase
(D)
G
2
checkpoint
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93
1. The answer is (D).
During meiosis I, the nondisjunction of the paired and doubled X and Y
would cause them to go into one of the daughter cells with no X or Y chromosomes going to
the other daughter cell. During meiosis II, the doubled X chromosome and the doubled Y
chromosome would go to separate daughter cells and the cell with no sex chromosomes
would give rise to two daughter cells with no sex chromosomes. Because the X and Y chro-
mosomes are doubled, the daughter cell receiving the X chromosomes will have two copies
and the daughter cell receiving the Y chromosomes will have two copies.
2. The answer is (A).
Homologous chromosomes pair during meiosis but not during mitosis.
The number of chromosomes is reduced by half, from 46 to 23 during meiosis and the
daughter cells are genetically different, but during mitosis, the chromosome number of 46 is
maintained and the daughter cells are genetically identical.
3. The answer is (B).
Crossing-over and random segregation of the maternal and paternal
chromosomes occur during meiosis.
4. The answer is (C).
The 46 doubled chromosomes separate during anaphase, resulting in 92
chromosomes, and if cytokinesis (cell division) fails, one cell with a tetraploid number of
chromosomes, 92, is the result instead of two daughter cells with the normal diploid num-
ber of 46 in each.
5. The answer is (B).
During meiosis I, the 23 paired, doubled homologs randomly separate,
resulting in two daughter cells with 23 chromosomes each.
6. The answer is (A).
Only one mature oocyte results from each primary oocyte. At each of the
two cell divisions in meiosis, a polar body is formed, which usually degenerates.
7. The answer is (B).
A secondary spermatocyte results from meiosis I, the reduction division,
in a primary spermatocyte and the chromosome number is reduced from 46 to 23.
8. The answer is (C).
Prometaphase and metaphase of mitosis is when the chromosomes are
condensed enough to visualize for cytogenetic analysis.
9. The answer is (D).
The other time in the cell cycle when there is a response to DNA damage
is at the G
1
checkpoint before the S (synthesis) phase begins.
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94
■
Mitotic chromosomes are fairly easy to study because they can be observed in any cell
undergoing mitosis.
■
Meiotic chromosomes are much more difficult to study because they can be observed
only in ovarian or testicular samples. In the female, meiosis is especially difficult
because meiosis occurs during fetal development. In the male, meiotic chromosomes
can be studied only in a testicular biopsy of an adult male.
■
Any tissue that can be grown in culture can be used for
karyotype analysis
,
but only cer-
tain tissue samples are suitable for some kinds of studies. For example, chorionic villi or
amniocytes from amniotic fluid are used for prenatal studies; bone marrow is usually the
most appropriate tissue for leukemia studies; skin or placenta is used for miscarriage
studies; and blood for patients with dysmorphic features, unexplained mental retarda-
tion, or any other suspected genetic conditions.
■
Whatever the tissue used, the cells must be grown in tissue culture for some period of
time until optimal growth occurs. Blood cells must have a mitogen added to the culture
media to stimulate the
mitosis
of lympocytes, but other tissues can be grown without
such stimulation.
■
Once a tissue has reached its optimal time for a harvest,
colchicine
(Colcemid) is added
to the media, which arrests the cells in
metaphase
.
■
The cells are then concentrated, treated with a hypotonic solution, which aids in the
spreading of the chromosomes, and finally fixed with an acetic acid/methanol solution.
■
The cell preparation is then dropped onto microscope slides and stained by a variety of
methods (see below).
■
It is often preferable to use
prometaphase
chromosomes in cytogenetic analysis as they
are less condensed and therefore show more detail. In cytogenetic analysis, separated
prometaphase or metaphase chromosomes are identified and photographed or digi-
tized.
■
The chromosomes in the photograph of the metaphase are then cut out and arranged in
a standard pattern called the
karyotype
,
or in the case of digital images, arranged into a
karyotype with the assistance of a computer.
Metaphase or prometaphase chromosomes may be prepared for karyotype analysis and
then stained by various techniques. In addition, one of the great advantages of some staining
techniques is that metaphase or prometaphase chromosomes are not required.
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