P. 64, #3. What is the difference between sister and nonsister
chromatids? Between homologous and nonhomologous chromosomes?
Chapter 3 -- Mitosis and Meiosis
Sister chromatids are identical to each other. During S
phase of the cell cycle the DNA is replicated and an identical
copy of the chromatid is made. These two chromatids are then
called sister chromatids. Nonsister chromatids are not identical
to each other as they represent different but homologous
chromosomes. The nonsister chromatid will carry the same type of
genetic information, but not exactly the same information.
Homologous chromosomes carry the same type of genetic
information, for example information for seed shape in peas, but
they may not carry exactly the same information, for example one
has information for round seeds and one for wrinkled seeds.
Homologous chromosome will also have the same size and position of
the centromere. Nonhomologous chromosomes have different types of
genetic information, and they are probably not the same size and
with the centromere in the same position.
P. 64, #4. You are working with a species with 2N = 6, in which one
pair of chromosomes is telocentric, one pair is subtelocentric,
and one pair is metacentric. The A,B, and C loci, each
segregating a dominant and recessive allele (A and a, B and b, C
and c), are each located on a different chromosome. Draw the
stages of mitosis.
During PROPHASE of mitosis the chromosomes are coiling but they do not pair. So you have six chromosomes all behaving independantly of the others. Remember that each chromosome exists as a pair of identical sister chromatids. Thus, each chromatid of a single chromosome will carry the same allele.
During METAPHASE each chromosome is moved to the equatorial plane of the cell. The chromosomes are still not paired in anyway and they are still behaving independantly of their homologous chromosome.
During ANAPHASE the centromere of each chromosome replicates and the sister chromatids separate from each other. Thus during anaphase you have 12 things (chromosomes) segregating to opposite ends of the cell. Each daughter cell will get one chromatid from a pair of sister chromatids, thus each daughter cell will be genetically identicle to the other daughter cell.
P. 64, #6. Given the following stages in nuclear division,
identify the process, stage, and diploid number (for example,
meiosis I, prophase, 2N = 10). Keep in mind that one picture
could possibly represent more than one process and stage.
A) Two possibilities:
Prophase II, 2N = 12
Prophase, 2N = 6.
You know that it must be either prophase of mitosis or of meiosis
II because a) the chromosomes are not synapsised (paired), b) the
chromosomes exist as pairs of sister chromatids, and c) the
centrioles are just replicating. IF it is prophase II then the
normal diploid number for the organism must be 12. The cell has
been through one reductional division (meiosis I) and there are
only 6 chromosomes remaining in the cell. If it is prophase of
mitosis then there has not been any previous cell divisions and
the normal diploid number must be 6.
B) Two possibilities:
Metaphase II, 2N = 12
Metaphase, 2N = 6
You know that it must be either metaphase of mitosis or of meiosis
II because a) the chromosomes are not lined-up in pairs and b) the
chromosomes exist as pairs of sister chromatids. IF it is
metaphase II then the normal diploid number for the organism must
be 12. The cell has been through one reductional division
(meiosis I) and there are only 6 chromosomes remaining in the
cell. If it is metaphase of mitosis then there has not been any
previous cell divisions and the normal diploid number must be 6.
C) Two possibilities
Anaphase II, 2N = 12
Anaphase, 2N =6
You know that it must be either anaphase of mitosis or of meiosis
II because the centromeres have replicated and the sister
chromatids have separated from each other. IF it is anaphase II
then the normal diploid number for the organism must be 12. The
cell has been through one reductional division (meiosis I) and
there are only 6 chromosomes remaining in the cell. If it is
anaphase of mitosis then there has not been any previous cell
divisions and the normal diploid number must be 6.
D) Only one possibility:
Prophase II, 2N = 6
You know that it must be prophase of the second meiotic division
because a) the homologous chromosomes are not synapsised (paired), b) there is only 1 copy of each chromosome meaning that 1 reductional division has already occurred, and
c) the centrioles are just beginning to replicate and move to
opposite ends of the cell. As it is prophase II, then there has
been a previous cell division and the normal diploid number
must be 6.
E) Only one possibility:
Anaphase I, 2N = 6
You know that it must be anaphase of the first meiotic division
because the homologous chromosomes are separating from each other.
Replication of the centromeres did not occur. If it is anaphase I
then there has not been any previous cell divisions and the normal
diploid number must be 6.
F) Only one possibility:
Anaphase II, 2N = 6
You know that it must anaphase of the second meiotic division
because a) there are two cells that are undergoing the same
process that will eventually give rise to four cells and b) the
centromeres have replicated and the sister chromatids are
separating from each other. If it is anaphase II then there has
been a previous cell division and the normal diploid number must
be 6. In this case each of the four final cells will have three
chromosomes (haploid) indicating that the normal diploid cell has
P. 64, #7. In human beings 2N = 46. How many chromosomes would
you find in a
a) brain cell? 46 -- A brain cell is a normal somatic cell
and as most somatic cells have the same number of chromosomes, it
will have 46.
B) red blood cell? 0 -- A characteristic of mammals is that
they have enucleated red blood cells. As there is no nucleus, the
cell can not have any chromosomes, thus 0.
C) polar body? 23 -- The polar body is one of those cells
produced during oogenesis after each meiotic division. As all
products of either meiotic division have a haploid number of
chromosomes, the chromosome number must be 23.
D) sperm cell? 23 -- As a sperm cell is a functional gamete,
which is the product of meiosis, it will have a haploid number of
chromosomes. For humans this is 23.
E) secondary oocyte? 23 -- This cell is produced during
oogenesis after the first meiotic division. As the first meiotic
division is a reductional division, this cell must have a haploid
number of chromosomes.
P. 65, #12. What are the differences between a reductional
division and an equational division? What do the terms refer to?
During a reductional division the number of chromosomes is
reduced to half of the normal diploid number, that is reduced to a
haploid number, because the homologous chromosome separate from
each other into separate cells. During an equational division,
there is no reduction in the chromosome number, because the
centromeres replicate and the sister chromatids separate from each
other into separate cells. The terms reductional and equational
refer to the number of chromosomes.
P. 65, #21. Drosophila has four pairs of chromosomes. Let
chromosomes from the male parent be A, B, C, and D and those from
the female parent be a, b, c, d. What fraction of the gametes
from an Aa Bb Cc Dd individual will be
a) all of paternal origin? 1/16 -- This probability is
determined by assuming that any given chromosome has a 1/2
probability of being in any given cell after anaphase I of
meiosis. Thus the probability that all of the parental
chromosomes would go into the same cell is given by multiplying
the probabilities of each individual chromosomes going into that
cell. Thus 1/2 X 1/2 X 1/2 X 1/2 = 1/16
b) all of maternal origin? 1/16 -- The reasoning is the same
as that for the gamete with all paternal chromosomes.
C) half of maternal origin and half of paternal origin? 6/16
There are six ways that this can occur (abCD, aBcD, AbcD, aBCd,
AbCd, ABcd). Each occurrence has a probability of 1/16 and the
sum of the six probabilities is 6/16.
P. 65, #28. A mature human sperm cell has c amount of DNA. How
much DNA (c, 2c, 4c, etc.) Will a somatic cell have if it is
a) in G1? 2c -- A sperm cell has a haploid number of
chromosomes with each chromosome existing as a single chromatids.
A somatic cell in G1 has a diploid number of chromosomes with each
chromosome existing as a single chromatid. Thus a somatic cell in
G1 Has twice as much DNA as a sperm cell.
B) in G2? 4c --A somatic cell in G2 has a diploid number of
chromosomes with each chromosome existing as a pair of sister
chromatids. Thus a somatic cell in G2 has four times as much DNA
as a sperm cell.
C) How much DNA will be in a cell at the end of meiosis I?
2c -- At the end of meiosis I, a cell has a haploid number of
chromosomes, the same number as a sperm cell, however, each
chromosome exists as a pair of chromatids, thus a cell after
meiosis I has twice the amount of DNA that a sperm cell has.
P. 65, #29. A hypothetical organism has two distinct chromosomes
(2N =4) and 50 known genes, each of which has two different
alleles. If an individual is heterozygous at all known loci, how
many gametes can be produced if
a) all genes behave independently? 1,125,899,906,843,000 -- The
answer is gotten by raising 2 to the 50th power. If each locus
assorts independently because of recombination during meiosis,
then the possible number of gametes is as above.
B) all genes are completely linked? 2 -- The answer is due to
that fact that recombination does not occur during meiosis, thus
each gamete gets one of the two sets of chromosomes.
Last updated on 23 August 1996.
Provide comments to Dwight Moore at firstname.lastname@example.org.
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