Answers to All Questions and Problems WC-3
(c) condensation of the chromosomes, (d) formation of
the mitotic spindle, (e) movement of chromosomes to
the equatorial plane, (f) movement of chromosomes to
the poles, (g) decondensation of the chromosomes, (h)
splitting of the centromere, and (i) attachment of micro-
tubules to the kinetochore.
ANS: (1) Anaphase: (f), (h); (2) metaphase: (e), (i); (3) prophase:
(b), (c), (d); (4) telophase: (a), (g).
2.10 Arrange the following events in the correct temporal
sequence during eukaryotic cell division, starting with
the earliest: (a) condensation of the chromosomes,
(b) movement of chromosomes to the poles, (c) duplica-
tion of the chromosomes, (d) formation of the nuclear
membrane, (e) attachment of microtubules to the kineto-
chores, and (f) migration of centrosomes to positions on
opposite sides of the nucleus.
ANS: (c), (f), (a), (e), (b), (d).
2.11 In human beings, the gene for b-globin is located on chro-
mosome 11, and the gene for a-globin, which is another
component of the hemoglobin protein, is located on chro-
mosome 16. Would these two chromosomes be expected to
pair with each other during meiosis? Explain your answer.
ANS: Chromosomes 11 and 16 would not be expected to pair
with each other during meiosis; these chromosomes are
heterologues, not homologues.
2.12 A sperm cell from the fruit y Drosophila melanogaster
contains four chromosomes. How many chromosomes
would be present in a spermatogonial cell about to enter
meiosis? How many chromatids would be present in a
spermatogonial cell at metaphase I of meiosis? How
many would be present at metaphase II?
ANS: There are eight chromosomes in a Drosophila spermato-
gonial cell about to enter meiosis. There are 16 chroma-
tids in a Drosophilia spermatogonial cell at metaphase I of
meiosis. There are eight chromatids in a Drosophilia cell
at metaphase II of meiosis.
2.13 Does crossing over occur before or after chromosome
duplication in cells going through meiosis?
ANS: Crossing over occurs after chromosomes have duplicated
in cells going through meiosis.
2.14 What visible characteristics of chromosomes indicate
that they have undergone crossing over during meiosis?
ANS: The chiasmata, which are visible late in prophase I of
meiosis, indicate that chromosomes have crossed over.
2.15 During meiosis, when does chromosome disjunction
occur? When does chromatid disjunction occur?
ANS: Chromosome disjunction occurs during anaphase I.
Chromatid disjunction occurs during anaphase II.
2.16 In Arabidopsis, is leaf tissue haploid or diploid? How
many nuclei are present in the female gametophyte?
How many are present in the male gametophyte? Are
these nuclei haploid or diploid?
ANS: Leaf tissue is diploid. The female gametophyte contains
eight identical haploid nuclei. The male gametophyte
contains three identical haploid nuclei.
2.17 From the information given in Table 2.1 in this chapter,
is there a relationship between genome size (measured in
base pairs of DNA) and gene number? Explain.
ANS: Among eukaryotes, there does not seem to be a clear
relationship between genome size and gene number. For
example, humans, with 3.2 billion base pairs of genomic
DNA, have about 20,500 genes, and Arabidopsis plants,
with about 150 million base pairs of genomic DNA, have
roughly the same number of genes as humans. However,
among prokaryotes, gene number is rather tightly cor-
related with genome size, probably because there is so
little nongenic DNA.
2.18 Are the synergid cells in an Arabidopsis female gameto-
phyte genetically identical to the egg cell nestled between
them?
ANS: Yes.
2.19 A cell of the bacterium Escherichia coli, a prokaryote, con-
tains one chromosome with about 4.6 million base pairs
of DNA comprising 4288 protein-encoding genes. A cell
of the yeast Saccharomyces cerevisiae, a eukaryote, contains
about 12 million base pairs of DNA comprising 6268
genes, and this DNA is distributed over 16 distinct chro-
mosomes. Are you surprised that the chromosome of a
prokaryote is larger than some of the chromosomes of a
eukaryote? Explain your answer.
ANS: It is a bit surprising that yeast chromosomes are, on aver-
age, smaller than E. coli chromosomes because, as a rule,
eukaryotic chromosomes are larger than prokaryotic
chromosomes. Yeast is an exception because its genome—
not quite three times the size of the E. coli genome—is
distributed over 16 separate chromosomes.
2.20 Given the way that chromosomes behave during meiosis,
is there any advantage for an organism to have an even
number of chromosome pairs (such as Drosophila does),
as opposed to an odd number of chromosome pairs (such
as human beings do)?
ANS: No, there is no advantage associated with an even num-
ber of chromosomes. As long as the chromosomes come
in pairs, they will be able to synapse during prophase I
and then disjoin during anaphase I to distribute the
genetic material properly to the two daughter cells.
2.21 In owering plants, two nuclei from the pollen grain par-
ticipate in the events of fertilization. With which nuclei
from the female gametophyte do these nuclei combine?
What tissues are formed from the fertilization events?
ANS: One of the pollen nuclei fuses with the egg nucleus in the
female gametophyte to form the zygote, which then
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