2.1 Carbohydrates and proteins are linear polymers. What types of molecules
combine to form these polymers?
ANS: Sugars combine to form carbohydrates; amino acids combine to form proteins.
2.2 All cells are surrounded by a membrane; some cells are surrounded by a wall.
What are the differences between cell membranes and cell walls?
ANS: Cell membranes are made of lipids and proteins; they have a fluid structure. Cell
walls are made of more rigid materials such as cellulose.
2.3 What are the principal differences between prokaryotic and eukaryotic cells?
ANS: In a eukaryotic cell the many chromosomes are contained within a membrane-
bounded structure called the nucleus; the chromosomes of prokaryotic cells are not
contained within a special subcellular compartment. Eukaryotic cells usually possess a
well-developed internal system of membranes and they also have membrane-bounded
subcellular organelles such as mitochondria and chloroplasts; prokaryotic cells do not
typically have a system of internal membranes (although some do), nor do they possess
2.4 Distinguish between the haploid and diploid states. What types of cells are
haploid? What types of cells are diploid?
ANS: In the haploid state, each chromosome is represented once; in the diploid state,
each chromosome is represented twice. Among multicellular eukaryotes, gametes are
haploid and somatic cells are diploid.
2.5 Compare the sizes and structures of prokaryotic and eukaryotic chromosomes.
ANS: Prokaryotic chromosomes are typically (but not always) smaller than eukaryotic
chromosomes; in addition, prokaryotic chromosomes are circular, whereas eukaryotic
chromosomes are linear. For example, the circular chromosome of E. coli, a prokaryote,
is about 1.4 mm in circumference. By contrast, a linear human chromosome may be 10 to
30 cm long. Prokaryotic chromosomes also have a comparatively simple composition:
DNA, some RNA, and some protein. Eukaryotic chromosomes are more complex: DNA,
some RNA, and lots of protein.
2.6 With a focus on the chromosomes, what are the key events during interphase and
M phase in the eukaryotic cell cycle?
ANS: During interphase, the chromosomes duplicate. During M phase (mitosis), the
duplicated chromosomes, each consisting of two identical sister chromatids, condense (a
feature of prophase), migrate to the equatorial plane of the cell (a feature of metaphase),
and then split so that their constituent sister chromatids are separated into different
daughter cells (a feature of anaphase); this last process is called sister chromatid
2.7 Which typically lasts longer, interphase or M phase? Can you explain why one of
these phases lasts longer than the other?
ANS: Interphase typically lasts longer than M phase. During interphase, DNA must be
synthesized to replicate all the chromosomes. Other materials must also be synthesized to
prepare for the upcoming cell division.
2.8 In what way do the microtubule organizing centers of plant and animal cells
ANS: The microtubule organizing centers of animal cells have distinct centrosomes,
whereas the microtubule organizing centers of plant cells do not.
2.9 Match the stages of mitosis with the events they encompass: Stages: (1)
anaphase, (2) metaphase, (3) prophase, (4) telophase. Events: (a) reformation of the
nucleolus, (b) disappearance of the nuclear membrane, (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, (i) attachment of microtubules to the
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) duplication of the chromosomes, (d)
formation of the nuclear membrane, (e) attachment of microtubules to the kinetochores,
(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 -globin is located on chromosome 11, and the
gene for -globin, which is another component of the hemoglobin protein, is located on
chromosome 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 Drosophila sperm cell 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 8 chromosomes in a Drosophila spermatogonial cell about to enter
meiosis. There are 16 chromatids in a Drosophilia spermatogonial cell at metaphase I of
meiosis. There are 8 chromatids in a Drosophilia cell at metaphase II of meiosis.
2.13 Does crossing over occur before or after chromosome duplication in cells going
ANS: Crossing over occurs after chromosomes have duplicated in cells going through
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
ANS: Chromosome disjunction occurs during anaphase I. Chromatid disjunction occurs
during anaphase II.
2.16 In flowering plants, is sporophytic 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: In flowering plants sporophytic tissue is diploid. Eight nuclei are present in the
female gametophyte; three are present in the male gametophyte. All the gametophytic
nuclei are haploid, although in the female gametophyte, two of these nuclei fuse to form a
diploid endosperm nucleus.
2.17 From the information given in this chapter, is there a relationship between
genome size (measured in base pairs of DNA) and gene number? Explain.
ANS: Among eukaryotes, there doesn’t 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 between 20,000 and 25,000 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 correlated with genome size,
probably because there is so little nongenic DNA.
2.18 T1 bacteriophages infect E. coli cells and destroys them. The infection process is
so efficient that with equal numbers of phage and bacteria in a culture tube, all the
bacterial cells are killed in less than 12 hours. Suppose that a mixture of large numbers of
phage and bacteria is incubated for 12 hours and then is spread over the surface of a
culture plate to determine whether any bacterial cells have survived. The next day, a
single colony of cells is visible on the surface of the plate. How would you interpret this
ANS: The single surviving colony may represent a mutant strain that is resistant to
infection by the bacteriophage. The mutation probably occurred in a single bacterial cell,
which then divided to produce the colony. Cells from this colony could be grown on
another plate in the presence of the bacteriophage to confirm that they are resistant to
2.19 In terms of DNA content, are yeast chromosomes larger or smaller than the E. coli
chromosome? What is the significance of your answer?
ANS: Yeast chromosomes are, on average, smaller than E. coli chromosomes. Thus,
some eukaryotic chromosomes are smaller than some prokaryotic 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 isn’t any advantage associated with an even number 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
2.21 In flowering plants, two nuclei from the pollen grain participate 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 develops into an embryo and ultimately into a sporophyte.
The other genetically functional pollen nucleus fuses with two nuclei in the female
gametophyte to form a triploid nucleus, which then develops into a triploid tissue, the
endosperm; this tissue nourishes the developing plant embryo.
2.22 The mouse haploid genome contains about 2.9 x 109 nucleotide pairs of DNA. How
many nucleotide pairs of DNA are present in each of the following mouse cells: (a)
somatic cell, (b) sperm cell, (c) fertilized egg, (d) primary oocyte, (e) fist polar body, (f)
ANS: (a) 5.8 109 nucleotide pairs (np); (b) 2.9 109 np; (c) 5.8 109 np; (d) 11.6 109
np; (e) 5.8 109 np; (f) 5.8 109 np
2.23 Arabidopsis plants have 10 chromosomes (5 pairs) in their somatic cells. How
many chromosomes are present in each of the following: (a) egg cell nucleus in the
female gametophyte, (b) generative cell nucleus in a pollen grain, (c) fertilized
endosperm nucleus, (d) fertilized egg nucleus?
ANS: (a) 5, (b) 5, (c) 15, (d) 10.