Solutions to Problems in Chapter 2 My comments/additions/corrections are in BOLDFACE. 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. Cell membranes are semi-permeable and play an important role in regulating the uptake and loss of molecular constituents from cells. This is not true of cell walls. Cell membranes are fluid, and are not strong enough to resist osmotic pressure. Walls are rigid and strong enough to withstand the osmotic pressure experienced when a cell is in a hypotonic environment. 2.3 What are the principal differences between prokaryotic and eukaryotic cells? ANS: In a eukaryotic cell the many chromosomes are contained within a membranebounded 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 membrane-bounded organelles. 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. In a haploid cell each chromosome is different (has a different set of genes) than any other chromosome in the cell. In a diploid cell there are two copies of each chromosome of the haploid set. For most higher (multicellular) eukaryotes, gametes are haploid and somatic cells of the multicelluar individual are usually diploid. There are exceptions to this found among lower plants such as mosses, where we find multicellar individuals composed of both diploid and haploid tissues. 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 disjunction. DNA is replicated during the S period of interphase. After S phase, each chromosome consists of 2 attached chromatids. During mitosis, the duplicated chromosomes condense and line up in a
planar array at the center of the cell. The chromosomes then split, and the now separate chromatids segregate to opposite poles of the cell so that they are destined to be in different daughter cells when cell division is complete. FEEDBACK: 2.2 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. FEEDBACK: 2.2 2.8 In what way do the microtubule organizing centers of plant and animal cells differ? ANS: The microtubule organizing centers of animal cells have distinct centrosomes, whereas the microtubule organizing centers of plant cells do not. Centrosomes are found only in animal cells. FEEDBACK: 2.2 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 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) 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 β-globin is located on chromosome 11, and the gene for α-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 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 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. -2.4
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 result? 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 infection. 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 daughter cells. There is no advantage to having either an even or odd number of chromosomes in the haploid set. 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 10 9 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) secondary spermatocyte? ANS: (a) 5.8 10 9 nucleotide pairs (np); (b) 2.9 10 9 np; (c) 5.8 10 9 np; (d) 11.6 10 9 np; (e) 5.8 10 9 np; (f) 5.8 10 9 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? i.e. n=5; 2n=10. ANS: (a) 5, (b) 5, (c) 15, (d) 10.