Chapter Questions
What role do the following cellular components play in the storage, expression, or transmission of genetic information: (a) chromatin, (b) nucleolus, (c) ribosome, (d) mitochondrion, (e) centriole, (f) centromere?
Discuss the concepts of homologous chromosomes, diploidy, and haploidy. What characteristics do two homologous chromosomes share?
If two chromosomes of a species are the same length and have similar centromere placements and yet are not homologous, what is different about them?
Describe the events that characterize each stage of mitosis.
If an organism has a diploid number of 16 , how many chromatids are visible at the end of mitotic prophase? How many chromosomes are moving to each pole during anaphase of mitosis?
What designations are assigned to chromosomes on the basis of their centromere placement, and where is the centromere located in each case?
Contrast telophase in plant and animal mitosis.
Describe the phases of the cell cycle and the events that characterize each phase.
Examine Figure 2-11, which shows oogenesis in animal cells. Will the genotype of the second polar body (derived from meiosis II) always be identical to that of the ootid? Why or why not?
Contrast the end results of meiosis with those of mitosis.
Define and discuss these terms: (a) synapsis, (b) bivalents, (c) chiasmata, (d) crossing over, (e) chromomeres, (f) sister chromatids, (g) tetrads, (h) dyads, (i) monads.
Contrast the genetic content and the origin of sister versus nonsister chromatids during their earliest appearance in prophase I of meiosis. How might the genetic content of these change by the time tetrads have aligned at the equatorial plate during metaphase I?
Given the end results of the two types of division, why is it necessary for homologs to pair during meiosis and not desirable for them to pair during mitosis?
An organism has a diploid number of 16 in a primary oocyte. (a) How many tetrads are present in the first meiotic prophase? (b) How many dyads are present in the second meiotic prophase? (c) How many monads migrate to each pole during the second meiotic anaphase?
Contrast spermatogenesis and oogenesis. What is the significance of the formation of polar bodies?
Explain why meiosis leads to significant genetic variation while mitosis does not.
A diploid cell contains three pairs of homologous chromosomes designated $\mathrm{Cl}$ and $\mathrm{C} 2, \mathrm{M} 1$ and $\mathrm{M} 2$, and $\mathrm{S} 1$ and $\mathrm{S} 2$. No crossing over occurs. What combinations of chromosomes are possible in (a) daughter cells following mitosis? (b) cells undergoing the first meiotic metaphase? (c) haploid cells following both divisions of meiosis?
Considering the preceding problem, predict the number of different haploid cells that could be produced by meiosis if a fourth chromosome pair (W1 and W2) were added.
During oogenesis in an animal species with a haploid number of 6, one dyad undergoes nondisjunction during meiosis II. Following the second meiotic division, this dyad ends up intact in the ovum. How many chromosomes are present in (a) the mature ovum and (b) the second polar body? (c) Following fertilization by a normal sperm, what chromosome condition is created?
What is the probability that, in an organism with a haploid number of 10 , a sperm will be formed that contains all 10 chromosomes whose centromeres were derived from maternal homologs?
During the first meiotic prophase, (a) when does crossing over occur; (b) when does synapsis occur; (c) during which stage are the chromosomes least condensed; and (d) when are chiasmata first visible?
Describe the role of meiosis in the life cycle of a vascular plant.
Contrast the chromatin fiber with the mitotic chromosome. How are the two structures related?
Describe the "folded-fiber" model of the mitotic chromosome.
You are given a metaphase chromosome preparation (a slide) from an unknown organism that contains 12 chromosomes. Two that are clearly smaller than the rest appear identical in length and centromere placement. Describe all that you can about these chromosomes.
In this chapter, we focused on how chromosomes are distributed during cell division, both in dividing somatic cells (mitosis) and in gamete- and spore-forming cells (meiosis). At the same time, we found many opportunities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter, what answers would you propose to the following fundamental questions:(a) How do we know that chromosomes exist in homologous pairs?(b) How do we know that DNA replication occurs during interphase, not early in mitosis?(c) How do we know that mitotic chromosomes are derived from chromatin?
In mitosis, what chromatid combination(s) will be present during metaphase? What combination(s) will be present at each pole at the completion of anaphase?
During meiosis I, assuming no crossing over, what chromatid combination(s) will be present at the completion of prophase? Draw all possible alignments of chromatids as migration begins during early anaphase.
Are there any possible combinations present during prophase of meiosis II other than those that you drew in Problem 28? If so, draw them.
Draw all possible combinations of chromatids during the early phases of anaphase in meiosis II.
Assume that during meiosis I none of the C chromosomes disjoin at metaphase, but they separate into dyads (instead of monads) during meiosis II. How would this change the alignments that you constructed during the anaphase stages in meiosis I and II? Draw them.
Assume that each gamete resulting from Problem 31 fuses, in fertilization, with a normal haploid gamete. What combinations will result? What percentage of zygotes will be diploid, containing one paternal and one maternal member of each chromosome pair?
A species of cereal rye (Secale cereale) has a chromosome number of 14 , while a species of Canadian wild rye (Elymus canadensis) has a chromosome number of 28 . Sterile hybrids can be produced by crossing Secale with Elymus.(a) What would be the expected chromosome number in the somatic cells of the hybrids?(b) Assume that the GI nuclear DNA content of Elymus is 25.5 picograms and that the G1 nuclear DNA content of Secale is 16.8 picograms. What would be the expected DNA content in a metaphase somatic cell of the hybrid?(c) Given that none of the chromosomes pair at meiosis I in the sterile hybrid (Hang and Franckowlak, 1984), speculate on the anaphase I separation patterns of these chromosomes.
An interesting procedure has been applied for assessing the chromosomal balance of potential secondary oocytes for use in human in vitro fertilization. Using fluorescence in situ hybridization (FISH), Kuliev and Verlinsky (2004) were able to identify individual chromosomes in first polar bodies and thereby infer the chromosomal makeup of "sister" oocytes.(a) Assume that when examining a first polar body you saw that it had one copy (dyad) of each chromosome but two dyads of chromosome 21. What would you expect to be the chromosomal $21 \mathrm{com}$ plement in the secondary oocyte? What consequences are likely in the resulting zygote, if the secondary oocyte was fertilized?(b) Assume that you were examining a first polar body and noted that it had one copy (dyad) of each chromosome except chromosome 21. Chromosome 21 was completely absent. What would you expect to be the chromosome 21 complement (only with respect to chromosome 21) in the secondary oocyte? What consequences are likely in the resulting zygote if the secondary oocyte was fertilized?(c) The authors state that there was a relatively high number of separation errors at meiosis I. In these cases the centromere underwent a premature division, occurring at meiosis I rather than meiosis II. Regarding chromosome 21 , what would you expect to be the chromosome 21 complement in the secondary oocyte in which you saw a single chromatid (monad) for chromosome 21 in the first polar body? If this secondary oocyte was involved in fertilization, what would be the expected consequences?