7.01 A species of wasp has three pairs of chromosomes (2n = 6). Chromosome 1 is the longest and is acrocentric, chromosome 2 is somewhat shorter and metcentric, and chromosome 3 is the shortest and is telocentric. Draw a cell of this species as it would appear at metaphase of mitosis. Use one line for each chromatid, and indicate the number (1, 2, or 3) of each chromosome.

Answer:

7.02 The cells shown in the diagram are in various stages of mitosis or meiosis. All the cells come the same individual (a mammal).

• (a) What is the diploid number of chromosomes in this animal? Answer: 6

• (b) How did you arrive at your answer to part (a)?

Answer: There are a total of 6 chromosomes in the left and middle cells, and 3 in the right cell, so 6 must be 2n and 3 is n.

• (c) Give the names of each stage shown.

Answer: Left cell: anaphase I of meiosis, middle cell: anaphase of mitosis, right cell: anaphase II of meiosis.

7.03 In a particular plant species, 2n = 12. For this organism, How many chromosomes, and how many DNA molecules will be present per cell for each of the following?

• (a) Leaf cell in G₁ Answer: 12, 12

• (b) Leaf cell in G₂ Answer: 12, 24

• (c) Root meristem cell in metaphase of mitosis. Answer: 12, 24

• (d) Root meristem cell in anaphase of mitosis. Answer: 24, 24

• (e) Root meristem cell in telophase of mitosis (cytokinesis complete) Answer: 12, 12

• (f) Microspore mother cell (in anther) in prophase I of meiosis. Answer: 12, 24

• (g) Microspore mother cell in metaphase II of meiosis. Answer: 6, 12

• (h) Pollen grain (after meiosis and cytokinesis complete) Answer: 6, 6

7.04 The haploid chromosome number in cats is 19. Sex is determined by the XY system as in humans. Answer the following:

• (a) How many chromosomes are present in a zygote? Answer: 38

• (b) How many sex chromosomes are present in a sperm cell? Answer: 1

• (c) How many autosomes are present in a liver cell? Answer: 36

• (d) How many X chromosomes are present in an egg cell? Answer: 1

• (e) How many Y chromosomes are present in an egg cell? Answer: 0

• (f) During meiosis in the female cat, how many bivalents would be present in the primary oocyte? Answer: 19

7.05 A certain species of animal has 18 chromosomes in its diploid cells (2n=18). Describe what the metaphase chromosome arrangement would look like for this species in each of the following stages: mitosis, meiosis I, and meiosis II.

Answer: Mitosis: There would be 18 individual chromosomes, each with two chromatids, lined up at the metaphase plate. Meiosis I: There would be 9 bivalents lined up, each consisting of 2 paired homologues, each homologue with two chromatids. Meiosis II: There would be 9 individual chromosomes lined up, each with two chromatids (like mitosis, but cells now haploid).

7.06 Explain why the number of chromosomes per cell (N level) remains constant through the cell cycle, but and the amount of DNA per cell (C level) does not. Think how these values change or do not change during G₁, S, G₂, mitosis, and cytokinesis.

Answer: Before replication, each chromosome has only one strand of DNA, but after replication each chromosome has 2 identical strands (chromatids). The amount of DNA has doubled, but the number of chromosomes is the same. When mitosis takes place, the amount of DNA per nucleus is one-half of what it was before mitosis because the chromatids have separated and are now in different nuclei. But the number of chromosomes per nucleus is the same as before mitosis. Each chromosome is now a single strand.

7.07 Consider an individual that is heterozygous for albinism, (Aa).

• (a) What gamete genotypes would you expect this individual to produce, and in what proportions?

Answer: Gametes could be A or a and should be present in equal numbers

• (b) Diagram how the chromosomes behave during meiosis to explain your answer to a.

Answer:

• (c) If this individual is heterozygous for a second gene (Bb) whose locus is on a different chromosome, what gamete genotypes would you expect, and in what proportions?

Answer: Gametes could be AB, Ab, aB, or ab, and should be present in equal numbers

• (d) As you did in part b, illustrate the meiotic chromosome behavior to explain your answer to c. Be sure you understand how this accounts for the principle of independent assortment.

Answer:

OR

7.08 A certain individual has the genotype MmTt. These two genes are located on different chromosomes. This individual can produce four possible types of gametes, but each time a single mother cell undergoes meiosis, only two of the four possible types are produced. Explain why a single meiosis can produce two and only two combinations.

Answer: Examine the diagrams in the previous question. Although there are two ways in which the bivalents may align at metaphase I, only one can happen during a single meiosis. After meiosis I, the two daughter cells each have one of the four possible allele combinations. Meiosis II doubles the number of cells but does not produce new combinations. (Actually, it is possible for one meiosis to produce all four allele combinations, but only when crossing over occurs. We will investigate this in the chapter on genetic linkage.)

7.09 Many lower organisms, like algae and fungi, have life cycles based on zygotic meiosis rather than gametic meiosis, which is common in higher animals.

• (a) How is the sexual process of algae and fungi similar to that of animals?

Answer: Sexual reproduction is similar to that of other organisms in that there is an alternation between haploid and diploid phases. Like animals and plants, algae and fungi have life cycles where at some point meiosis occurs to reduce the chromosome number from diploid to haploid, and at some other point, fertilization occurs producing a diploid zygote from the fusion of two haploid gametes.

• (b) How is it different?

Answer: What’s different about zygotic meiosis is that the zygote is the only diploid cell in the life cycle. The first division of the zygote is meiosis. The haploid cells produced then lead to an organism that is completely haploid. The is just backwards from the animal cycle where the gametes are the only haploid cells and everything else is diploid.

7.10 Ferns, like all vascular plants, have life cycles based on sporic meiosis. For each of the plant parts listed, tell whether cells of that part would be haploid or diploid: leaf, root, egg, antheridium, sporangium, archegonium, sperm, spore.

Answer: Leaves, roots, and sporangia are diploid (parts of the sporophyte). The antheridium and archegonium are haploid (parts of the gametophyte). Spores are haploid. They are the direct products of meiosis. Eggs and sperm are haploid as gametes always are.

7.11 Female honey bees are diploid; male bees are haploid. Male bees produce sperm and can successfully mate with females (queens), eggs can also develop without fertilization (parthenogenesis). Fertilized eggs develop into females, while unfertilized eggs develop into males. How do you think the process of sperm production in male bees differs from sperm production in other insects such as Drosophila?

Answer: Because male bees are haploid, meiosis would not be required to produce sperm cells. No reduction of chromosome number would be needed. Therefore, sperm cells would be produced by mitosis rather than meiosis. In this way, both sperm and egg would have the correct haploid number of chromosomes. Meiosis would occur as usual in the female to produce the haploid egg from diploid oocytes.