Biology 111G Chapter 10 concept checks and end of chapter review

A human cell containing 22 autosomes and a Y chromosome is A-a sperm. B-an egg. C-a zygote. D-a somatic cell of a male.

A-a sperm.

Compare the life cycles of animals and plants, mentioning their similarities and differences.

Animals and plants both reproduce sexually, alternating meiosis with fertilization. Both have haploid gametes that unite to form a diploid zygote, which then goes on to divide mitotically, forming a diploid multicellular organism. In animals, haploid cells become gametes and don't undergo mitosis, while in plants, the haploid cells resulting from meiosis undergo mitosis to form a haploid multicellular organism, the gametophyte. This organism then goes on to generate haploid gametes. (In plants such as trees, the gametophyte is quite reduced in size and not obvious to the casual observer.)

In prophase I, homologous chromosomes pair up and undergo crossing over. Can this also occur during prophase II? Explain.

At the end of meiosis I, the two members of a homologous pair end up in different cells, so they cannot pair up and undergo crossing over.

Homologous chromosomes move toward opposite poles of a dividing cell during A-mitosis. B-meiosis I. C-meiosis II. D-fertilization.

B-meiosis I.

If we continued to follow the cell lineage from question 3, the DNA content of a cell at metaphase of meiosis II would be A-0.25x. B-0.5x. C-x. D-2x.

C-x

If the DNA content of a diploid cell in the G1 phase of the cell cycle is represented by x, then the DNA content of the same cell at metaphase of meiosis I would be A-0.25x. B-0.5x. C-x. D-2x.

D-2x

Concept 10.1: Offspring acquire genes from parents by inheriting chromosomes

Each gene in an organism's DNA exists at a specific locus on a certain chromosome. In asexual reproduction, a single parent produces genetically identical offspring by mitosis. Sexual reproduction combines genes from two parents, leading to genetically diverse offspring.

In Figure 10.4, how many DNA molecules (double helices) are present (see Figure 9.5)? What is the haploid number of this cell? Is a set of chromosomes haploid or diploid?

Each of the six chromosomes is duplicated, so each contains two DNA double helices. Therefore, there are 12 DNA molecules in the cell. The haploid number, n, is 3. One set is always haploid.

Explain how three processes unique to sexual reproduction generate a great deal of genetic variation.

First, during independent assortment in metaphase I, each pair of homologous chromosomes lines up independent of every other pair at the metaphase plate, so a daughter cell of meiosis I randomly inherits either a maternal or paternal chromosome. Second, due to crossing over, each chromosome is not exclusively maternal or paternal, but includes regions at the ends of the chromatid from a nonsister chromatid (a chromatid of the other homolog). (The nonsister segment can also be in an internal region of the chromatid if a second crossover occurs beyond the first one before the end of the chromatid.) This provides much additional diversity in the form of new combinations of alleles. Third, random fertilization ensures even more variation, since any sperm of a large number containing many possible genetic combinations can fertilize any egg of a similarly large number of possible combinations.

Explain why human offspring resemble their parents but are not identical to them.

Genes program specific traits, and offspring inherit their genes from each parent, accounting for similarities in their appearance to one or the other parent. Humans reproduce sexually, which ensures new combinations of genes (and thus traits) in the offspring. Consequently, the offspring are not clones of their parents (which would be the case if humans reproduced asexually).

WHAT IF? After the synaptonemal complex disappears, how would any pair of homologous chromosomes be associated if crossing over did not occur? What effect might this have on gamete formation?

If crossing over did not occur, the two homologs would not be associated in any way; each sister chromatid would be either all maternal or all paternal, and would only be attached to its sister, not to a non-sister chromatid. This might result in incorrect arrangement of homologs during metaphase I and ultimately in formation of gametes with an abnormal number of chromosomes.

If maternal and paternal chromatids have the identical two alleles for every gene, will crossing over lead to genetic variation?

If the segments of the maternal and paternal chromatids that undergo crossing over are genetically identical and thus have the same two alleles for every gene, then the recombinant chromosomes will be genetically equivalent to the parental chromosomes. Crossing over contributes to genetic variation only when it involves the rearrangement of different alleles.

Concept 10.3: Meiosis reduces the number of chromosome sets from diploid to haploid

Meiosis I and meiosis II produce four haploid daughter cells. The number of chromosome sets is reduced from two (diploid) to one (haploid) during meiosis I. Meiosis is distinguished from mitosis by three events of meiosis I:(SEE ATTACHED PICTURE) Meiosis II separates the sister chromatids. Sister chromatid cohesion and crossing over allow chiasmata to hold homologs together until anaphase I. Cohesions are cleaved along the arms at anaphase I, allowing homologs to separate, and at the centromeres in anaphase II, releasing sister chromatids.

What is the source of variation among alleles of a gene?

Mutations in a gene lead to the different versions (alleles) of that gene.

Concept 10.2: Fertilization and meiosis alternate in sexual life cycles

Normal human somatic cells are diploid. They have 46 chromosomes made up of two sets of 23 chromosomes, one set from each parent. Human diploid cells have 22 homologous pairs of autosomes and one pair of sex chromosomes; the latter determines whether the person is female (XX) or male (XY). In humans, ovaries and testes produce haploid gametes by meiosis, each gamete containing a single set of 23 chromosomes (n = 23). During fertilization, an egg and sperm unite, forming a diploid (2n = 46) single-celled zygote, which develops into a multicellular organism by mitosis. Sexual life cycles differ in the timing of meiosis relative to fertilization and in the point(s) of the cycle at which a multicellular organism is produced by mitosis.

Using what you know of gene expression in a cell, explain what causes traits of parents (such as hair color) to show up in their offspring.

Parents pass genes to their offspring; by dictating the production of messenger RNAs (mRNAs), the genes program cells to make specific enzymes and other proteins, whose cumulative action produces an individual's inherited traits.

A horticulturalist breeds orchids, trying to obtain a plant with a unique combination of desirable traits. After many years, she finally succeeds. To produce more plants like this one, should she crossbreed it with another plant or clone it? Why?

She should clone it. Crossbreeding it with another plant would generate offspring that have additional variation, which she no longer desires now that she has obtained her ideal orchid.

How does an asexually reproducing eukaryotic organism produce offspring that are genetically identical to each other and to their parent?

Such organisms reproduce by mitosis, which generates offspring whose genomes are exact copies of the parent's genome (in the absence of mutations).

Compare the chromosomes in a cell at metaphase of mitosis with those in a cell at metaphase of meiosis II.

The chromosomes are similar in that each is composed of two sister chromatids, and the individual chromosomes are positioned similarly at the metaphase plate. The chromosomes differ in that in a mitotically dividing cell, sister chromatids of each chromosome are genetically identical, but in a meiotically dividing cell, sister chromatids are genetically distinct because of crossing over in meiosis I. Moreover, the chromosomes in metaphase of mitosis can be a diploid set or a haploid set, but the chromosomes in metaphase of meiosis II always consist of a haploid set.

Each sperm of a pea plant contains seven chromosomes. What are the haploid and diploid numbers for this species?

The haploid number (n) is 7; the diploid number (2n) is 14. 4. This organism has the life cycle shown in Figure 10.6c. Therefore, it must be a fungus or a protist, perhaps an alga.

In the karyotype shown in Figure 10.3, how many pairs of chromosomes are present? How many sets?

There are 23 pairs of chromosomes and two sets.

Explain how you can tell that the cell in question 5 is undergoing meiosis, not mitosis.

This cell must be undergoing meiosis because homologous chromosomes are associated with each other at the metaphase plate; this does not occur in mitosis.

WHAT IF? A certain eukaryote lives as a unicellular organism, but during environmental stress, it produces gametes. The gametes fuse, and the resulting zygote undergoes meiosis, generating new single cells. What type of organism could this be?

This organism has the life cycle shown in Figure 10.6c. Therefore, it must be a fungus or a protist, perhaps an alga.

Concept 10.4: Genetic variation produced in sexual life cycles contributes to evolution

Three events in sexual reproduction contribute to genetic variation in a population: independent assortment of chromosomes during meiosis, crossing over during meiosis I, and random fertilization of egg cells by sperm. During crossing over, DNA of nonsister chromatids in a homologous pair is broken and rejoined. Genetic variation is the raw material for evolution by natural selection. Mutations are the original source of this variation; recombination of variant genes generates additional diversity.

The diagram shows a cell in meiosis. (a) Label the appropriate structures with these terms: chromosome (label as duplicated or unduplicated), centromere, kinetochore, sister chromatids, nonsister chromatids, homologous pair (use a bracket when labeling), homolog (label each one), chiasma, sister chromatid cohesion, gene loci, alleles of the F gene, alleles of the H gene. (b) Identify the stage of meiosis shown. (c) Describe the makeup of a haploid set and a diploid set.

b) Metaphase I (c) A haploid set is made up of one long, one medium, and one short chromosome, no matter what combination of colors. For example, one red long, one blue medium, and one red short chromosome make up a haploid set. (In cases where crossovers have occurred, a haploid set of one color may include segments of chromatids of the other color.) All red and blue chromosomes together make up a diploid set.