Chapter 13 Critical Thinking

A species has 2n = 8. How many homologous pairs does it have?

4 homologous pairs.

A species has 2n = 8. How many chromosomes are in one gamete?

4 chromosomes.

A 2n = 8 cell has completed DNA replication. Chromosomes?

8 chromosomes.

A 2n = 8 cell has completed DNA replication. Chromatids?

16 chromatids.

A 2n = 8 cell in metaphase I contains how many tetrads?

4 tetrads.

A 2n = 8 cell in metaphase I contains how many chromosomes?

8 chromosomes.

A 2n = 8 cell in metaphase I contains how many chromatids?

16 chromatids.

A 2n = 8 cell after meiosis I has how many chromosomes per cell?

4 duplicated chromosomes.

A 2n = 8 cell after meiosis I has how many chromatids per cell?

8 chromatids.

A 2n = 8 cell after meiosis II has how many chromosomes per gamete?

4 unduplicated chromosomes.

A 2n = 8 cell after meiosis II has how many chromatids per gamete?

4 chromatids.

A cell has six X-shaped chromosomes. How many chromosomes are present?

6 chromosomes.

A cell has six X-shaped chromosomes. How many chromatids are present?

12 chromatids.

A student says an X-shaped chromosome counts as two chromosomes. Why is this wrong?

It is one chromosome until sister chromatids separate.

A duplicated chromosome becomes two chromosomes when what happens?

Sister chromatids separate at the centromere.

A cell contains 46 chromosomes and 92 chromatids. What happened earlier?

DNA replication occurred.

A human cell has 23 chromosomes and 46 chromatids. What is its likely state?

A haploid cell with duplicated chromosomes.

A cell contains 23 chromosomes with one chromosome from each homologous pair. What is its ploidy?

Haploid.

A cell contains 23 duplicated chromosomes after meiosis I. Why is it still haploid?

It has one homolog from each pair.

A cell contains two copies of each homolog but each chromosome is unreplicated. What is its ploidy?

Diploid.

Homologs separate while sister chromatids stay joined. What phase is this?

Anaphase I.

Sister chromatids separate and move to opposite poles in a haploid cell. What phase is this?

Anaphase II.

Homologous pairs line up side by side at the equator. What phase is this?

Metaphase I.

Individual duplicated chromosomes line up at the equator in a haploid cell. What phase is this?

Metaphase II.

Individual duplicated chromosomes line up at the equator in a diploid cell. What phase could this be?

Mitotic metaphase.

Two cells are haploid but each chromosome is still X-shaped. What stage just ended?

Meiosis I.

Four cells are haploid and each chromosome is a single chromatid. What stage just ended?

Meiosis II.

Why does chromosome number decrease during meiosis I rather than meiosis II?

Homologs separate during meiosis I.

Why does meiosis II not reduce ploidy further?

It separates sister chromatids, not homologs.

Why is DNA not replicated between meiosis I and meiosis II?

Replication would prevent the final cells from staying haploid.

A student says meiosis has two DNA replications because it has two divisions. Correct?

No; DNA replicates once before meiosis I.

A student says meiosis II is identical to mitosis. Best correction?

Both separate sisters, but meiosis II begins with haploid cells.

A student says mitosis can only occur in diploid cells. Correct?

No; mitosis can occur in haploid or diploid cells.

A student says meiosis can occur in haploid cells. Correct?

No; meiosis begins with diploid cells.

A cell cannot form tetrads. Which event is most directly disrupted?

Synapsis of homologous chromosomes.

A defective synaptonemal complex most directly affects what?

Tight pairing of homologous chromosomes.

A defective synaptonemal complex would likely reduce what source of variation?

Crossing over.

What happens if crossing over occurs between sister chromatids only?

It produces little or no new allele combination.

Why must crossing over occur between nonsister chromatids?

They may carry different alleles.

Crossing over occurs in what phase?

Prophase I.

Why would crossing over in prophase II be ineffective?

Homologs have already separated.

What structure visibly marks a crossover site?

A chiasma.

What holds sister chromatids together before they separate?

Cohesins.

Why are chiasmata important in meiosis I?

They help keep homologs associated until separation.

A chromosome has maternal DNA on one section and paternal DNA on another. What is it?

A recombinant chromosome.

Does a recombinant chromosome mean an offspring inherited half a chromosome directly from each parent?

No; recombination occurred earlier between homologs.

A child's chromosome inherited from the mother may contain DNA from whom?

Both of the mother's parents.

A student says chromosomes are inherited as completely maternal or paternal units. What corrects this?

Crossing over creates recombinant chromosomes.

A species has n = 3. Independent assortment alone produces how many gamete types?

8 possible types.

A species has 2n = 12. Independent assortment alone produces how many gamete types?

64 possible types.

A human has 23 homologous pairs. Independent assortment produces how many possible gametes?

8,388,608.

Why is the actual number of possible human gametes greater than 2^23?

Crossing over adds more combinations.

Without crossing over, what produces about 70 trillion potential human zygotes?

Random fertilization combined with independent assortment.

Why does random fertilization increase variation?

Any genetically different sperm can fuse with any genetically different egg.

Independent assortment rearranges what level of genetic material?

Whole maternal and paternal chromosomes.

Crossing over rearranges what level of genetic material?

DNA segments within homologous chromosomes.

Mutation differs from independent assortment because mutation does what?

It can create new alleles.

Independent assortment differs from mutation because it does what?

It reshuffles existing alleles.

A species reproduces sexually but skips meiosis. What happens to chromosome number over generations?

It would tend to double after each fertilization.

A species reproduces sexually but skips fertilization. What happens to chromosome number over generations?

It would keep being halved by meiosis.

Why must fertilization and meiosis alternate?

They maintain a stable chromosome number across generations.

An animal has a multicellular haploid stage. Does this match the usual animal life cycle?

No; animal haploid cells are usually only gametes.

In animals, a haploid cell divides by mitosis before forming the adult. Is this the usual life cycle?

No; the diploid zygote develops into the adult.

A plant gametophyte makes gametes by meiosis. Correct?

No; it makes gametes by mitosis.

Why does a plant gametophyte use mitosis to make gametes?

The gametophyte is already haploid.

A plant sporophyte makes gametes by meiosis. Correct?

No; it makes spores by meiosis.

What does a plant sporophyte produce by meiosis?

Haploid spores.

What develops from a plant spore by mitosis?

A haploid gametophyte.

What develops after plant gamete fertilization?

A diploid sporophyte.

A fungus has a long-lived multicellular diploid adult. Does this match the typical fungal life cycle described?

No; the zygote is usually the only diploid stage.

In most fungi, what happens soon after the diploid zygote forms?

It undergoes meiosis.

A dog muscle cell has 78 chromosomes. How many chromosomes are in a dog sperm cell?

39 chromosomes.

A dog muscle cell has 78 chromosomes. How many chromosomes are in a dog zygote?

78 chromosomes.

A dog cell has 39 chromosomes. Could it be a normal somatic cell?

No; it is likely haploid.

Privet shrubs and humans can both have 2n = 46. Why are they still very different?

They have appreciably different genes.

Why is chromosome number alone a poor measure of organism complexity?

Different species can have the same chromosome number but different genes.

A karyotype shows chromosome pairs. Why is it usually made from somatic cells?

Homologous pairs can be examined together.

A cell has one X chromosome and 22 autosomes. What kind of human cell is it?

An egg cell or an X-bearing sperm cell.

A cell has one Y chromosome and 22 autosomes. What kind of human cell is it?

A Y-bearing sperm cell.

A cell has XX sex chromosomes and 44 autosomes. What kind of human cell is it?

A diploid female somatic cell.

A cell has XY sex chromosomes and 44 autosomes. What kind of human cell is it?

A diploid male somatic cell.

Why are normal gametes from one meiosis event not necessarily identical?

Crossing over and independent assortment produce different allele combinations.

What feature must all normal gametes from the same meiosis event share?

The same haploid chromosome number.

Why does sexual reproduction usually produce genetically varied offspring?

It combines independent assortment, crossing over, and random fertilization.

Why can asexual reproduction be efficient in stable conditions?

One parent passes on all of its genes without finding a mate.

Why can sexual reproduction be advantageous in changing environments?

Genetic variation increases the chance that some offspring will be suited to new conditions.

A wound needs replacement cells. Why is meiosis unsuitable?

It makes haploid genetically varied cells instead of matching body cells.

A zygote needs to become a multicellular adult. Why is mitosis suitable?

It preserves chromosome number while producing matching body cells.

Egg production needs meiosis rather than mitosis. Why?

Eggs must be haploid so fertilization restores diploidy.

A scientist observes pairing of homologs and chiasmata. What conclusion is strongest?

The cell is in prophase I of meiosis.

A scientist observes homologs moving apart but X-shaped chromosomes remain intact. What conclusion is strongest?

The cell is in anaphase I.

A scientist observes chromatids moving apart as individual chromosomes. What conclusion is strongest?

The cell is in anaphase II or mitotic anaphase.

How can you distinguish anaphase I from anaphase II?

In anaphase I homologs separate; in anaphase II sisters separate.

How can you distinguish metaphase I from mitotic metaphase?

Metaphase I has homologous pairs; mitosis has individual chromosomes.

How can you distinguish meiosis I from meiosis II?

Meiosis I separates homologs; meiosis II separates sister chromatids.