Meiosis

For crossing over to occur, homologous chromosomes must align precisely early in prophase I so that nonsister chromatids can exchange corresponding segments of DNA.

True

Dogs have 78 chromosomes in their diploid cells. If a diploid dog cell enters meiosis, how many chromosomes and double-helical molecules of DNA will be present in each daughter cell at the end of meiosis II?

39 chromosomes and 39 double-helical DNA molecules

Which statement is correct concerning the relationship between chromosomes and chromatids?

A replicated chromosome contains two sister chromatids. A replicated chromosome contains two identical DNA double helices that correspond to each sister chromatid.

How would genetic variation be affected if, during meiosis I, maternal chromosomes always lined up together on one side of the metaphase plate and paternal chromosomes always lined up on the other side (ignoring the effects of crossing over)?

Gametes would contain either maternal chromosomes only or paternal chromosomes only. All maternal chromosomes would go to one daughter cell and all paternal chromosomes to the other, so different combinations would not be produced in gametes.

Meiotic nondisjunction could be a result of __________.

Incorrect segregation of homologous chromosomes during meiosis I. If one set of homologous chromosomes does not segregate properly, the result will be one cell with two sets of homologous chromosomes, and one cell with none.

For the male Drosophila shown in Figure 13.1, what are the (1) haploid number, (2) ploidy, and (3) total number of chromosomes?

(1) 4; (2) 2n; (3) 8 This individual is diploid (2n) with two versions of each of the four types of chromosome.

Which of the following statements about the meiosis are correct?

-Non-sister chromatids are found on two different homologs. -A chiasma forms between two of the four molecules of double-stranded DNA on duplicated homologous chromosomes.

Normal human gametes carry _____ chromosomes.

23. This is the number of chromosomes in a single set of human chromosomes.

How many genetically unique gametes can be produced by independent assortment from a 2n=6 cell?

8 Based on the different arrangements of homologous pairs during meiosis I, 8 unique gametes can be produced.

Which statement about the difference between homologous chromosomes and sister chromatids is correct?

Homologous chromosomes contain the same genes but often different alleles. Sister chromatids contain the same genes and the same alleles. Sister chromatids are produced by the process of DNA replication, whereas homologous chromosomes are inherited from the mother and father.

As a result of crossing over, sister chromatids are no longer identical to each other

True

Sort each process into the appropriate bin according to whether it contributes to heredity only, genetic variation only, or both.

heredity only -none genetic variation only -none both -DNA Replication -Fertilization -Crossing Over -Metaphase I -> Anaphase I -Metaphase II -> Anaphase II

Human gametes are produced by _____.

meiosis. Meiosis produces haploid gametes from a diploid parental cell.

Assume that an organism exists in which crossing over does not occur, but that all other processes associated with meiosis occur normally. Consider how the absence of crossing over would affect the outcome of meiosis. If crossing over did not occur, which of the following statements about meiosis would be true? Select all that apply.

There would be less genetic variation among gametes. Crossing over contributes significantly to the genetic variation seen in gametes. This is because the exchange of maternal and paternal genes between the nonsister chromatids of a homologous chromosome pair creates recombinant chromosomes with unique combinations of alleles. However, crossing over is not the only process that introduces genetic variation in meiosis I. The independent assortment of homologous chromosomes (which are never identical) in meiosis I produces daughter cells that differ from each other. The effect of crossing over on genetic variation is shown below. Without crossing over, sister chromatids remain identical and thus, pairs of daughter cells would be identical. With crossing over, however, all four daughter cells are genetically unique.

A unique feature of meiosis I is the attachment of both kinetochores of a duplicated chromosome to spindle fibers that come from a single spindle pole. Why is this unusual attachment important?

This attachment is necessary to pull each homologous pair of chromosomes apart, moving one homolog to one pole and the other homolog to the other pole.

In the life cycle of an organism, meiosis is paired with the process of fertilization. Understanding the life cycle of an organism is the key to understanding how sexual reproduction ensures the inheritance of traits from both parents and also introduces genetic variation. Complete the diagram to show the life cycle of a typical animal.

a) 2n b) 2n c) meiosis d) meiosis e) n f) egg g) sperm h) n i) fertilization j) 2n k) zygote l) mitosis m) 2n n) meiosis o) n p) egg

This diagram shows a diploid nucleus (2n=8) in which chromosome replication has occurred in preparation for mitosis (top) and meiosis (bottom). The nucleus at top right is now in prophase of mitosis; the nucleus at bottom right is now in prophase I of meiosis.

a) non-homologous chromosomes b) sister chromatids c) homologous chromosomes d) centromere e) nonsister chromatids f) homologous chromosomes g) sister chromatids

In Figure 13.14, every asexual individual or sexually reproducing couple produces 4 offspring over a lifetime, sexually reproducing individuals produce half males and half females, and all offspring survive to breed. How many (1) asexually and (2) sexually produced offspring would be present in generation four?

(1) 64; (2) 16

How many double-helical DNA molecules will be present in the sperm and egg cells of dogs that are diploid animals that contain 78 replicated chromosomes in each cell that enters meiosis?

39 There would 78 replicated chromosomes at the start of meiosis I, each with two molecules of DNA. The number of replicated chromosomes is reduced to 39 at the end of meiosis I. During meiosis II, the sister chromatids of the replicated chromosomes are separated to become individual daughter chromosomes, each with one molecule of DNA. This means that each of the daughter cells at the end of meiosis contains 39 unreplicated chromosomes and 39 molecules of double-stranded DNA.

Sort each daughter cell into the appropriate bin depending on which arrangement at metaphase I would create it.

ARRANGEMENT 1: 3 & 8 ARRANGEMENT 2: 1 & 5 ARRANGEMENT 3: 6 & 2 ARRANGEMENT 4: 4 & 7

Which statement best explains why meiosis will be problematic in a triploid (3n) organism?

Chromosomes will not align properly in meiosis I. Since there are three chromosome sets, some chromosomes will not form bivalents in meiosis I.

The parent cell that enters meiosis is diploid, whereas the four daughter cells that result are haploid. Which statement correctly describes how cellular DNA content and ploidy levels change during meiosis I and meiosis II?

DNA content is halved in both meiosis I and meiosis II. Ploidy level changes from diploid to haploid in meiosis I, and remains haploid in meiosis II.

Crossing over prevents homologous chromosomes from separating during meiosis I

False

Predict how many double helical molecules of DNA will be present in the gametes of the fruit fly, Drosophila, a diploid organism that has eight replicated chromosomes in each cell that enters meiosis.

Four double-helical molecules of DNA, which correspond to four unreplicated chromosomes Meiosis will reduce the chromosome number by half.

Can you recognize the eight stages of meiosis based on the location and behavior of the chromosomes?

Meiosis involves two sequential cellular divisions. In meiosis I, homologous chromosomes pair and then separate. Thus, although the parent cell is diploid (containing two chromosome sets, one maternal and one paternal), each of the two daughter cells is haploid (containing only a single chromosome set). In meiosis II, the sister chromatids separate. The four daughter cells that result are haploid.

Which event makes meiosis a reductional division and why?

Separation of homologs in meiosis I, because it produces 2 haploid (n) daughter cells from a single diploid (2n) parent cell Separation of homologs results in a reduction in chromosome number.

Which statement best describes unreplicated chromosomes in a 2n cell where n=3 and in a 3n=6 cell.

The 2n and 3n cells will both have 6 chromosomes; however, the 2n cell will have 3 different types of chromosomes and the 3n cell will have 2 different types of chromosomes. When we write ploidy number the "n" refers to the number of different types of chromosomes in a set and the number that precedes "n" indicates the number of sets present.

For the experiment shown in Figure 13.15, what would the results be if a non-evolving pathogen were used instead of an evolving pathogen?

The outcrossing rate would initially increase significantly then return to around 0.2 over subsequent generations. Exposure to either an evolving or non-evolving pathogen will create an initial advantage for roundworms that produce genetically varied offspring by outcrossing. But, over subsequent generations, this advantage will persist only with an evolving (changing) pathogen.

Crossing over occurs at the ends of chromosomes, rather than near the centromeres.

True

According to Figure 13.13, the chances that a woman who is age 45 will have a child with Down syndrome is somewhere around ______________.

1 in 30 There is a strong correlation between trisomy-21 and maternal age.

What proportion of chromosomes in a man's skin cell are maternal chromosomes?

1/2

Meiosis results in independent assortment of maternal and paternal chromosomes. If n=3 for a given organism and there is no crossing over, what is the chance that a gamete will receive only paternal chromosomes?

1/8

Meiosis results in independent assortment of maternal and paternal chromosomes. If 2n = 6 for a given organism and there is no crossing over, what is the chance that a gamete produced by this diploid organism will receive only paternal chromosomes?

1/8 Because n = 3, independent assortment creates 2 n = 23 = 8 different combinations of chromosomes. One of these combinations is made up of paternal chromosomes only. Therefore, a haploid cell has a 1/8 chance of receiving only paternal chromosomes.

Some plants are triploid. If a plant has a haploid chromosome number of 42, how many chromosomes are present in its somatic cells?

126 Three copies of each chromosome are in the somatic cells: 3 × 42 = 126.

A diploid organism whose somatic (nonsex) cells each contain 32 chromosomes produces gametes containing _____ chromosomes.

16 Gametes have fewer chromosomes than do somatic cells.

Predict how many genetically different offspring could be produced by outcrossing two 2n=4 individuals (just account for independent assortment and not crossing over).

16 genetically unique offspring Each parent has the potential to produce 4 genetically unique gametes via independent assortment. When you multiple this potential for each parent, 4 x 4, you get the potential to produce 16 genetically unique offspring.