ch 13 mb

asexual reproduction

produces offspring genetically identical to the parent (Only one individual makes a genetic contribution to the offspring.)

During _____ a spindle forms in a haploid cell.

prophase II Prophase II is essentially the same as mitotic prophase except that the cells are haploid.

Which processes lead to most genetic variation in sexually reproducing organisms?

random fertilization,independent orientation of chromosomes in meiosis,crossing over

What is crossing over?

the exchange of homologous portions of nonsister chromatids -The result is new combinations of genetic material (genetic recombination).

Meiosis I produces _____ cells, each of which is _____.

two ... haploid

What number and types of chromosomes are found in a human somatic cell

-44 autosomes and 2 sex chromosomes (Human somatic cells contain 22 pairs of autosomes and either two X chromosomes (in females) or an X and a Y chromosome (in males).)

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

16 is half of 32.

At the end of meiosis I there are ______ haploid cells.

2

in diploid cells mitosis results in the formation of how many cells__________

2 diploid

In diploid cells: mitosis results in the formation of how many cells; meiosis results in the formation of how many cells?

2 dipoid...4 haploid In mitosis a cell that has doubled its genetic material divides to produce two diploid daughter cells. In meiosis a cell that has doubled its genetic material undergoes two rounds of division, producing four haploid cells.

Normal human gametes carry _____ chromosomes.

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

At the end of _____ and cytokinesis there are four haploid cells.

At the end of telophase II and cytokinesis there are four haploid cells.

Which of these cells is (are) haploid?

C and D Once meiosis I is completed, cells are haploid.

drag the labels to fill in the targets beneath each diagram of a cell. Note that the diagrams are in no particular order.

Crossing over occurs during prophase I when homologous chromosomes loosely pair up along their lengths. Crossing over occurs only between nonsister chromatids within a homologous pair of chromosomes, not between the sister chromatids of a replicated chromosome. Only segments near the ends of the chromatids, not segments nearest the centromeres, can exchange DNA.

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. (During anaphase of both meiosis I and meiosis II, the DNA content (number of copies of chromosomes) in a cell is halved. However, the ploidy level changes only when the number of unique chromosome sets in the cell changes. This occurs only in meiosis I (where separation of homologous chromosomes decreases the ploidy level from 2n to n and produces daughter cells with a single chromosome set).

How are sister chromatids and homologous chromosomes different from each other?

Homologous chromosomes contain the same gene loci but may have different alleles of a particular gene. Sister chromatids are identical copies of each other produced during DNA replication. (One homologous chromosome comes from the father, and the other comes from the mother. Sister chromatids are identical copies of each other.)

Meiosis guarantees that in a sexual life cycle, offspring will inherit one

In organisms that reproduce sexually, the processes of DNA replication, the precise pairing of homologs during crossing over, chromosome alignment and separation in meiosis I and II, and fertilization ensure that traits pass from one generation to the next. Unlike with asexual reproduction, offspring of sexual reproduction are genetically different from each other and from both of their parents. Mechanisms that contribute to genetic variation include errors (mutations) that occur during DNA replication the production of recombinant chromosomes due to crossing over the independent assortment of homologous chromosomes in meiosis I the separation of sister chromatids (which are no longer identical due to crossing over) in meiosis II the random fusion of male and female gametes during fertilization

Which of the following phases make up the stages of mitosis?

Prophase, prometaphase, metaphase, anaphase, and telophase

Complete the diagram to show the life cycle of a typical animal.

Meiosis creates gametes (eggs and sperm) with only a single chromosome set (haploid or n) from parental cells with two chromosome sets (diploid or 2n). During fertilization, the haploid sperm (n) and egg (n) fuse, producing a diploid zygote (2n). The cells of the zygote then divide by mitosis (which does not change the ploidy level) to produce an adult organism (still 2n) of the next generation. In sexual life cycles, meiosis and fertilization keep the number of chromosomes constant from generation to generation.

During _____ chromosomes align single file along the equator of a haploid cell.

Metaphase II

Synapsis occurs during _____.

Prophase I (Synapsis, the pairing of homologous chromosomes, occurs during prophase I.)

At the end of _____ and cytokinesis, haploid cells contain chromosomes that each consist of two sister chromatids.

Telophase 1

Look at the cell in the figure. Based on this figure, which of the following statements is true?

The cell is diploid (This cell contains two copies of each chromosome, one from the male parent and the other from the female parent, making it diploid.)

Which of the following occurs during meiosis but not during mitosis?

The pairing of homologous chromosomes that only occurs during prophase I of meiosis is called synapsis. Synapsis

What must happen to a chromosome before a cell starts mitosis?

The single DNA molecule in the chromosome must be replicated. (Before a cell starts mitosis, its chromosomes must be duplicated. This includes replicating the single DNA molecule of a chromosome, resulting in two sister chromatids of a duplicated chromosome that are attached along their lengths. The chromosomes condense during prophase of mitosis.)

Drag the labels to their appropriate targets to correctly identify the various chromosome structures. Labels can be used more than once.

To understand the process of meiosis, it is essential that you can differentiate between sister chromatids, nonsister chromatids, homologous chromosomes, and non-homologous chromosomes.

A human cell containing 22 autosomes and a Y chromosome is

a sperm

Which of the following events characterizes metaphase of mitosis?

alignment of the chromosomes at the equator

Homologous chromosomes migrate to opposite poles during _____.

anaphase 1 (During anaphase I sister chromatids remain attached at their centromeres, and homologous chromosomes move to opposite poles.)

During _____ sister chromatids separate.

anaphase II (Anaphase II is essentially the same as mitotic anaphase except that the cell is haploid.)

Two sister chromatids are joined at the centromere prior to meiosis. Which statement is correct?

barring mutation the two sister chromatids must be identical

Heritable variation is required for which of the following?

evolution Variation is the raw material of evolution.

Gametes are produced by the process of

meiosis

The shuffling of chromosomes that occurs during both fertilization and _____ can lead to genetic variation.

meiosis -Variation is produced as a result of independent assortment and crossing over.

The two homologs of a pair move toward opposite poles of a dividing cell during

meiosis I.

Human gametes are produced by _____.

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

Which life cycle stage is found in plants but not animals?

multicellular haploid

If the DNA content of a diploid cell in the G1 phase of the cell cycle is 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. E) 4x.

2x.

Meiosis II typically produces _____ cells, each of which is _____.

4....... halploid

Meiosis II is similar to mitosis in that A) sister chromatids separate during anaphase. B) DNA replicates before the division. C) the daughter cells are diploid. D) homologous chromosomes synapse. E) the chromosome number is reduced.

A) sister chromatids separate during anaphase.

Which of these gametes contains one or more recombinant chromosomes?

Each of the chromosomes in gametes B and C are composed of material derived from both parents.

Drag the labels from the left to their correct locations in the concept map on the right.

Knowing the terms and relationships shown in this concept map will help you understand the role that meiosis plays in heredity, sexual reproduction, and genetic variability.

Drag the diagrams of the stages of meiosis onto the targets so that the four stages of meiosis I and the four stages of meiosis II are in the proper sequence from left to right. (Note that only one of the two daughter cells is shown for meiosis II.)

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.

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

One aspect of meiosis that generates genetic variation is the random orientation of homologous pairs of chromosomes at metaphase I. Each pair can orient with either its maternal or paternal homolog closer to a given pole; as a result, each pair sorts into daughter cells independently of every other pair. Due to independent assortment alone, a diploid cell with 2n chromosomes can produce 2 n possible combinations of maternal and paternal chromosomes in its daughter cells. For the cell in this problem (n = 3), there are 23, or 8, possible combinations; for humans (n = 23), there are 223, or 8.4 million, possible combinations. Note that when crossing over occurs, the number of possible combinations is even greater.

This chromosome has two chromatids, joined at the centromere. What process led to the formation of the two chromatids?

The two chromatids were formed by the duplication of a chromosome

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.)

What is the best evidence telling you whether this cell is diploid or haploid?

the cell is diploid because it contains two sets of chromosomes (Only diploid cells contain two sets of chromosomes, one maternal and one paternal. Read about diploid cells.)