Mastering Biology Chapter 13

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

16 is half of 32.

Normal human gametes carry _____ chromosomes.

23

Recombinant Chromosomes

A chromosome created when crossing over combines the DNA from two parents into a single chromosome. Each of the chromosomes in gametes B and C are composed of material derived from both parents.

Which of these cells is (are) haploid?

After Meiosis I and Meiosis II (C &D)

Homologous chromosomes migrate to opposite poles during _____.

Anaphase I

During _____ sister chromatids separate.

Anaphase II

metaphase I

At metaphase I, the chromosome tetrads are aligned on the metaphase plate. For each tetrad, kinetochore microtubules from one pole of the cell are attached to one homologous chromosome, while kinetochore microtubules from the other pole of the cell are attached to the other chromosome of the pair. Thus, the homologous chromosomes are poised to move to opposite poles of the cell. During metaphase I tetrads align along the metaphase plate.

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.

anaphase I

During anaphase I, each pair of chromosomes is pulled apart and the homologous chromosomes move toward opposite poles. Note that sister chromatids remain attached at their centromeres and move as a unit toward the same pole. During anaphase I homologous chromosomes, consisting of sister chromatids, migrate to opposite poles.

Anaphase II

During anaphase II sister chromatids separate and migrate to opposite poles.

metaphase II

During metaphase II, the chromosomes align on the metaphase plate. Metaphase II is essentially the same as mitotic metaphase except that the cells are haploid.

telophase II and cytokinesis

In telophase II, nuclei form at opposite poles of each dividing cell, and cytokinesis splits the cells apart. Meiosis has produced four haploid cells, each with one set of chromosomes. Four haploid cells are present at the end of telophase II and cytokinesis.

interphase

Like mitosis, meiosis is preceded by an interphase, during which the chromosomes replicate. The centrosome also duplicates in preparation for cell division. During interphase the centrosome duplicates and the chromosomes are not condensed.

Human gametes are produced by _____.

Meiosis produces haploid gametes from a diploid parental cell.

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

Metaphase II

Crossing Over

Pairing of homologous chromosomes is followed by crossing over, the exchange of genetic material between nonsister chromatids of homologous chromosomes.

Synapsis occurs during _____.

Prophase I

prophase I

Prophase I begins with condensation of the chromosomes. Homologous chromosomes, each made up of two sister chromatids, come together in pairs. This pairing is called synapsis. Each chromosome pair is called a tetrad, a complex of four chromatids. Chromatids of homologous chromosomes cross over each other and exchange parts at chiasmata (singular, chiasma). Meanwhile, other cellular components prepare for the division of the nucleus. The centrosomes move away from each other, and spindle microtubules form between them. The nuclear envelope and nucleoli disperse. Finally, spindle microtubules capture the kinetochores that form on the chromosomes, and the chromosomes begin moving to the metaphase plate. Homologous chromosomes pair during prophase I..

During _____ a spindle forms in a haploid cell.

Prophase II

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

Telophase I

telophase I and cytokinesis

The chromosomes finish their journey during telophase I, and cytokinesis occurs, producing two haploid daughter cells. Note that each chromosome still consists of two sister chromatids. Meiosis isn't over yet; remember that it consists of two consecutive divisions. During meiosis II, the sister chromatids will be separated. At the end of telophase I and cytokinesis there are two haploid cells.

Prophase II

The first phase of meiosis II. Prophase II is identical to mitotic prophase, except that the number of chromosomes was reduced by half during meiosis I. The events of prophase II are essentially the same as those of mitotic prophase except that prophase II cells are haploid.

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

four, haploid

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

telophase II, telophase II

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

two, haploid