Biology Chapter 13

Meiosis produces haploid gametes from a diploid parental cell.

23

16 is half of 32.

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.

After Meiosis I and Meiosis II (C &D)

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

two, haploid

four, haploid

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.

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

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.

Anaphase II

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.

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

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.

Prophase I

Anaphase I

Metaphase II

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.

telophase II, telophase II

Prophase II

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.

25 picograms

G1 comes before S phase, when DNA is replicated. As the cell goes into G2, it has 100 pg of DNA, so it had to have 50 pg of DNA in G1. Each of the two daughter cells will have 50 pg of DNA at the end of meiosis I, and each of the four daughter cells will have 25 pg of DNA at the end of meiosis II.

is haploid and has 2 pg of DNA

During meiosis I, the diploid (2n) parent cell produces two haploid (n) daughter cells. Notice, however, that each chromosome still consists of two sister chromatids, meaning that chromosomes are still replicated at the end of meiosis I. Remember that G1 comes before S phase, when DNA is replicated. As the cell goes into G2, it has 4 pg of DNA.

THE CORRECT ANSWER 24, 24, 22, and 22

Remember that n = 23 in humans.

At the end of meiosis II, the former will divide into two sperm cells that have two copies of chromosome 22 (for a total of 24 chromosomes), whereas the latter will result in sperm cells that have zero copies of chromosome 22 (for a total of 22 chromosomes).

Prophase 1

Anaphase 1

6

Homologous Chromosomes -is a set of one maternal chromosome and one paternal chromosome that pair up with each other inside a cell during mitosis.

is haploid and has 1 pg of DNA

In meiosis, one diploid cell with replicated chromosomes gives rise to four haploid cells with unreplicated chromosomes. If the parental cell has 2 pg of DNA in G1, it has 4 pg of DNA in G2. At the end of meiosis II, each gamete has 1 pg of DNA and is haploid.

The homologous chromosomes are paired in meiosis, but not mitosis.

Genetic variation would decrease.

Prophase

Genetic variation would decrease.

The chromosomes in each homologous pair don't separate from each other during the first meiotic division.

Failure of the zygote to develop into an embryo

Anaphase II

The random separation of maternal and paternal homologous chromosomes during meiosis I.

Sister chromatids result from DNA replication, which occurs prior to both mitosis and meiosis. Mitosis and meiosis are not defined by the ploidy state of the parent cell.

Sister chromatids must separate from each other and move to opposite poles of the dividing cell during meiosis II. If both homologs in meiosis I or both sister chromatids in meiosis II move to the same pole of the parent cell, the products of meiosis will be abnormal. The replication of DNA is required for the cell cycle to continue into mitosis or meiosis. It will not affect the occurrence of trisomy.

Sister chromatids separate in anaphase II. Crossing over produces new combinations of alleles within a chromosome—combinations that did not exist in either parent. This phenomenon is known as recombination.

Trisomy-21 is common most likely because it involves a small chromosome with a correspondingly small number of genes. With the exception of trisomy-21, most trisomies and monosomies observed in humans involve the sex chromosomes. In all these cases, the result of trisomy was still viable and does not represent all instances of trisomy in the gametes.

Synapsis is the physical pairing of two homologous chromosomes during prophase I of meiosis; a chiasma is the x-shaped structure formed during meiosis by crossing over between the non-sister chromatids of homologous chromosomes.

They are degraded at the kinetochore. Chromosome movement is driven by fraying of the ends of microtubules at each kinetochore, just as it is in mitosis (shown below for reference).

Haploid -having one of each type of chromosome (n) Bacteria and archaea are haploid, as are many plant and animal gametes are haploid.

Trisomy

A normal number of chromosomes in all gametes

A zygote with a genetic profile of 2n-1 is called a monosomy. Cells that have too many or too few chromosomes of a particular type are said to be aneuploid ("without-form"). Trisomy ("three-bodies") occurs when a cell has three copies of the chromosome.

Centrosomes are a microtubule organizing center located near the nucleus that contain two bundles of microtubules called centrioles. The two centrioles inside a centrosome consist of microtubules as triplets arranged in a circle.

centromere