When is chromosome number reduced in meiosis

The crossover events are the first source of genetic variation produced by meiosis. A single crossover event between homologous non-sister chromatids leads to an exchange of DNA between chromosomes.

Following crossover, the synaptonemal complex breaks down and the cohesin connection between homologous pairs is also removed. At the end of prophase I, the pairs are held together only at the chiasmata; they are called tetrads because the four sister chromatids of each pair of homologous chromosomes are now visible. Crossover between homologous chromosomes : Crossover occurs between non-sister chromatids of homologous chromosomes. The result is an exchange of genetic material between homologous chromosomes.

Synapsis holds pairs of homologous chromosomes together : Early in prophase I, homologous chromosomes come together to form a synapse. The chromosomes are bound tightly together and in perfect alignment by a protein lattice called a synaptonemal complex and by cohesin proteins at the centromere.

The key event in prometaphase I is the formation of the spindle fiber apparatus where spindle fiber microtubules attach to the kinetochore proteins at the centromeres. Microtubules grow from centrosomes placed at opposite poles of the cell. The microtubules move toward the middle of the cell and attach to one of the two fused homologous chromosomes at the kinetochores.

At the end of prometaphase I, each tetrad is attached to microtubules from both poles, with one homologous chromosome facing each pole. In addition, the nuclear membrane has broken down entirely. During metaphase I, the tetrads move to the metaphase plate with kinetochores facing opposite poles. The homologous pairs orient themselves randomly at the equator.

This event is the second mechanism that introduces variation into the gametes or spores. In each cell that undergoes meiosis, the arrangement of the tetrads is different. The number of variations is dependent on the number of chromosomes making up a set. There are two possibilities for orientation at the metaphase plate. The possible number of alignments, therefore, equals 2n, where n is the number of chromosomes per set.

Given these two mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will have the same genetic composition. In this case, there are two possible arrangements at the equatorial plane in metaphase I. The total possible number of different gametes is 2n, where n equals the number of chromosomes in a set.

In this example, there are four possible genetic combinations for the gametes. In anaphase I, the microtubules pull the attached chromosomes apart. The sister chromatids remain tightly bound together at the centromere. The chiasmata are broken in anaphase I as the microtubules attached to the fused kinetochores pull the homologous chromosomes apart. In telophase I, the separated chromosomes arrive at opposite poles.

In some organisms, the chromosomes decondense and nuclear envelopes form around the chromatids in telophase I. Then cytokinesis, the physical separation of the cytoplasmic components into two daughter cells, occurs without reformation of the nuclei. In nearly all species of animals and some fungi, cytokinesis separates the cell contents via a cleavage furrow constriction of the actin ring that leads to cytoplasmic division.

In plants, a cell plate is formed during cell cytokinesis by Golgi vesicles fusing at the metaphase plate. This cell plate will ultimately lead to the formation of cell walls that separate the two daughter cells. Two haploid cells are the end result of the first meiotic division. The cells are haploid because at each pole there is just one of each pair of the homologous chromosomes.

Therefore, only one full set of the chromosomes is present. Although there is only one chromosome set, each homolog still consists of two sister chromatids. During meiosis II, the sister chromatids within the two daughter cells separate, forming four new haploid gametes. Meiosis II initiates immediately after cytokinesis, usually before the chromosomes have fully decondensed.

In contrast to meiosis I, meiosis II resembles a normal mitosis. In some species, cells enter a brief interphase, or interkinesis, before entering meiosis II. Interkinesis lacks an S phase, so chromosomes are not duplicated. The two cells produced in meiosis I go through the events of meiosis II together. The mechanics of meiosis II is similar to mitosis, except that each dividing cell has only one set of homologous chromosomes.

If the chromosomes decondensed in telophase I, they condense again. If nuclear envelopes were formed, they fragment into vesicles. The centrosomes that were duplicated during interphase I move away from each other toward opposite poles and new spindles are formed. The nuclear envelopes are completely broken down and the spindle is fully formed.

Each sister chromatid forms an individual kinetochore that attaches to microtubules from opposite poles. The sister chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles. Non-kinetochore microtubules elongate the cell.

Meiosis I vs. In prometaphase I, microtubules attach to the fused kinetochores of homologous chromosomes, and the homologous chromosomes are arranged at the midpoint of the cell in metaphase I. This is different from metaphase in mitosis, where all chromosomes align single file on the metaphase plate. The position of each chromosome in the bivalents is random - either parental homolog can appear on each side.

This means that there is a chance for the daughter cells to get either the mother's or father's homolog for each chromosome see figure below. As shown in the below figure, during metaphase I, bivalents from either parent can align on either side of the cell.

In an organism with two sets of chromosomes, there are four ways in which the chromosomes can be arranged, resulting in differences in chromosomal distribution in daughter cells after meiosis I.

A diploid organism with 2 n chromosomes will have 2 n possible combinations or ways of arranging its chromosomes during metaphase I. In a diploid cell with 2 pairs of chromosomes, there are 4 ways to arrange the chromosomes during metaphase I. In anaphase I, homologous chromosomes separate. Homologous chromosomes, each containing two chromatids, move to separate poles. Each sperm and egg will end up with either B or b from mom and either B or b from dad.

It's a flip of the coin. But this happens independently for each trait, so just because you got your dad's brown eyes doesn't mean you'll get his blond hair too. This shuffling process is known as recombination or "crossing over" and occurs while the chromome pairs are lined up in Metaphase I.

In Metaphase I, homologous chromosome pairs line up. Homologous chromosomes can exchange parts in a process called "crossing over. Purpose : Meiosis is a special version of cell division that occurs only in the testes and ovaries; the organs that produce the male and female reproductive cells; the sperm and eggs. Why is this different? Ordinary body cells have a complete set of chromosomes.

If body cells from mom and dad fused to form a baby, the fertilized egg would have twice as many chromosomes as it should. Meiosis is sometimes called "reduction division" because it reduces the number of chromosomes to half the normal number so that, when fusion of sperm and egg occurs, baby will have the correct number.