![]() ![]() In some species, the nuclear membrane briefly forms around the chromosomes, while in others it does not. Each of the two daughter cells is now haploid ( n), with half the number of chromosomes per nucleus as in meiosis I. Cytokinesis then divides the cell into two daughter cells. In telophase I, the homologs of each bivalent arrive at opposite poles of the cell, and a new nuclear membrane forms around each set of chromosomes. Unlike in mitosis, the centromeres do not split and sister chromatids remain paired in anaphase I. ![]() Homologous chromosomes, each containing two chromatids, move to separate poles. In anaphase I, homologous chromosomes separate. In a diploid cell with 2 pairs of chromosomes, there are 4 ways to arrange the chromosomes during metaphase I. (A diploid organism with 2 n chromosomes will have 2 n possible combinations or ways of arranging its chromosomes during metaphase I.) 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. ![]() As shown in the below figure, during metaphase I, bivalents from either parent can align on either side of the cell. This means that there is a 50-50 chance for the daughter cells to get either the mother's or father's homolog for each chromosome (see figure below). The position of each chromosome in the bivalents is random - either parental homolog can appear on each side. This is different from metaphase in mitosis, where all chromosomes align single file on the metaphase plate. In metaphase I, each pair of bivalents (two chromosomes, four chromatids total) align on the metaphase plate. Note that these bivalents have two chromosomes and four chromatids, with one chromosome originating from each parent. In figure below, following crossing over, the blue and red chromosomes, which originally carried AA and aa alleles, respectively, now carry Aa alleles in both chromosomes at the end of prophase I. The point where a crossover occurs is called a chiasma (plural chiasmata) (see below figure). During synapsis, crossovers – cross-connections that form from breakage and rejoining between sister chromatids – can occur between the paired bivalents, leading to genetic recombination (exchange of genetic material) between the strands involved. At this time they are said to be in synapsis. In late prophase I, homologous chromosomes (also called bivalent chromosomes, or bivalents) pair laterally, or side-by-side. (See figure below, where meiosis I begins with a diploid (2 n = 4) cell and ends with two haploid ( n = 2) cells.) In humans (2 n = 46), who have 23 pairs of chromosomes, the number of chromosomes is reduced by half at the end of meiosis I ( n = 23).ĭuring prophase I, chromosomal condensation allows chromosomes to be viewed under the microscope. In addition, in meiosis I, the chromosomal number is reduced from diploid (2 n) to haploid ( n) during this process. Meiosis I is unique in that genetic diversity is generated through crossing over and random positioning of homologous chromosomes (bivalent chromosomes). Meiosis I proceeds directly to meiosis II without going through interphase. In meiosis I, the phases are analogous to mitosis: prophase I, metaphase I, anaphase I, and telophase I (below figure). The amount of DNA in the cell has doubled, and the ploidy of the cell remains the same as before, at 2 n. The steps leading up to meiosis are similar to those of mitosis – the centrioles and chromosomes are replicated. However, several features, namely, the pairing and genetic recombination between homologous chromosomes, are unique to meiosis. Meiosis uses similar mechanisms as those employed during mitosis to accomplish the separation and redistribution of chromosomes. Halving the ploidy in meiocytes is essential for restoring the genetic content of the zygote to that of the parents. Diploid (2 n) organisms rely on meiosis to produce meiocytes, which have half the ploidy of the parents, for sexual reproduction. Meiosis is the process by which replicated chromosomes undergo two nuclear divisions to produce four haploid cells, also called meiocytes (sperms and eggs). ![]()
0 Comments
Leave a Reply. |