Meiosis :: essays research papers

Meiosis is a specialized change of thermo atomic division in which there two successive nuclear divisions (meiosis I and II) without any chromosome replication between them. Each division flowerpot be divided into 4 phases similar to those of mitosis (pro-, meta-, ana- and telophase). Meiosis occurs during the composition of gametes in animals.     Meiosis is a special type of carrel division that occurs during formation of sperm and nut case cells and gives them the correct enumerate of chromosomes. Since a sperm and egg unite during fertilization, from each one must have only half the number of chromosomes other body cells have. Otherwise, the fertilized cell would have too many.      Inside the cells that nominate sperm and eggs, chromosomes become twined. While they be pressed together, the chromosomes may break, and each may swap a portion of its transmittable material for the co-ordinated portion from its mate. This form of r ecombination is called crossing-over. When the chromosomes glue themselves back together and separate, each has picked up new genetic material from the other. The constellation of physical characteristics it determines is now antithetic than before crossing-over.      In Meiosis 1, chromosomes in a diploid cell resegregate, producing four haploid female child cells. It is this step in Meiosis that generates genetic diversity.Meiosis 2 is similar to mitosis. However, there is no "S" phase. The chromatids of each chromosome are no longer identical because of recombination. Meiosis II separates the chromatids producing two daughter cells each with 23 chromosomes (haploid), and each chromosome has only one chromatid.      During prophase I, homologous chromosomes pair and form snynapses. The paired chromosomes are called trebles, and the formation of chiasmata caused by genetic recombination becomes apparent. The bivalent has two chromo somes and four chromatids, with one chromosome coming from each parent.      In prometaphase I, the nuclear membrane disappears. One kinetochore forms per chromosome , and the chromosomes attached to spindle fibers begin to move.      In metaphase I, bivalents, each composed of two chromosomes, align at the metaphase plate. The orientation is random, with any parental homologue on a side. This means that there is a 50-50 hap for the daughter cells to get either the mothers or fathers homologue for each chromosome.      In anaphase I, chiasmata separate.