Meiosis :: essays research papers

Meiosis is a specialized configuration of thermonuclear division in which there two successive nuclear divisions (meiosis I and II) without any chromosome replication between them. Each division female genital organ be divided into 4 phases similar to those of mitosis (pro-, meta-, ana- and telophase). Meiosis occurs during the brass of gametes in animals.     Meiosis is a special type of electric cell division that occurs during formation of sperm and ballock cells and gives them the correct deem of chromosomes. Since a sperm and egg unite during fertilization, to 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 offer sperm and eggs, chromosomes become matesed. While they ar pressed together, the chromosomes may break, and each may swap a portion of its inheritable material for the interconnected portion from its mate. This form of recombination 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 assorted than before crossing-over.      In Meiosis 1, chromosomes in a diploid cell resegregate, producing four haploid lady friend 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 doubleds, and the formation of chiasmata caused by genetic recombination becomes apparent. The bivalent has two chromosomes 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 each parental homologue on a side. This means that there is a 50-50 line up for the daughter cells to get either the mothers or fathers homologue for each chromosome.      In anaphase I, chiasmata separate.