Meiosis :: essays research papers

Meiosis is a specialized form of nuclear division in which there two successive nuclear divisions (meiosis I and II) without any chromosome replication between them. Each division can be divided into 4 phases analogous to those of mitosis (pro-, meta-, ana- and telophase). Meiosis occurs during the formation of gametes in animals.     Meiosis is a special type of cell division that occurs during formation of sperm and egg cells and gives them the correct number of chromosomes. Since a sperm and egg unite during fertilization, each(prenominal) must have only half the number of chromosomes other body cells have. Otherwise, the fertilized cell would have too many.       inner(a) the cells that produce sperm and eggs, chromosomes become paired. While they are pressed together, the chromosomes may break, and each may swap a portion of its genetic material for the co-ordinated portion from its mate. This form of recombination is called crossing-ov er. 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 different than onward crossing-over.      In Meiosis 1, chromosomes in a diploid cell resegregate, producing four haploid daughter cells. It is this step in Meiosis that generates genetic diversity.Meiosis 2 is alike 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 bivalents, and the formation of chiasmata caused by genetic recombination becomes apparent. The bivalent has two chromosomes and four chromatids, with one chro mosome plan of attack 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 either parental homologue on a side. This means that there is a 50-50 chance for the daughter cells to get either the mothers or fathers homologue for each chromosome.      In anaphase I, chiasmata separate.