Cell Division
Recall that most cells of the body undergo cell division, the process of reproducing themselves. There are 2 types: somatic cell division and reproductive cell division. A somatic cell is any body cell other than a germ cell. A germ cell is a gamete, or any precursor destined to be a gamete. Equine cells contain 32 pairs of chromosomes for a total of 64. Thirty-two single chromosomes are inherited from each parent. Somatic cells contain 2 sets of 32 chromosomes are called diploid (2n). In somatic cell division, a somatic cell undergoes nuclear division called Mitosis, and cytoplasmic division called cytokinesis to produce 2 IDENTICAL cells each with the same number of chromosomes as the original cell. Each new organism is the result of the union of 2 different gametes (fertilization). If gametes had the same number of chromosomes as somatic cells, the number of chromosomes would double at fertilization. In reproductive cell division that occurs in the gonads, the gametes undergo Meiosis, which produces gametes in which the number of chromosomes is reduced by half. As a result, gametes contain a single set of 32 chromosomes and thus are called haploid (n). Fertilization restores the diploid number of chromosomes. Meiosis occurs in 2 successive stages: meiosis I and meiosis II. During interphase that precedes meiosis I, the chromosomes of the diploid cell start to replicate. As a result of replication each chromosome consists of 2 sister chromatids (genetically identical) attached at their centromeres.
Meiosis I begins once chromosomes have replicated within the parent cell.
A) Prophase I: chromosomes condense, nuclear envelope and nucleolus disappear, and mitotic spindle develops. Synapsis occurs in which 2 sister chromatids of each pair of homologous chromosomes pair off. The resulting chromatids form a tetrad. Crossing over , the exchange
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