Meiosis is a type of reproductive cell division that occurs after the joining of the egg and sperm cell. Meiosis has two phases, meiosis I and II, each containing interphase, prophase, metaphase, anaphase and telophase. It is during meiosis I where genetic diversity or variability occurs. The chromosomes in the diploid cell (joined egg and sperm cell or 2N), re-segregate and produce four genetically different haploid (N) daughter cells.
DNA replication begins with interphase I before the start of meiosis I. During interphase I, there is a duplication of genetic material. This prepares the cells for the start of the Meiosis I phases, prophase I, metaphase I and anaphase I, where genetic variability in the cells occur.
After the genetic material is duplicated during interphase I, homologous chromosomes (identical chromosomes) pair and form synapses, which link the homologous chromosomes together forming a tetrad, or four-sided structure, with two chromatids on each end. Crossing over occurs when the chromatids on the homologous chromosomes break and reattach to a different homologous chromosome, creating genetic diversity. This process is also referred to as genetic recombination. Genetic recombination produces four different types of chromosomes, instead of two types of chromosomes, increasing the genetic variability of the gamete genotype (sex cell genes).
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The chromosomes align in the middle of the cell at the metaphase plate during metaphase I for independent assortment. This occurs when the homologous chromosome line up in a random fashion. There is a 50-50 chance that the daughter cell gets the mother or father homologue chromosome, thereby increasing genetic variability.
Anaphase I and Telophase I
During anaphase I, homologous chromosome pairs are pulled away from each other to each pole of the cell. During this phase, a cleavage furrow forms to allow cytokinesis, or cell splitting, to occur. By the end of telophase I, each of the four daughter cells created have a single set of chromosomes and half the total number of genetic material the original cell had. These are the daughter cells, each with different genetic material.