Meiosis

3.2.1 Explain the role of meiosis in sexual reproduction and genetic variation.

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Meiosis Meiosis is the process by which a diploid cell (2n) divides twice to produce 4 haploid cells (n). The 2 divisions are called Meiosis I and Meiosis II. Products of meiosis are gametes-eggs or sperm.

Meiosis is a Reduction Division Prior to sexual reproduction meiosis reduces the number of chromosomes by half and fertilization restores the original number.

Meiosis Overview Meiosis I Meiosis II Interphase II Interphase I Prophase I Metaphase I Anaphase I Telophase I Cytokinesis I Meiosis II Interphase II Prophase II Metaphase II Anaphase II Telophase II Cytokinesis II

Chromosomes Homologous chromosomes are replicated as in mitosis which are joined at the centromere. The 4 sister chromatids form a tetrad.

Meiosis I Tetrads line up at the metaphase plate and are separated. Homologous chromosomes are separated.

Prophase I Crossing over occurs between non-sister chromatids in which they break and exchange genetic material.

Meiosis II Meiosis II separates sister chromatids so that the 4 daughter cells each contain half the amount of chromosomes (n).

Independent Assortment The random distribution of homologous chromosomes during meiosis. Meaning each of the 23 pairs of chromosomes segregates independently which can form 2 23 (about 8 million) gamete combinations of genes.

Spermatogenesis Meiosis in males forming sperm. Occurs in testes. 1 diploid cell will form 4 haploid sperm.

Oogenesis Meiosis in women forming an egg. Occurs in the ovaries. 1 diploid cell forms 1 egg, and 3 polar bodies which will die.

Sources of Variation 1. Crossing Over- During Prophase 1 homologous chromosomes exchange DNA. 2. Random assortment- Chromosomes randomly line up at the equator during Metaphase. 3. Mutations- DNA can be incorrectly copied during Interphase 1 4. Fertilization- only 1 sperm fertilizes an egg