Meiosis

What is Meiosis? Quick Review: What organelle is responsible for holding the cell’s genetic code? Meiosis is the process of creating 4 haploid cells (n) from one diploid cell (2n) from a reproductive organ....Say What? –Nucleus

Haploid cells (n) contain only one set of chromosomes from ONE parent –one set of chromosomes (one set of genes). Diploid cells (2n) contains one set of chromosomes from EACH parent –two sets of chromosomes (two sets of genes). –Most cells are diploid Gametes: (sex cells) produced by meiosis. Are gametes haploid or diploid cells?

Phases of Meiosis Meiosis II Prophase II MetaphaseII AnaphaseII Telophase II Cytokenesis Why do you think Meiosis has two cycles while mitosis only has one? Meiosis I Interphase Prophase I Metaphase I Anaphase I Telophase I Cytokenesis The purpose of meiosis is to produce haploid gametes.

1. Interphase Cell growth DNA replication Cell prepares for meiosis

2. Prophase I Tetrad: Chromosome pairs with homologous chromosome to form a tetrad. Each tetrad has 4 chromatids. Crossing-over: Exchange of alleles between homologous chromosomes during pairing.

3. Metaphase I Tetrads line up at the “equator.” Spindle fibers attach to the chromosomes at the centromere.

4. Anaphase I Tetrads break apart and move toward opposite poles. Differs from mitosis by sister chromatids stay together.

5. Telophase I Spindle fiber breaks down. Chromosomes uncoil. Cytoplasm divides to form 2 new cells. Each cell has ½ the genetic information as the original cell. Cells are diploid because the DNA was replicated before meiosis I.

Meiosis II The cell does not go into interphase between meiosis I and meiosis II. Meiosis II is the same as mitosis. The only difference is the end product: 4 haploid cells. Recall: What was the end product of mitosis?

Metaphase meiosis I Metaphase meiosis II

Critical Thinking: Why do you think gametes (reproductive cells) are haploid when all other cells are diploid?  When 2 haploid gametes join, they form one diploid cell (zygote)!