MEIOSIS: PROCESS OF CELL DIVISION THAT REDUCES THE # OF CHROMOSOMES IN THE NEW

CELLS TO HALF

produces “gametes”: sex cellsMale: sperm X and Y

Female: eggs

Meiosis: divided into 2 stages:

Meiosis I Meiosis II

Meiosis I

PROPHASE I:spindle fibers appear

homologous chromosomes pair

**this happens only in meiosis!

Synapsis: the pairing of homologous chromosomes

Tetrad: each pair of homologous chromosomes

Aligned so that genes line up

Prophase I

alignment

Prophase I (con’t)

Crossing-over occurs: Chromatids twist

around each other Portions of

chromatids break off and attach to the adjacent chromosome

Creates “genetic recombination”: new mix of genetic material

Metaphase I

Tetrads line up along equator of cell

Spindle fibers are attached to centromere

Anaphase I

Each homologous chromosome moves to the opposite pole

“INDEPENDENT ASSORTMENT”: random separation of chromosomes

Telophase I

Chromosomes reach opposite ends

Cytokinesis I begins

These cells are HAPLOID! But ….. Each cell contain 2 copies (chromatids)

Meisosis II

Some organsism, interphase first occurs

In other organisms, you go straight from telophase I to prophase II

Prophase II

Spindle fibers form

Metaphase II

Chromosomes line up along the midline

Each chromatid faces the opposite poles

Anaphase II

Chromatids separate and move toward opposite poles

Telophase II

Nuclear memb forms around the chromosome in the 4 new cells

Cytokinesis II occurs

Each cell is haploid

Mitosis and Meiosis

Spermatogenesis: the production of sperm

Occurs in the testes

4 sperm created

Oogenesis: production of “ova” or eggs

3 polar bodies made and 1 egg