MEIOSIS IS A SPECIAL FORM OF CELL DIVISION MEIOSIS IS NECESSARY FOR SEXUAL REPRODUCTION CELLS DIVIDE TWICE DURING MEIOSIS. –Before meiosis starts, the chromosomes of the parent cell are copied. (so a cell ready to divide has twice as many chromosomes as usual) MEISOIS & MITOSIS DIFFER IN SOME IMPORTANT WAYS

SEXUAL REPRODUCTION Human body cells have 46 chromosomes, its “diploid” number or 2n (the full number of chromosomes for the species- the 2n for a fruit fly is 8) GAMETES: cells with ½ the usual number of chromosomes (one from each pair). Are 1n or haploid cells (sex cells). Human gametes have 23 chromosomes. EGG is gamete formed in reproductive organ of a female; SPERM is gamete formed in male reproductive organs. FERTILIZATION: process where a sperm (1n) and an egg (1n) combine to form one new cell (2n).

Sexual Reproduction In humans the egg has 23 chromosomes, the sperm also has 23 chromosomes, and they form a fertilized egg with 46 chromosomes (the diploid number or 2n). MEIOSIS is a special kind of cell division that makes haploid (1n) cells. During meiosis a single cell (2n) goes through 2 cell divisions: meiosis I and meiosis II. This only occurs in reproductive tissue of an organism.

2 CELL DIVISIONS Before meiosis begins, the chromosomes of parent cell are copied (interphase); so twice as many chromosomes as normal (4n) At the beginning of meiosis I, the cell has 2 copies of each homolog. These get separated not once, but twice during meiosis to make 4 haploid (1n) cells. Each haploid cell has one UNPAIRED set of chromosomes

Meiosis I Prophase I: chromosomes pair up with partners; 2 sets of chromosome pairs in parent cell now (doubled homologs or 4n) Metaphase I: each set of chromosome pairs line up along center of cell. Anaphase I: 2 copies of one homolog are pulled apart from 2 copies of other homolog (to opposite ends of parent cell) This is most critical time. Telophase I: a new cell membrane forms at the center of cell, dividing parent cell into 2 daughter cells (each 2n now)

Meiosis II Prophase II: in each daughter cell are 2 copies each of n chromosomes (2n) still attached to each other. Metaphase II: chromosomes line up in center of cell. Anaphase II: the 2 copies of each chromosome separate & pull to opposite ends of the cell. Telophase II: new cell membrane forms in center of cell, so each cell divides into 2 (1n) daughter cells, making a total of 4 (1n) cells. In humans (& other species) only 1 of the 4 daughter cells made by a female becomes an egg. The rest dissolve or are never formed.

Meiosis & Mitosis Differ Only cells becoming gametes go through meiosis. All other cells go through mitosis A cell dividing by meiosis goes through 2 cell divisions, but chromosomes aren’t copied before the second division. In mitosis, chromosomes are always copied before division. Meioisis daughter cells are haploid (1n) and only have ½ of parent cells genetic material, while mitosis daughter cells are diploid (2n) and have exactly the same genetic material as the parent cell.