Notes E 4.3

1. Notes E 4.3 Meiosis is a special form of cell division.

2. Meiosis is necessary for sexual reproduction. Organisms that reproduce sexually produce gametes, sex cells that contain ½ as many chromosomes as body cells. • Eggs—female gametes • Sperm—male gametes • 1n—gametes, also called haploid cells, produced by the process of meiosis

3. In fertilization, 2 gametes join. The resulting cell is 2n, or diploid. I has pairs of chromosomes, with a homolog from each parent. 1n+1n=2n

4. Cells divide twice during meiosis. Before meiosis, the DNA in each homolog is copied . The copies are referred to as chromatids and held together by a centromere. Meiosis differs from mitosis because: • The genetic material is divided not once, but twice • Four 1n (haploid) daughter cells are formed. In meiosis I, the first 2 daughter cells are produced. They receive one of the homologs.

5. In meiosis II, these 2 daughter cells divide again, splitting the homolog so that each cell gets one of the copies. Each of the 4 resulting daughter cells has half the number of chromosomes needed for offspring.

6. Meiosis and mitosis differ in important ways. • Cells that become gametes go through meiosis; all other cells divide by mitosis. • In meiosis, DNA is copied; then the cells divide twice. • Each cell has only one homolog from one parent. • In mitosis, DNA is copied, and the cells divide just once. Each cell has 2 homologs, one from each parent.

7. Meiosis produces haploid (1n) daughter cells that have half of the genetic material of the parents. • Mitosis produces diploid (2n) daughter cells that have the same genetic material as the parents.

8. Meiosis produces gametes, cells specialized for reproduction. • During fertilization, 2 gametes combine and 2 haploid cells form into one diploid cells. • The cell then divides by mitosis, developing into a multicellular organism.