Presentation on theme: "Notes E 4.3 Meiosis is a special form of cell division."— Presentation transcript:
Notes E 4.3 Meiosis is a special form of cell division.
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
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
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.
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.
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.
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.
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.