MEIOSIS (textbook section 3.1 pg 74)

Sexual reproduction produces offspring that is genetically similar, but different from the parents. This is accomplished by having two sex cells that each have half the information (half the chromosomes) combine to form a cell. The process to make a sex cell is called meiosis.

Multi-cellular organisms (like you) have two cell types: Somatic cells – called body cells (things like skin, stomach,

etc) reproduce by mitosis. They each have 23 pairs of chromosomes for a total of 46 chromosomes.

Gamete cells – called sex cells (sperm and egg) reproduce by meiosis. They each have one half of each of the 23 chromosome pairs.

Members of the same species have the same number of chromosomes. Humans

have 46. Gametes cannot have 46 chromosomes or else when they combined the new cell would have 92 chromosomes. Since gametes have half the number of chromosomes they are called haploid (23 chromosomes). Somatic cells are diploid (46 chromosomes).

HAPLOID CELLS HAVE ONE SET OF EACH CHROMOSOME (23 TOTAL FOR HUMANS) AND DIPLOID CELLS HAVE TWO SETS OF EACH CHROMOSOME (46 TOTAL FOR HUMANS)

In diploid cells chromosomes come in pairs. These are called homologous pairs (homologous chromosomes). One half of the pair comes from the mom, the other from the dad. Each half contains genes for the same protein at the same location.

Meiosis is similar to mitosis, but it differs in some very important ways. First is that Meiosis results in the creation of 4 haploid cells, not 2 diploid cells. Secondly, Meiosis creates 4 daughter cells with half the number of original chromosomes, whereas Meiosis produces 2 cells that contain the same amount of chromosomes (See handout on Meiosis).