Creating gametes (sex cells) MEIOSIS Creating gametes (sex cells) Page 84 EQ: What is meiosis, and how is it different from mitosis? EQ: Why is meiosis important for sexually reproducing organisms?

Gametes: Somatic Cells: Types of Cells Egg & sperm cells Called haploid - contain 1 copy of every chromosome; “n”; 1/2 the amount of DNA for the organism DNA can be passed to offspring Somatic Cells: ALL cells in your body except sex cells Called diploid – contains 2 copies of every chromosome; “2n”; complete amount of DNA for the organism

B A Total number of chromosomes for the organism Somatic Cell n Matching Total number of chromosomes for the organism B A Somatic Cell n Dipl oid Haplo id 2n Half the number of chromosomes for the organism Game te liver cell egg cell

Chromosomes Homologous chromosomes are similar, but not identical Carry same genes in same order, but the form of the gene may not be the same Ex: blue eye color gene vs brown eye color gene Sister chromatids are identical copies of the same chromosome A human body cell has 23 pairs of homologous chromosomes for a total of 46 chromosomes.

What is Meiosis? Meiosis is a two-part cell division process occurring in organisms who sexually reproduce that results in cells with half the number of chromosomes as a normal cell The point of meiosis is to create gametes (egg & sperm cells) so that babies can be made The random formation of gametes during meiosis creates genetic diversity in a population.

Mitosis vs. Meiosis Meiosis Mitosis Type of nuclear division Results in 4 genetically different haploid cells Involves sexual reproduction 2 divisions Function: create sex cells Mitosis Type of nuclear division Results in 2 genetically identical diploid cells Involves asexual reproduction 1 division Function: growth & repair

Mitosis vs. Meiosis

Phases of Meiosis: Meiosis 1 Occurs after DNA has been replicated Divides homologus chromosomes in 4 phases (PMAT!)

Crossing Over Occurs in Meiosis 1 a. Homologous chromosomes form a tetrad b. Chromatids cross over one another c. The crossed sections of chromatids are exchanged ***This produces a new combination of genes which contributes to diversity within a population. ***

Phases of Meiosis: Meiosis 2 NO DNA replication between meiosis 1 & meiosis 2 Divides sister chromatids in 4 phases (PMAT!)

Gametogenesis in males & females Gametogenesis – production of gametes Females: 1 diploid cell becomes 1 mature haploid cell Males: 1 diploid cell becomes 4 mature haploid cells This difference is because an egg cell contributes cytoplasm, organelles, and DNA to an embryo while a sperm contributes only DNA