Meiosis

Chromosomes Key feature of a chromosome: centromere (where spindle attaches) Chromosomes come in 2 forms, depending on the stage of the cell cycle. The monad form consists of a single chromatid.The dyad form consists of 2 identical chromatids (sister chromatids) attached together at the centromere. Chromosomes are in the monad and dyad forms at different times during the cell cycle. The dyad form is the result of DNA replication: a single piece of DNA (the monad chromosome) replicated to form 2 identical DNA molecules (the 2 sister chromatids of the dyad chromosome).

More Chromosomes Diploid organisms have 2 copies of each chromosome, one from each parent. The two members of a pair of chromosomes are called homologues. Each species has a characteristic number of chromosomes, its haploid number n. Humans have n=23, that is, we have 23 pairs of chromosomes. Drosophila have n=4, 4 pairs of chromosomes.

Human Karyotype

Meiosis Purpose – produce gametes (sex cells) for reproduction. Location – gonads / sex organs only Meiosis is the special cell division that converts diploid body cells into the haploid gametes. Only occurs in specialized cells. Takes 2 divisions, Meiosis1 and M2 In humans, cells start with 46 chromosomes (23 pairs) in dyad state. After M1, there are 2 cells with 23 dyad chromosomes each. After M2 there are 4 cells with 23 monad chromosomes each.

Summary of Main Events Monad chromosomes duplicate to form dyad Homologous chromosomes fuse to form tetrad/homologous pairs Crossing over between non-sister chromatids Independent assortment during anaphase I ~forms 2 different cells Sister chromatids separate during anaphase II ~forms 4 different cells

Comparing Mitosis / Meiosis http://www.pbs.org/wgbh/nova/baby/divi_flash.html

Genetic Recombination Purpose: to create chromosomes that are a new combination of paternal and maternal material Genetic variation is important to the survival of every species.

Nice "current event" tie in to mitosis/stem cells/etc… http://video