Transcript Meiosis
Meiosis – A Source of Distinction Why do you share some but not all characters of each parent? What are the rules of this sharing game? At one level, the answers lie in meiosis. Meiosis – A Source of Distinction Meiosis does two things - 1) Meiosis takes a cell with two copies of every chromosome (diploid) and makes cells with a single copy of every chromosome (haploid). This is a good idea if you’re going to combine two cells to make a new organism. This trick is accomplished by halving chromosome number. In meiosis, one diploid cells produces four haploid cells. Reduction of Chromosome Number in the First Division of Meiosis Meiosis – A Source of Distinction The second accomplishment of meiosis: 2) Meiosis scrambles the specific forms of each gene that each sex cell (egg or sperm) receives. This makes for a lot of genetic diversity. This trick is accomplished through independent assortment and crossing-over. Genetic diversity is important for the evolution of populations and species. Keys to Understanding Meiosis Chromosomes are paired. Chromosomes carry genes. The gene forms on a pair of chromosomes may be identical .. or different. Brown eyes Brown eyes Tall Tall Brown eyes Blue eyes Tall Short One Way Meiosis Makes Lots of Different Sex Cells (Gametes) – Independent Assortment Independent assortment produces 2n distinct gametes, where n = the number of unique chromosomes. In humans, n = 23 and 223 ≈ 8,000,0000. That’s a lot of diversity by this mechanism alone. Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over Crossing-over multiplies the already huge number of different gamete types produced by independent assortment. Between Independent Assortment and Crossing-Over, No Two Gametes Are Identical. Meiosis Following a Single Chromosome Pair Crossing over The Key Difference Between Mitosis and Meiosis is the Way Chromosomes Uniquely Pair and Align in Meiosis Mitosis The first (and distinguishing) division of meiosis Boy or Girl? The Y Chromosome “Decides” Y chromosome X chromosome Boy or Girl? The Y Chromosome “Decides” Oogenesis – A Path of Meiosis in Humans Woman are less busy in meiosis than men – meiosis produces only a few hundred mature eggs over a lifetime. Spermatogenesis – Another Path of Meiosis in Humans Men are busy – meiosis produces roughly 250,000,000 sperm per day. “Putting It All Together” - Fertilization What Meiosis is About Meiosis allows the creation of unique individuals through sexual reproduction.