2. Meiosis: reduction division that results in half the number of chromosomes to make g ametes

3. What’s a gamete? Male = sperm consists of the nucleus (in the head), a tail, and mitochondria for energy Female = eggs Nucleus, mitochondria, ribosomes and a reserve supply of food for developing organism

4. How are gametes different from somatic cells? Somatic cells Gamete cells Body cells 46 c’somes Diploid – 2 sets of chromosomes C’somes in pairs Sex cells 23 c’somes Haploid- only 1 set of c’somes C’somes not in pairs

5. Meiosis consists of ONE replication of c’somes followed by TWO divisions

6. MEIOSIS I Homologous chromosomes separate Daughter cells each have one DUPLICATED homolog Crossing over occurs MEIOSIS II Each of the two daughter cells divide again just like mitosis but with 23 chromosomes instead of 46 Results in 4 daughter cells with ½ the # of c’somes as the parent cell In males: all 4 cells become sperm In females: one cell becomes an egg, the other 3 die

8. Comparison of Mitosis & Meiosis Mitosis 2 daughter cells 1 cell division Daughter cells are diploid DC=46 c’somes DC-identical to each other and to parent Meiosis 4 daughter cells 2 cell divisions Daughter cells are haploid DC=23 c’somes DC-different from each other and from parent

9. CROSSING OVER Happens in Prophase of Meiosis I Paired Homologous chromosomes exchange genes while next to each other You don’t inherit entire chromosomes from your parents— only pieces of them Produces new combinations of genes creating more variation and diversity within a species Important for natural selection and evolution

10. Crossovers are completely random!

11. Meiosis animation Meiosis Square Dance

12. Determination of Sex Females: two X chromosomes (XX) Males: XY All offspring receive an X chromosome from mom and either an X or a Y from dad

13. X chromosome: Essential for LIFE large and contains thousands of genes Y chromosome: Not essential for life Small and contains only 23 genes

15. CHECK YOUR UNDERSTANDING What are the male and female gametes? How many chromosomes does a gamete have? What is the ploidy of a gamete? When does crossing over occur? Why is it important?

16. How many sperm cells are formed by meiosis of one cell? How many egg cells? Summarize the differences between mitosis and meiosis What sex chromosome is always passed down by a mother? What 2 sex chromosomes may be passed down by a father?