1. Meiosis: the creation of sex cells

2. Meiosis defined The production of gametes (sperm and eggs) by dividing the genetic material in half.The production of gametes (sperm and eggs) by dividing the genetic material in half. Meiosis is a process by which the chromosome number is reduced by half.Meiosis is a process by which the chromosome number is reduced by half. –Only occurs in testes and ovaries. in cells called “germ” cells. Increases genetic diversity of offspring Think: why is it important for the cell to have half as many chromosomes in the gametes?

3. Cells undergo the phases G1, S, & G2 prior to meiosis. Cells that undergo meiosis start as diploid (2n) cells. Meiosis results in four haploid (1n) cells being produced. Before Meiosis

4. Splitting chromosomes Body cells are diploid, (2n) Chromosomes are in homologous pairs. One “homologue” of a pair came from mom and the other from dad. These pairs must be split during the process of meiosis Think: “homo”logous mean they are…

5. After meiosis is complete only one of each type of chromosome is present in a sperm or egg. –i.e. one of each Autosome and 1 sex chromosome Haploid result Think: can you get an X from your mom or dad? What about the Y?

6. Chromosomes # is reduced from 2n to 1n. Divisions of the cell result in 4 cells being produced. reduction and division

7. Creating variation The purpose of meiosis is to create sex cells – the purpose of sex is to create variation. Unlike mitosis, which is to create 2 identical cells with the same DNA, meiosis has steps to create 4 cells with different DNA.

8. Achieving variation: crossing over At one stage of meiosis homologous pairs align with each other. Each pair of homologues chromosomes is called a tetrad. (4) Crossing over occurs during tetrad positioning of chromosomes. Portions of the DNA is swapped from one chromosome to another

9. Independent assortment & segregation There are 2 cell division in meiosis. The first separates homologous pairs - independently from each other. The second division separates the chromatids - segregation

10. Nondisjunction Sometimes when the germ cells divided the chromosomes are not distributed evenly –One gamete can get an extra chromosome and another can have one missing. Nondisjunction results in missing or extra chromosomes

11. Disorders Due to Nondisjunction Trisomy means 3 chromosomes instead of normal pair. Monosomy means 1 chromosome instead of normal pair. An extra chromosome #21 causes Down Syndrome –Most of the time nondisjunction causes and embryo to die Look closely: Each of these karyotypes shows a nondisjunction – where are the problems in each?

12. Contrasting Mitosis & Meiosis MitosisMitosis –Occurs each time a “body cell” replicates –Homologues are randomly distributed across metaphase plate –Crossing over never occurs. –Sister chromatids remain identical. –Sister chromatids separate in anaphase I. MeiosisMeiosis –Occurs only in germ cells of ovaries and testes. –Homologous pairs align (synapsis) –Crossing over occurs. –Sister chromatids are genetically different –Sister chromatids do not separate in anaphase I –Don’t know what all of the above means? – don’t worry it is on the next lecture.

13. Summary Review 1.Cells with a pair of chromosomes is called _____. 2.What is the purpose of meiosis? 3.A) How many cells result from meiosis? B) Are the resulting cells haploid or diploid? 4. What are the 3 ways variations is achieved?