Quick review: identify this stage of the diploid cell cycle.

1. KEY CONCEPT During meiosis, diploid cells undergo two cell divisions that result in haploid cells. In this picture, sperm are trying to fertilize an egg. Only one sperm can do this, resulting in a diploid cell (46 chromosomes). If 2 sperm fertilize this, there will be 69 chromosomes, reslting in triploidy (a disorder with 0.01% survival)

2. Quick review: identify this stage of the diploid cell cycle. Prophase

3. Quick review: identify this stage of the diploid cell cycle. telophase

4. Quick review: identify this stage of the diploid cell cycle. anaphase

5. Quick review: identify this stage of the diploid cell cycle. Prophase

6. Quick review: identify this stage of the diploid cell cycle. metaphase

7. Quick review: identify this stage of the diploid cell cycle. interphase

8. Cells go through two rounds of division in meiosis. • Meiosis reduces chromosome number and creates genetic diversity. This is how we are all so different, and why we can adapt to our environment

9. Meiosis I occurs after DNA has been replicated. • Pairs of homologous chromosomes separate inmeiosis I. • Homologous chromosomes are similar but not identical. • Meiosis I divides homologous chromosomes in four phases (PMAT) • Meiosis II divides sister chromatids in four phases (PMAT) • DNA is not replicated between meiosis I andmeiosis II • Sister chromatids divide in meiosis II. • Sister chromatids are copies of the same chromosome. • Meiosis I and meiosis II each have four phases, similar to those in mitosis. homologouschromosomes sister chromatids sister chromatids

10. Interphase • Similar to mitosis • G1: growth, organelle replication, normal function • S: DNA synthesis/replication • G2: growth, normal function

11. Prophase 1 • Similar as mitosis, except… • Two Events: 1) Synapsis: Pairing of homologous chromosomes • Dad’s #1 chromosome pairs with Mom’s #1, etc… • The four paired chromatids equals a TETRAD 2) Crossing Over: Exchange of genes

12. Metaphase 1 • Homologous chromosomes (tetrads) align along cell equator

13. Anaphase 1 • Homologous chromosomes are pulled apart to opposite ends • Tetrads separated

14. Telophase 1 • Cytokinesis divides the cells • Each cell contains 1 of the homologous chromosomes

15. Prophase 2 • Same as mitosis • Note: No new DNA was created (no 2nd interphase)

16. Metaphase 2 • Remaining chromosomes are aligned at cell equator

17. Anaphase 2 • Chromatids are pulled back to opposite ends • (remember, the prefix “ana-” means “back”)

18. Telophase 2 • Nucleus reforms on the four new cells • Cytokinesis divides the cytoplasm • Chromatids uncoil • End Result: Four haploid cells (23 chromosomes in each cell)

19. Haploid cells develop into mature gametes. • Gametogenesis is the production of gametes. • Gametogenesis differs between females and males. • In males it is called Spermatogenesis • In females, it is called Oogenesis

20. Spermatogenesis • Defined: Production of sperm cells • in male testes • Meiosis produces four haploid sperm cells at a time • Sperm become streamlined and motile. • Sperm primarily contribute DNA to an embryo.

21. Oogenesis • Defined: Production of the egg cell (ovum) • In female ovaries • Eggs contribute DNA, cytoplasm, and organelles to an embryo. • During meiosis, the egg gets most of the contents; the other cells form polar bodies. • During meiosis I: Cytoplasm distributed unequally • After meiosis II: • 1 cell matures • 3 “Polar Bodies” dissolve

22. Meiosis differs from mitosis in significant ways. • Meiosis has two cell divisions while mitosis has one. • In mitosis, homologous chromosomes never pair up. • Meiosis results in haploid cells; mitosis results in diploid cells.