1. Cell Division

2. Interphase Mitotic Cell Cycle 2 distinct phases What's the most important event of interphase? Chromosome duplication S G2G2

3. All chromosomes are duplicated Do they contain identical genes? What is significant about DNA in the S and G 2 phases? That means: two copies of each chromosome What are the copies called? Sister chromatids Yes, but… What about meiosis?

4. Some terminology Double-chromatid chromosomes Single-chromatid chromosomes Chromatin Homologous chromosomes Chromatid When do each of these occur?

5. Interphase (actually, this is not part of mitosis itself) Prophase Metaphase Anaphase Telophase Interphase (actually, this is not part of mitosis itself) Prophase Metaphase Anaphase Telophase THE STEPS OF MITOSIS

6. Onion root tip Whitefish blastula Interphase Nuclear envelop intact Chromosomes duplicated but not apparent 2 nucleoli visible in onion root tip Nucleoli not visible in whitefish blastula Mitosis DNA loose, uncondensed, called chromatin

7. Onion root tip Whitefish blastula Prophase Mitosis Double-chromatid chromosomes evident Chromatin becomes super-coiled & compact Nuclear envelop breaks down Nucleoli disappear in onion root tip Centrioles migrate toward poles of cell forming the spindle

8. Double-chromatid chromosomes

9. Onion root tip Whitefish blastula Metaphase Mitosis Double-chromatid chromosomes line up on equatorial plate of cell The spindle fibers attach to the chromosomes at the centromeres

10. Onion root tip Whitefish blastula Anaphase Mitosis Centromeres are pulled apart Groups of single-chromatid chromosomes move opposite poles of the cell Spindle fibers pull chromatids apart

11. Onion root tip Whitefish blastula Telophase Mitosis Cytokinesis begins with appearance of cleavage furrow Cytokinesis begins with appearance of cell plate Groups of single-chromatid chromosomes reach poles of cell Nuclear envelop begins to reform Nucleoli reform 2 new daughter cells formed

12. Onion root tip Whitefish blastula Returning to Interphase Mitosis Cytokinesis completes Chromosomes ‘disappear’ as interphase chromatin reforms 2 new daughter cells enter G 1 phase of the cell cycle

13. A SUMMARY OF MITOSIS

14. SOMATIC CELLS What is the process of somatic cell duplication called? What are typical body cells called? These cells divide continuously The new cells receive an exact copy of all the parent cell’s: Mitosis DNA

15. What are these somatic cells? Diploid or 2n What does this mean? They contain the full number of chromosomes in pairs How many in humans? 46 23 pairs

16. Mitosis occurs only in somatic cells Gametes are not diploid (2n) Instead, they are haploid (n) Gametes are not diploid (2n) Instead, they are haploid (n) What about sex cells? Called gametes Eggs and sperm Produced in ovaries or testes

17. Our haploid (n) number is 23 So our eggs and sperm have how many chromosomes? Half the number Why?

18. Fertilization is the union of an egg and a sperm If the egg and sperm were both diploid, what would the fertilized egg (zygote) be?

19. It would be a genetic mess! At fertilization, n + n = 23; 23 + 23 = 46! Instead, gametes are haploid (n). Egg and sperm both have exactly half the number of chromosomes of somatic cells

20. Note how mitosis and meiosis differ: Number of divisions? Number of chromosomes? Number of products? Number of divisions? Number of chromosomes? Number of products?

21. Meiosis ! Meiosis is the cell division process that enables the transformation from 2n to n Somehow somatic cells (2n) in our ovaries or testes must produce gametes (n)

22. How is meiosis more complicated than mitosis? Each chromosome has a partner Gametes must contain precisely half the diploid number of chromosomes They must contain one of each homologous pair of chromosomes Remember karyotypes? They come in pairs One from mom One from dad

23. Human Karyotype

24. Which pair of chromosomes in us in not homologous? All our other pairs of chromosomes are homologous It’s the 23rd pair in males, the XY pair Remember what homologous means?

25. What are the divisions called? How many divisions does meiosis have?

26. 1234 1.One pair of homologues to start 2.DNA is replicated 3.Meiosis 1 = 2n ---> n 4.Meiosis 2 = chromatids separate into 4 products

28. In males, how many progeny are produced? Typically 4 viable sperm are produced following each Meiosis 2

29. In females, how many progeny are produced? Just one viable ovum (egg) is produced, plus 3 small polar bodies

30. Crossing over When does it occur? Only during Prophase of Meiosis 1 Homologous chromosomes get together in temporary tetrads Overlap (cross over) and trade their DNA Why is this a good thing to do, generally?

31. Meiosis Summary 1.Meiosis 1 a.DNA replication takes place b.A parent cell produces two daughter cells each with one member of each original pair of homologous chromosomes (to create haploid daughter cells) c.Crossing over may occur 1.Meiosis 1 a.DNA replication takes place b.A parent cell produces two daughter cells each with one member of each original pair of homologous chromosomes (to create haploid daughter cells) c.Crossing over may occur

32. Meiosis Summary 2.Meiosis 2 a.There is no more DNA replication b.The chromatids of each chromosome separate and each daughter cell divides c.At the end of Meiosis 2, there are 4 daughter cells from each parent cell. Each daughter cell has half the number of chromosomes as the parent cell 2.Meiosis 2 a.There is no more DNA replication b.The chromatids of each chromosome separate and each daughter cell divides c.At the end of Meiosis 2, there are 4 daughter cells from each parent cell. Each daughter cell has half the number of chromosomes as the parent cell