M E I O S I S

1. M E I O S I S

2. Why don’t cells get bigger forever?? • Let’s do a lab! • With some math! • Surface area = 6 (l x w) • Volume = l x w x h

3. Calculations • Cell 2 • Sides: 1 cm • SA= • Volume = • When cut… • Cell 1 • Sides: .5 cm • SA= • Volume = • When cut… • Cell 3 • Sides: 2 cm • SA= • Volume = • When cut…

4. Limits to cell growth • The larger a cell becomes, the more demands the cell places on its DNA. • In addition, the cell has more trouble moving enough nutrients and wastes across its cell membrane • Activity

5. Surface Area to volume ratio • We want surface area to be bigger then volume • WHY? (Think about the lab)

6. So how do cells solve this problem? • Before they get too large… • They divide! • Into two “daughter” cells • Called CELL DIVISION

7. What steps may be involved?

8. Two stages • Stage 1: MITOSIS • Dividing nucleus • Stage 2: CYTOKINESIS • Dividing cytoplasm

9. Chromosome • We have 46! • Super condensed DNA • In mitosis, each chromosome gets copied • Now called two SISTER CHROMATIDS held together by a centromere

10. Animation 1 Animation 2

11. Sketch it out!

13. The Cell Cycle • Series of events that cells go through as they grow and divide • Grows, prepares to divide, and divides • Two main parts • Interphase • Mitosis

15. Divided into 2 parts • Interphase • G1 - “Gap” - Cell grows from last cell cycle • S – Synthesis of DNA (DNA makes a copy of itself) • G2 – Cell grows and gets ready for cell division • G0 – No cell division – typical of cells like nerve cells that stop dividing at maturity • Mitosis – M phase • Prophase • Metaphase • Anaphase • Telophase • Cytokinesis

16. Interphase • Longest phase in the Cell Cycle – over half the time is spent in this phase

17. Mitosis • Prophase – longest phase in Mitosis • Copied chromosomes pair up. Held together by Centromere (each is called a chromatid) • Shortening up of chromosomes, makes them visible • Replicated Centrioles (of animal cells) migrate to opposite poles • Nuclear envelope & nucleolus start to break down • Spindle fibers (microtubules) extend from centrioles to centromeres

18. Metaphase – shortest part of Mitosis • Chromosomes line up on the equator (middle)

19. Anaphase – “And away they go!” • Chromatids separate and go to opposite poles. Now called chromosomes!! • Spindle fibers shorten up

20. Telophase • “the end” – Final phase • Chromosomes reach opposite ends • Cell membrane pinches in (cleavage furrow) or Cell plate forms (if plants) • Chromosomes uncoil • Nuclear membrane & nucleolus reappear • Spindle fibers disassembles

21. Cytokinesis –Dividing cellular contents. Occurs simultaneously with Telophase MITOSIS OVERVIEW

22. Difference between Plant and Animal mitosis • No centrioles in plant cells • A cell plate forms in plant cells

23. Difference between Plant and Animal mitosis • Animal cells have a cleavage furrow thatsplits the two cells

24. How do cells… grow? • Know when to • Found important factors • Contact prevents growing

25. Cyclin is a protein that regulates the timing of the cell cycle. • Amount of cyclin in a cell rises and falls with the steps in the cell cycle. • There are other internal and external proteins that are also involved the cell cycle. • Cancer – uncontrolled cell growth. Cells do not respond to normal signals and grow out of control. Cause masses called tumors.

26. Cancer • Cancer occurs when cells do not respond to signals that tell them to stop growing

28. Count those chromosomes • We have 46 • If we made a baby, how many would they have? • 46 from mom 46 • 46 from dad + 46 • 92 for baby • No longer human!! 92

29. What do we do? • We go through MEIOSIS • Process that takes our two sets of chromosomes (diploid number{2N}) and cuts them into one set (haploid number{1N}) • 4623 • 23 from mom + 23 from dad = 46! • Normal baby!

30. MeiosisSteps • Unlike mitosis, meiosis has two stages of division • Meiosis I • Meiosis II

31. Meiosis I: Interphase I: • Replication of chromosomes. Like chromosomes are called homologous chromosomes

32. Prophase I: • The homologous chromosomes come together (Synapsis) and form a group of 4 called a tetrad. • Twisting & exchange parts of adjacent chromatids will occur – Crossing over. • This results in genetic recombination and unlinks genes that are located on the same chromosome. • HUGE GENETIC VARIETY!!!!!

33. Anaphase I & Telophase I • Chromosomes separate independently of each other. • This is known as Independent assortment. • Results in… • Random separation of chromosomes • Genetic recombination!!!!!

34. At the end of Meiosis I, there are 2 - 1N cells No interphase Meiosis I Results in 2 - 1N cells Meiosis II Results in 4-1N cells

35. Meiosis II • Occurs immediately following Telophase I. • NO INTERPHASE II !! • 1N cells immediately go into Prophase II, then Metaphase II, Anaphase II, and Telophase II • Split into four 1N cells, each with unpairedchromosomes • Haploid At the end of Meiosis II, there are four 1N cells.

37. Meiosis Animations • http://www.sumanasinc.com/webcontent/animations/content/meiosis.html • John Kryk • Cells Alive • You Tube • http://www.csuchico.edu/~jbell/Biol207/animations/meiosis.html • McGraw • Arizona • How Mitosis and Meiosis Differ

38. GametogenesisCreation of gametes (sex cells) • Spermatogenesis (testes) • Makes 4 haploid sperm cells from 1 - 2N primary sperm cell. Looks like normal Meiosis • Oogenesis (ovaries) • Produces ovum (eggs) from 1 diploid primary egg cell. Difference occurs during cytokinesis during meiosis I and meiosis II. • Unequal distribution of the cytoplasm resulting in 1 large ovum and 3 small polar bodies. • Polar bodies serve no function in animals but are needed in to plant embryos.

39. 1st Meiotic Division 2nd Meiotic Division

40. Comparing mitosis and meiosis • Be sure to know… • How many divisions for each? • How many starting cells? • How many cells at the end? • What N are those cells?