1. Meiosis Premed 1 Biology

2. CHAPTER 21 MEIOSIS There is more to lectures than the power point slides! Engage your mind

3. 2006-2007 Meiosis & Sexual Reproduction

4. Cell division / Asexual reproduction Mitosis produce cells with same information o identical daughter cells exact copies o clones same number of chromosomes o same genetic information Aaaargh! I’m seeing double!

5. Asexual reproduction Budding Single-celled eukaryotes o Yeast, Paramecium o Amoeba Simple multicellular eukaryotes o Hydra What are the disadvantages of asexual reproduction? What are the advantages?

6. Binary Fission Prokaryotes (bacteria and archae) undergo reproduction in which cell grows to double its size and divide to form two cells (genetic information are exact copies of parent) Initiated with circular DNA of bacterial chromosome replicates at a specific place on chromosome called “Origin of replication” 2 origins of replication While replicating, the cell elongates Ending replication, plasma membrane pinches inward dividing the parent into two daughter cells

8. How about the rest of us? What if a complex multicellular organism (like us) wants to reproduce? o joining of egg + sperm Do we make egg & sperm by mitosis? 46 + 92 eggspermzygote What if we did, then…. Doesn’t work! No! 

9. Transmission of hereditary traits is through replication of DNA ------ producing copies of genes that can be passed from parents to offspring Reproductive cells “GAMETES” are vehicles that transmit genes from one generation to next During fertilization – Male & Female gametes (sperm and eggs) unite, passing genes of both parents to offspring

10. Human life cycle beings when a haploid sperm cell from father fuses with a haploid egg from mother Union of gametes is called FERTILIZATION resulting in “Zygote” Zygote is diploid because it contains two haploid sets of chromosomes representing the maternal and paternal family * Only cells of human body not produced by mitosis are Gametes

11. 46 chromosomes 23 pairs XY Human male karyotype diploid = 2 copies 2n

12. Human female karyotype 46 chromosomes 23 pairs XX diploid = 2 copies 2n

13. Paired chromosomes Homologous chromosomes o both chromosomes of a pair carry“matching” genes  One from mom, one from dad  control same inherited characters (ie. eye color)  homologous = same information diploid 2n 2n = 4 homologous chromosomes double stranded homologous chromosomes eye color (brown?) eye color (blue?)

14. gametes How do we make sperm & eggs? Must reduce 46 chromosomes  23 o must half the number of chromosomes o haploid 23 46 egg sperm 46 meiosis 46 fertilization 23 zygote

15. Meiosis makes sperm & eggs 46 chromosomes to 23 chromosomes o half the number of chromosomes 23 46 egg sperm 46 meiosis haploiddiploid

17. MEIOSIS

19. Type of cell division reducing the number of sets of chromosomes from two to one in the gametes Result: Sperm and egg are Haploid (n=23) MEIOSIS I & MEIOSIS II  4 daughter cells

20. Meiosis 1 overview 1st division of meiosis 4 chromosomes diploid 2n 2 chromosomes haploid 1n double stranded Copy DNALine Up 1 Divide 1 prophase 1metaphase 1 telophase 1

21. Meiosis 2 overview 2nd division of meiosis o looks like mitosis 2 chromosomes haploid 1n 4 gametes Line Up 2 Bye Bye 2 telophase 1 metaphase 2 telophase 2

22. Meiosis = reduction division Meiosis o special cell division in sexually reproducing organisms o reduce number of chromosomes by half  2n  1n  diploid  haploid o makes gametes ( sperm, eggs)

23. Meiosis & mitosis Meiosis to make gametes o sperm & egg Mitosis to make copies of ALL OTHER cells for: o growth o repair o development

24. Sexual reproduction lifecycle 1 copy haploid 1n 2 copies diploid 2n 1 copy haploid 1n meiosisfertilization We’re mixing things up here! A good thing?

25. mitosis zygote Putting it all together… meiosis  fertilization  mitosis + development 23 46 egg sperm 46 meiosis 46 23 fertilization mitosis & development 46 gametes

26. The value of meiosis 1 meiosis keeps chromosome number same from generation to generation from Mom from Dad offspring Dad Mom

27. The value of meiosis 2 Change over time o meiosis introduces genetic variation  new combinations of traits from generation to generation from Mom from Dad new gametes made by offspring offspring We’re mixing things up here! variation

28. How does this explain why kids are similar to their parents but not the same? Martin & Charlie Sheen, Emilio Estevez Michael & Kirk Douglas Baldwin brothers

29. Meiosis I Purpose: Meiosis I has two main purposes: It is the reduction division, so it reduces the number of chromosomes in half, making the daughter cells haploid (when the parent cell was diploid). It is during meiosis I that most of the genetic recombination occurs.

30. Phases: Keep in mind that before meiosis begins at all, the DNA undergoes replication, just like it did before mitosis started. So, when you first see chromosomes in meiosis I, they have sister chromatids, just like in mitosis. It is just that in meiosis I, we will be talking about tetrads (4 chromatids) becoming visible, lining up, separating, and decondensing (rather than chromosomes, like in mitosis). Finally, cytokinesis occurs, too, any time after the tetrads have moved out of the equator (just like in mitosis).

31. Homologous Chromosome Homologous chromosomes: Two choromosomes of a pair that have same length, centromere position, and staining pattern Both chromosomes of each pair carry genes controlling same inherited characters These homologous chromosomes are duplicated at interphase (before Prophase I)

32. Prophase I Just like in mitosis, during prophase, DNA condensation occurs, the nuclear envelope and nucleoli disappear, and the spindle starts to form. The big difference is what is going on with the chromosomes themselves. As DNA condensation proceeds and the chromosomes first become visible, they are visible as tetrads (4 chromatids) So, tetrads become visible during prophase “ Crossing Over” (exchange of genetic material between two homologous chromosomes) “Chiasmata” – Site where crossovers had occurred

34. Crossing Over -> variation nonsister chromatids chiasmata: site of crossing over variation Tetrad Prophase 1

36. Metaphase I Tetrads line up at the equator. The spindle has completely formed. It is during prophase I and metaphase I that genetic recombination is occurring. Take a look at the genetic recombination page to find out about how that happens here. Keep in mind that it only happens when there are tetrads, so as soon as anaphase I gets going, genetic recombination is over.

37. Anaphase I Tetrads pull apart and chromosomes with two chromatids move toward the poles. The homologs move toward opposite poles, guided by the spindle apparatus

38. Telophase I Chromosomes with two chromatids decondense and a nuclear envelope reforms around them. Each nucleus is now haploid (haploid set of duplicated chromosomes). Keep in mind that it is not the number of chromatids per chromosome that determine whether a cell is diploid or haploid, but, it is the number of chromosomes and whether they are paired that determines this.

40. Meiosis II Purpose: At the end of meiosis I, each chromosome still had two chromatids. That is double the amount of DNA that a cell should have. So, the entire reason to go through meiosis II is to reduce the amount of DNA back to normal-- basically, to split the chromosomes so that each daughter cell has only one chromatid per chromosome (the normal genetic content).

41. Phases: As you read through the phases of meiosis II, you will see that it looks just like mitosis. It is really similar to mitosis-- so keep that in mind. The only difference is that the two chromatids per chromosome are not necessarily identical due to genetic recombination occuring in meiosis I.

42. Prophase II Chromosomes with two chromatids become visible as they condense (and the nuclear envelope and nucleoli disappear, and the spindle is forming).

43. Metaphase II Chromosomes with two chromatids line up at the equator. The spindle is fully formed. Although genetic recombination primarily occurs during meiosis I, the way the chromosomes line up during metaphase II can also help to make unique daughter cells (not identical). I mention this on the genetic recombination page.

44. Anaphase II Chromosomes split, so that a chromosome with only one chromatid heads toward each pole. Sister chromatids separate

45. Telophase II Chromosomes with only one chromatid decondense and get surrounded by new nuclear envelopes, nuclei form The four daughter cells are now all haploid and have the right amount of DNA. They are ready to develop into sperm or eggs now. * Four daughter cells are genetically different from one another and from parent cell