CH 13 Meiosis & Sexual Life Cycles

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1 CH 13 Meiosis & Sexual Life Cycles AP Biology

2 Cell division / Asexual reproduction Mitosis produce cells with same information identical daughter cells exact copies clones same amount of DNA same number of chromosomes same genetic information AP Biology Aaaargh! I m seeing double!

3 Asexual reproduction Single-celled eukaryotes reproduce asexually yeast Paramecium Amoeba Simple multicellular eukaryotes reproduce asexually Hydra budding What are the disadvantages of asexual reproduction? What are the AP Biology advantages?

4 How about the rest of us? What if a complex multicellular organism (like us) wants to reproduce? joining of egg + sperm Do we make egg & sperm by mitosis? egg sperm zygote AP Biology

5 How do we make sperm & eggs? reduce 46 chromosomes 23 chromosomes half the number of chromosomes egg meiosis fertilization sperm AP Biology

6 11-4 Meiosis Copyright Pearson Prentice Hall Slide 6 of 35 End Show

7 Meiosis = reduction division Alternating processes alternating stages meiosis reduces chromosome number diploid haploid makes gametes sperm, eggs 2n n humans: fertilization restores chromosome number haploid diploid n 2n Warning: meiosis evolved from mitosis, so stages & machinery are similar but the processes are radically different. Do not confuse the two! AP Biology

8 Homologous chromosomes Paired chromosomes both chromosomes of a pair carry genes diploid 2n control same inherited characters homologous = same information homologous chromosomes double stranded homologous chromosomes AP Biology

9 11-4 Meiosis A Homologous Chromosome Pair:- Slide 9 of 35 Copyright Pearson Prentice Hall End Show

10 11-4 Meiosis Chromosome Number A cell that contains both sets of homologous chromosomes is said to be diploid. The number of chromosomes in a diploid cell is sometimes represented by the symbol 2N. For Drosophila, the diploid number is 8, which can be written as 2N=8. Slide 10 of 35 Copyright Pearson Prentice Hall End Show

11 11-4 Meiosis Humans have 23 pairs of chromosomes, or a total of 46 chromosomes per cell (2N= 46). Each chromosome from a pair of Homologous Chromosomes comes from a different parent: one from the mother through the egg and the other from the father through the sperm. Homologous chromosomes carry alleles for the same genes. Slide 11 of 35 Copyright Pearson Prentice Hall End Show

12 11-4 Meiosis The gametes are haploid with one chromosome from each homologous pair. The total number of chromosomes in each gamete cell is 23 (N= 23). Slide 12 of 35 Copyright Pearson Prentice Hall End Show

13 Sexual reproduction: Fertilization from Mom - 1 copy - haploid - 1n from Dad - 2 copies - diploid - 2n AP Biology

14 Making gametes for the next generation - 2 copies - diploid - 2n We re mixing things up here! - 1 copy - haploid - 1n AP Biology

15 Double division of meiosis DNA replication 1st division of meiosis separates homologous pairs 2nd division of meiosis separates sister chromatids AP Biology

16 Preparing for meiosis 1st step of meiosis Duplication of DNA Why bother? meiosis evolved after mitosis convenient to use machinery of mitosis DNA replicated in S phase of interphase of MEIOSIS (just like in mitosis) 2n = 6 single stranded 2n = 6 double AP Biology M1 prophase stranded

17 Preparing for meiosis 1st division of meiosis separates homologous pairs prophase1 2n = 4 single stranded 2n = 4 double stranded synapsis metaphase1 2n = 4 double stranded tetrad telophase1 1n = 2 double AP Biology stranded

18 Meiosis 2 2nd division of meiosis separates sister chromatids 1n = 2 double stranded What does this division look like? 1n = 2 double stranded prophase2 metaphase2 1n = 2 single stranded AP Biology telophase2

19 Steps of meiosis AP Biology Meiosis 1 interphase prophase 1 metaphase 1 anaphase 1 telophase 1 Meiosis 2 prophase 2 metaphase 2 anaphase 2 telophase 2 1st division of meiosis separates homologous pairs (2n 1n) 2nd division of meiosis separates sister chromatids (1n 1n) * just like mitosis *

20 AP Biology

21 AP Biology

22 Meiosis 1 AP Biology

23 Meiosis 2 AP Biology

24 Mitosis vs. Meiosis Mitosis 1 division daughter cells genetically identical to parent cell produces 2 cells 2n 2n produces cells for growth & repair no crossing over Meiosis 2 divisions daughter cells genetically different from parent produces 4 cells 2n 1n produces gametes crossing over AP Biology

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26 Mitosis vs. Meiosis AP Biology

27 11-4 Meiosis Phases of Meiosis Meiosis I Interphase I Meiosis I Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Slide 27 of 35 Copyright Pearson Prentice Hall End Show

28 11-4 Meiosis Phases of Meiosis Cells undergo a round of DNA replication, forming duplicate chromosomes. Interphase I Slide 28 of 35 Copyright Pearson Prentice Hall End Show

29 11-4 Meiosis Phases of Meiosis Synapsis occurs that is the joining of homologous chromosomes along their length. This newly formed structure is called tetrad and precisely aligns the homologous chromosomes gene by gene. MEIOSIS I Prophase I There are 4 chromatids in a tetrad. Nuclear envelope disintegrates Copyright Pearson Prentice Hall Slide 29 of 35 End Show

30 Crossing over DNA from one homolog is cut and exchanged with an exact portion of DNA from another homolog. Where crossing over has occurred (2-3 times per homologous pair), crisscrossed regions termed chiasmata form, which hold the homologs together until anaphase I Crossing-over produces new combinations of alleles. Copyright Pearson Prentice Hall

31 11-4 Meiosis Phases of Meiosis The homologous pairs of chromosomes are lined up at the metaphase plate, and microtubules from each pole attach to each member of the homologous pairs in preparation for pulling them to opposite ends of the cell. MEIOSIS I Metaphase I Slide 31 of 35 Copyright Pearson Prentice Hall End Show

32 11-4 Meiosis Phases of Meiosis The fibers pull the homologous chromosomes toward opposite ends of the cell. MEIOSIS I Anaphase I Sister chromatids stay connected and move together toward the poles Slide 32 of 35 Copyright Pearson Prentice Hall End Show

33 11-4 Meiosis Phases of Meiosis Each pole contains haploid set of chromosomes. MEIOSIS I Telophase I and Cytokinesis Nuclear membranes form. The cell separates into two cells. The two cells produced by meiosis I have chromosomes and alleles that are different from each other and from the diploid cell that entered meiosis I. Copyright Pearson Prentice Hall Slide 33 of 35 End Show

34 11-4 Meiosis Phases of Meiosis Meiosis II The two cells produced by meiosis I now enter a second meiotic division. Unlike meiosis I, neither cell goes through chromosome replication. Each of the cell s chromosomes has 2 chromatids. Copyright Pearson Prentice Hall Slide 34 of 35 End Show

35 11-4 Meiosis Phases of Meiosis Meiosis II Telophase I and Cytokinesis I Meiosis II Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Slide 35 of 35 Copyright Pearson Prentice Hall End Show

36 11-4 Meiosis Phases of Meiosis Meiosis II results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell. MEIOSIS II Prophase II Slide 36 of 35 Copyright Pearson Prentice Hall End Show

37 11-4 Meiosis Phases of Meiosis The chromosomes line up in the center of cell. MEIOSIS II Metaphase II Slide 37 of 35 Copyright Pearson Prentice Hall End Show

38 11-4 Meiosis Phases of Meiosis The sister chromatids separate and move toward opposite ends of the cell. MEIOSIS II Anaphase II Slide 38 of 35 Copyright Pearson Prentice Hall End Show

39 11-4 Meiosis Phases of Meiosis Meiosis II results in four haploid (N) daughter cells. MEIOSIS II Telophase II and Cytokinesis Copyright Pearson Prentice Hall Slide 39 of 35 End Show

40 11-4 Meiosis Gamete Formation Gamete Formation In male animals, meiosis results in four equal-sized gametes called sperm. Slide 40 of 35 Copyright Pearson Prentice Hall End Show

41 11-4 Meiosis Gamete Formation In many female animals, only one egg results from meiosis. The other three cells, called polar bodies, are usually not involved in reproduction. Slide 41 of 35 Copyright Pearson Prentice Hall End Show

42 Sexual life cycles produce genetic variation among offspring The mechanisms contributing to genetic variation are: 1. crossing over 2. chromosome shuffling in independent assortment 3. random fertilization

43 Independent assortment of chromosomes contributes to genetic variability due to the random orientation of tetrads at the metaphase plate. There is a fifty-fifty chance that a particular daughter cell of meiosis I will get the maternal chromosome of a certain homologous pair and a fifty-fifty chance that it will receive the paternal chromosome.

44 In this simple example with 2 homologous chromosomes (n=2) there are 4 possible different gametes (2 2 ). In humans with n=23 there are over 8 million possible different gametes (2 23 ). Although this is an impressively large number, there is a limit to the mixing in that genes on the same chromosome must always stay together. This limitation is solved by crossing over.

45 Random fertilization Any 2 parents will produce a zygote with over 70 trillion (2 23 x 2 23 ) diploid combinations AP Biology

46 Sexual reproduction creates variability Sexual reproduction allows us to maintain both genetic similarity & differences. Michael & Kirk Douglas AP Biology Baldwin brothers Martin & Charlie Sheen, Emilio Estevez

47 Differences across kingdoms Not all organisms use haploid & diploid stages in same way which one is dominant (2n or n) differs but still alternate between haploid & diploid have to for sexual reproduction AP Biology