Meiosis & Sexual Life Cycle

Transcription

1 Chapter 13. Meiosis & Sexual Life Cycle 1

2 Cell reproduction Mitosis produce cells with same information identical daughter cells exact copies (clones) same amount of DNA same number of chromosomes asexual reproduction What are disadvantages of asexual reproduction? What are advantages? 2

3 Asexual reproduction Single-celled eukaryotes reproduce asexually yeast Paramecium Amoeba Simple multicellular eukaryotes reproduce asexually Hydra budding 3

4 Budding in Yeast Binary fission in Amoeba 4

5 Reproduction in multicellular organisms How is it possible to maintain such genetic continuity in a multicellular organism? Michael & Kirk Douglas Baldwin brothers Martin & Charlie Sheen, Emilio Estevez 5

6 Sexual reproduction But how does a complex multicellular organism reproduce? joining of gametes egg (or ovum) sperm What has to happen to make gametes? 6

7 Role of Meiosis in Sexual Life Cycles Alternating processes fertilization & meiosis alternate meiosis reduces chromosome number diploid haploid 2n n fertilization restores chromosome number haploid diploid n 2n 7

8 Meiosis & fertilization must alternate for sexual reproduction to continue Alternation of generation the dominant multicellular stage (n vs. 2n) differs amongst organisms Haploid (n) organism = gametophyte Diploid (2n) organism = sporophyte Alternation of generations: life history having a m ulticellular haploid stage which alternates with a m ulticellular diploid stage Evolutionary trends: Earlier organism have dominant gametophyte (n) individual the gametophyte generation is the major stage in the life of mosses, the visible stage in fungi, and is an independent plant in ferns However, the gametophyte is only an inconspicuous structure within the flower or other reproductive organs of "higher" plants 8

9 Visualizing chromosomes karyotype 9

10 23 pairs of chromosomes 10

11 Human female karyotype 11

12 Human male karyotype 12

13 What does karyotype reveal? Mutation in chromosome number or gross chromosomal abnormalities Down s syndrome 13

14 Homologous chromosomes Paired chromosomes both chromosomes of a pair carry genes controlling same inherited characters homologous = same information - 2 copies - diploid - 2n 14

15 Sexual reproduction: Fertilization from Mom - 1 copy - haploid - 1n from Dad What would happen if meiosis didn t occur? - 2 copies - diploid - 2n 15

16 Making gametes - 2 copies - diploid - 2n - 1 copy - haploid - 1n 16

17 Meiosis = reduction division Meiosis special cell division in sexually reproducing organisms reduce 2n 1n diploid haploid (half) makes gametes (sperm, eggs) Warning: meiosis evolved from mitosis, so stages & machinery are similar but the processes are radically different. Do not confuse! 17

18 Double division of meiosis DNA replication 1st division of meiosis separates homologous pairs 2nd division of meiosis separates sister chromatids 18

19 Steps of meiosis 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 * 19

20 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 meiosis interphase (just like in mitosis) 2n single stranded M1 prophase 2n double stranded 20

21 Preparing for meiosis 1st division of meiosis separates homologous pairs prophase1 2n single stranded 2n double stranded synapsis metaphase1 2n double stranded tetrad telophase1 1n single stranded 21

22 Meiosis 2 2nd division of meiosis separates sister chromatids What does this division look like? 1n double stranded metaphase2 1n single stranded anaphase1 22

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25 Mitosis vs. Meiosis 25

26 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 26

27 The value of meiosis Meiosis introduces genetic variation gametes of offspring do not have same genes as gametes from parents genetic recombination random assortment in humans produces 2 23 (8,388,608) different combinations from Mom from Dad new gametes made by offspring 27

28 Random fertilization Any 2 parents will produce a zygote with over 70 trillion (2 23 x 2 23 ) diploid combinations 28

29 And more variation During Prophase 1 homologous pairs swap pieces of chromosome tetrad sister chromatids intertwine crossing over 29

30 Crossing over 3 steps cross over breakage of DNA re-fusing of DNA New combinations of traits What are advantages of sexual reproduction? What are disadvantages? 30

31 Genetic variation Meiosis & crossing over introduce great genetic variation to population drives evolution Consider the greater variation with 23 pairs of chromosomes = mixing and matching 31

32 Sources of genetic variability Genetic variability in sexual reproduction independent assortment homologous chromosomes in M1 sister chromatids in M2 crossing over between homologous chromosomes in prophase1 random fertilization random ovum by a random sperm 32

33 Spermatogenesis Continuous & prolific process in the adult male Each ejaculation contains million sperm occurs in seminiferous tubules 33

34 Structure of sperm 34

35 Oogenesis At birth an ovary contains all of the primary oocytes it will ever have ~300 eggs released Unequal cytokinesis 1 egg 3 polar bodies polar bodies degenerate What is the advantage of 1 egg? 35