Chapter 6. Meiosis and Mendel

Transcription

1 Chapter 6 Meiosis and Mendel 1

2 6.1 Two groups of Cells 1) Somatic Cells body cells Can NOT pass DNA on to offspring 2) Gametes sex cells Develop from germ cells in reproductive organs DNA IS passed on to offspring 2

3 Somatic Cells Are diploid Full set of chromosomes Diploid number =2n Ex: 2n = 46 (humans) = 23 pairs 3

4 Sex Cells (Gametes) Egg (female) Sperm (male) Haploid Contain half the number of chromosomes NO pairs! Haploid number = n Ex: n = 23 in humans 4

5 Fertilization Fusion of egg and sperm forms a zygote Nuclei fuse and form one = diploid cell 1/2 DNA from each parent 5

6 Chromosome Numbers Each organism has a characteristic # Not related to complexity Consistent within a species 6

7 Homologous Chromosomes = a pair of chromosomes One from each parent Similar in: size shape genetic information 7

8 Types of Chromosomes Autosomes Chromosomes that do not determine gender Sex chromosomes Determine the sex of individual Only two sex chromosomes X and Y 8

9 Sex Chromosomes Male XY Y chromosome is smaller Determines gender of zygote Female XX 9

10 Human Chromosome Number 46 Essential for normal development and function 10

11 Down Syndrome Trisomy 21 Extra copy of chromosome 21 = Down Syndrome Caused by nondisjunction 11

12 Risk of Down Syndrome Increases with the mother s age All eggs present in ovaries at birth Can accumulate damage Women over 35 Advised to get prenatal testing 12

13 Nondisjunction Gametes form but chromosomes fail to separate properly One gamete gets both chromosomes Now 2 in one gamete and 0 in other 13

14 6.2 Meiosis Form of nuclear division that creates 4 haploid cells from 1 diploid cell 14

15 Mitosis (Review) Type of cell division Divides the nuclear material Maintains chromosome # Forms diploid cells Forms somatic cells 15

16 Meiosis From Greek meioun = to make smaller Type of cell division Divides nuclear material Cuts chromosome # in half Forms haploid reproductive cells Gametes Spores 16

17 Meiosis Sex Cells 17

18 Meiosis DNA replicated before the two divisions of nucleus Starts w homologous chromosomes 18

19 What s the difference???? Homologous chromosomes vs. sister chromatids? Chromosome = name given during anaphase of Mitosis or anaphase of Meiosis II 19

20 Homologous Chromosomes Two separate chromosomes One from mom, one from dad Very similar to each other Same features and functions Some instructions may be slightly different Same genes NOT exact copies! Divided in Meiosis I 20

21 Sister Chromatids Duplicated chromosomes that are attached by a centromere Exact copies of each other Separated in anaphase of Meiosis II OR Separated in anaphase of Mitosis 21

22 Gene for eye color Gene for hair color 22

23 Before Meiosis... Remember DNA has already been copied!!! Just like what happens before Mitosis! 23

24 Meiosis has 2 Stages: Meiosis 1 Prophase 1 Metaphase 1 Anaphase 1 Telophase1 and cytokinesis Meiosis 2 Prophase 2 Metaphase 2 Anaphase 2 Telophase 2 and cytokinesis 24

25 Meiosis 1 Prophase 1 Chromosome become visible Nuclear envelope breaks down Centrioles move Spindle fibers appear Homologous chromosomes pair Crossing over occurs 25

26 Prophase 1 UNLIKE mitosis, homologous chromosomes line up next to each other Called tetrads Process = synapsing 26

27 Prophase 1 When hanging out so close, sometimes parts are swapped = Crossing Over 27

28 More Crossing Over 28

29 Looking at Real Tetrads: 29

30 Crossing Over Increases genetic variation (diversity)... Why is this important in the long haul? 30

31 Metaphase 1 Homologous pairs moved to equator by spindles Random arrangement 31

32 Anaphase 1 Homologous chromosomes separate Move to opposite poles by spindle fibers Each chromosome still composed of 2 chromatids 32

33 Telophase 1 and Cytokinesis Chromosomes gather at poles Cytoplasm divides 2 new cells formed One chromosome from each pair in each *Chromosomes do NOT replicate bw meiosis 1 and meiosis 2 33

34 Beneath the microscope: 34

35 Result of Meiosis 1 Homologous chromosomes separated Two NON-identical daughter cells formed Chromosome # cut in half 35

36 Ponder... Why are chromosomes NOT replicated between meiosis I and meiosis 2? Think about it... Keep thinking... Got it! Makes sense, doesn t it?! 36

37 Meiosis 2 Starts with 2 daughter cells formed in Meiosis 1 Similar to mitosis 37

38 38

39 Meiosis 1 39

40 Prophase 2 Nuclear membrane breaks down Centrioles move Spindles assemble 40

41 Metaphase 2 Chromosomes lined up at equator Centromeres attached to spindles 41

42 Anaphase 2 Centromeres divide Sister chromatids move to opposite poles (Now called chromosomes) 42

43 Telophase 2 and cytokinesis Nuclear envelope reforms Spindle breaks down Cytokinesis occurs Two daughter cells formed for each cell Total: 4 haploid cells 43

44 44

45 Under the Microscope 45

46 Under the Microscope: 46

47 Thinker... What is the major difference between metaphase I and metaphase II? Metaphase I = pairs of chromosomes line up. Metaphase II = chromosomes are NOT paired 47

48 48

49 Gametogenesis Production of gametes 2 types: Spermatogenesis Oogenesis 49

50 Spermatogenesis Forms sperm Cells move quickly Small Compact Flagellum Main contribution = DNA Yields 4 haploid sperm 50

51 Oogenesis Forms Ova (eggs) Begins before birth DNA, cytoplasm, organelles, etc Yields 4 cells One egg 3 polar bodies smaller, break down, die 51

52 Meiosis: The Continuation of Life Meiosis - The Continuation of Life - 11:48 52

53 Basic Vocabulary Trait Inherited characteristics Heredity The passing on of traits from parents to offspring Genetics The study of heredity Cross Mating of two organisms 53

54 6.3 How it all Began... Gregor Mendel 1860s Austrian monk Father of Genetics Studied pea plants Pisum sativum Developed rules to predict patterns of heredity 54

55 Mendel s Background Mathematician Child of peasants Knew a lot about agriculture Studied theology Became a priest Went to U of Vienna Studied science and math 55

56 Mendel s Background Mendel repeated experiments of T. A. Knight British farmer Crossed pea plants Mendel counted offspring Analyzed data Mendel s work rediscovered in

57 Useful Features in Pea Plants Small Grows easily Mature quickly Lots of offspring Traits occur in two forms Enclosed flowers Tend to self-pollinate Can control crosses 57

58 Genetic Humor 58

59 Before Mendel Blending Hypothesis Tall X Short = Medium Did NOT agree with Mendel s findings! 59

60 Mendel s Experiment Monohybrid cross Cross one pair of contrasting traits Ex: Tall x Short Purple x White Round x Wrinkled 60

61 Mendel s Experiment 61

62 Mendel s Experiment Used Purebreds = True breeding Homozygous Always produce offspring with same characteristics P (parental) generation First 2 individuals in a cross 62

63 Mendel s Experiment F1 generation First filial Offspring of P generation Showed just one form of the trait Filial = Son 63

64 Mendel s Experiment F2 generation Second filial generation Offspring of the F1 generation Showed both forms of the trait again 64

65 Mendel s Law of Segregation Individuals get two copies of heritable factors (genes) Alternative versions of genes (alleles) One from each parent Dominant or recessive Gametes carry only one allele for each gene 65

66 Gregorian Chant... Mendel song 3:30 66

67 6.4 Alleles Different versions of gene Everyone has 2 for each trait One from each parent Together they code for expression of gene (trait) 67

68 Dominant Alleles Expressed If present, shows up! 68

69 Recessive Alleles Not expressed if dominant allele is present Shows up only if both alleles are recessive 69

70 Genotype Genetic makeup of organism Actual alleles inherited Letters represent alleles Capital letters = dominant Lower case = recessive 70

71 Homozygous Genotype Two similar alleles Two dominant = two capital letters Two recessive = two lower case letters 71

72 Heterozygous Genotype Two different alleles One dominant One recessive One capital letter & one lower case letter Dominant always written 1 st 72

73 Comparing Genotypes 73

74 Phenotype Physical appearance Determined by alleles Genotype determines Phenotype 74

75 6.5 Punnett Squares Grid system for predicting outcome of a cross Considers all possible gamete combinations 75

76 Punnett Squares Combine alleles Multiply and fill boxes in Shows all Possible genotypes of offspring 76

77 Practice Punnett Squares Determine genotypes and phenotypes Monohybrid cross One trait crossed Ex: Pure short pea plant x hybrid pea plant 77

78 BABY STEPS: 78

79 Practice Cross: homozygyous recessive individual with blue eyes with heterozygyous individual with brown eyes 79

80 Probability Likelihood that an event will happen Predicts average number of occurrences Practice: Determine the ratios 80

81 Testcross Used to figure out genotype of an organism Must cross unknown organism with homozygous recessive individual WHY?!!! 81

82 Dihybrid Cross Crossing two different traits Each gene still represented by 2 alleles 82

83 Law of Independent Assortment Mendel s 2 nd law Inheritance of one trait does not influence the inheritance of any other Alleles of different genes separate independently during meiosis 83

84 6.6 Genetic Variation Rapid because of meiosis Key contributors: 1) independent assortment 2) random fertilization 3) crossing-over 84 84

85 1) Independent assortment Random distribution of homologous chromosomes during meiosis Determined by chance 85

86 1) Independent Assortment # of different gametes possible from one original human cell: 2 23 (~8 million)!wowzers! That is a lot of possibilities! 86 86

87 2) Random Fertilization Zygote formed bw random gametes Fertilization by random sperm Now outcome: 2 23 x 2 23 > 70 trillion Human couple can produce a child w 1 of ~70 trillion possible combos 87

88 3)Crossing Over Results in Recombination Mixing of parental alleles DNA exchanged during prophase 1 # of possibilities is nearly unlimited 88 88

89 Genetic Linkage Genes close together tend to be inherited together Genes far apart sort independently Allows scientist to calculate distance bw genes Create genetic map 89 89

90 Interactive Website: NOVA Online 18 Ways to Make a Baby How Cells Divide: Mitosis vs. Meiosis (Flash) 90 90