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Meiosis and Sexual Life Cycles
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Fig. 13-12-2

Prophase I

of meiosis

Pair of

homologs

Nonsister

chromatids

held together

during synapsis

Chiasma

Centromere

TEM

Slide 78

Fig. 13-12-3

Fig. 13-12-3

Prophase I

of meiosis

Pair of

homologs

Nonsister

chromatids

held together

during synapsis

Chiasma

Centromere

Anaphase I

TEM

Slide 79

Fig. 13-12-4

Fig. 13-12-4

Prophase I

of meiosis

Pair of

homologs

Nonsister

chromatids

held together

during synapsis

Chiasma

Centromere

Anaphase I

Anaphase II

TEM

Slide 80

Fig. 13-12-5

Fig. 13-12-5

Prophase I

of meiosis

Pair of

homologs

Nonsister

chromatids

held together

during synapsis

Chiasma

Centromere

Anaphase I

Anaphase II

Daughter

cells

Recombinant chromosomes

TEM

Slide 81

Random Fertilization

Random Fertilization

Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg)

The fusion of two gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations

Slide 82

Crossing over adds even more variation

Crossing over adds even more variation

Each zygote has a unique genetic identity

Animation: Genetic Variation

Slide 83

The Evolutionary Significance of Genetic Variation Within Populations

The Evolutionary Significance of Genetic Variation Within Populations

Natural selection results in the accumulation of genetic variations favored by the environment

Sexual reproduction contributes to the genetic variation in a population, which originates from mutations

Slide 84

Fig. 13-UN1

Fig. 13-UN1

Prophase I: Each homologous pair undergoes

synapsis and crossing over between nonsister

chromatids.

Metaphase I: Chromosomes line up as homolo-

gous pairs on the metaphase plate.

Anaphase I: Homologs separate from each other;

sister chromatids remain joined at the centromere.

Slide 85

Fig. 13-UN2

Fig. 13-UN2

F

H

Slide 86

Fig. 13-UN3

Fig. 13-UN3

Slide 87

Fig. 13-UN4

Fig. 13-UN4

Slide 88

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