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Meiosis and Sexual Life Cycles
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Occurs during interphase before meiosis I begins

Two, each including prophase, metaphase, anaphase, and

telophase

Occurs during prophase I along with crossing over

between nonsister chromatids; resulting chiasmata

hold pairs together due to sister chromatid cohesion

Four, each haploid (n), containing half as many chromosomes

as the parent cell; genetically different from the parent

cell and from each other

Produces gametes; reduces number of chromosomes by half

and introduces genetic variability among the gametes

Slide 62

Three events are unique to meiosis, and all three occur in meiosis l:

Three events are unique to meiosis, and all three occur in meiosis l:

Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information

At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes

At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate

Slide 63

Sister chromatid cohesion allows sister chromatids of a single chromosome to stay together through meiosis I

Sister chromatid cohesion allows sister chromatids of a single chromosome to stay together through meiosis I

Protein complexes called cohesins are responsible for this cohesion

In mitosis, cohesins are cleaved at the end of metaphase

In meiosis, cohesins are cleaved along the chromosome arms in anaphase I (separation of homologs) and at the centromeres in anaphase II (separation of sister chromatids)

Slide 64

Fig. 13-10

Fig. 13-10

EXPERIMENT

RESULTS

Shugoshin+ (normal)+

Spore case

Fluorescent label

Metaphase I

Shugoshin

Anaphase I

Metaphase II

Anaphase II

Mature

spores

OR

Spore

Two of three possible arrange-

ments of labeled chromosomes

Shugoshin+

Shugoshin

Spore cases (%)

100

80

60

40

20

0

?

?

?

?

?

?

?

?

Slide 65

Fig. 13-10a

Fig. 13-10a

EXPERIMENT

Shugoshin+ (normal)

Spore case

Fluorescent label

Metaphase I

Anaphase I

Metaphase II

Anaphase II

Mature

spores

Spore

OR

Two of three possible arrange-

ments of labeled chromosomes

Shugoshin

?

?

?

?

?

?

?

?

Slide 66

Fig. 13-10b

Fig. 13-10b

RESULTS

Shugoshin+

Shugoshin

Spore cases (%)

100

80

60

40

20

0

Slide 67

Concept 13.4: Genetic variation produced in sexual life cycles contributes to evolution

Concept 13.4: Genetic variation produced in sexual life cycles contributes to evolution

Mutations (changes in an organisms DNA) are the original source of genetic diversity

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