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:

– 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

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

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

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

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

Mutations (changes in an organism’s DNA) are the original source of genetic diversity