Lecture 5 – Meiosis I. A few important points about meiosis

A. cell division that reduces chromosome number by 1/22n four 1n cells

B. crossing over used to increase genetic diversity of gametes produced

II. Overview of meiosisA. homologues pairB. 1st division: 2n cell divides to give two 1n cellsC. 2nd division: each 1n cell divides to give total of four 1n cells

Two stages:meiosis I and meiosis II

III. Meiosis in detailA. meiosis I

1. prophase Ia. leptotene

b. zygotene

synapsis =

c. pachytene

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bivalent =

tetrad =

d. diplotene

chiasmata =

crossing over =

e. diakinesis

2. metaphase I

3. anaphase I

4. telophase I

IV. Meiosis IIA. between meiosis I and II

1. no DNA replication occurs2. nuclear envelope may reform transiently3. chromosomes may decondense briefly4. meiosis II resembles mitosis, but:

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B. steps of meiosis II1. prophase II

2. metaphase II

3. anaphase II

4. telophase II

V. Differences in gametogenesisA. spermatogenesis vs. B. oogenesis

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VI. Mitosis versus meiosisA. side-by-side comparison

B.

final thoughts on mitosis vs. meiosis

VII. Principle of segregation – Hereditary determinants separate in such a way that each gamete is equally likely to inherit either member of the pair.

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mitosis meiosisreduces ploidy

# cells produced

homologues pair

VIII. Independent assortment – unlinked genes assort independently from one another.

A. Products depend on metaphase I alignment of chromosomes.

B. How metaphase I alignment determines gamete genotype.

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