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meiosis I

meiosis II

Lecture 5 – Meiosis I. A few important points about meiosis A. cell division that reduces chromosome number by 1/2 2n four 1n cells B. crossing over used to increase genetic diversity of gametes produced II. Overview of meiosis

A. homologues pair B. 1st division: 2n cell divides to give two 1n cells C. 2nd division: each 1n cell divides to give total of four 1n cells

Two stages: meiosis I and meiosis II III. Meiosis in detail A. meiosis I 1. prophase I a. leptotene b. zygotene synapsis = c. pachytene bivalent =

tetrad =

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d. diplotene chiasmata = crossing over =

e. diakinesis 2. metaphase I

3. anaphase I

4. telophase I IV. Meiosis II A. between meiosis I and II 1. no DNA replication occurs 2. nuclear envelope may reform transiently 3. chromosomes may decondense briefly 4. meiosis II resembles mitosis, but:

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B. steps of meiosis II 1. prophase II

2. metaphase II 3. anaphase II 4. telophase II V. Differences in gametogenesis A. spermatogenesis vs. B. oogenesis

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VI. Mitosis versus meiosis A. 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.

mitosis meiosis reduces ploidy

# cells produced

homologues pair

A A aa

A aA a

A aA a

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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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