Chapter 7 Meiosis

Cell division / Asexual reproduction Mitosis produce cells with same information identical daughter cells exact copies clones same number of chromosomes same genetic information Aaaargh! I’m seeing double!

Asexual reproduction Binary Fission Fission Fragmentation Budding Prokaryotes Fission Amoeba Fragmentation Starfish (Echinoderms) Budding Yeast, Hydra What are the disadvantages of asexual reproduction? What are the advantages? 2005-2006

Budding in Yeast Fission in Amoeba 2005-2006

Asexual Reproduction Sexual Reproduction Simple and very primitive Complex series of nuclear and cellular division Allows for the production of many offspring in a short period of time Production of offspring requires more time and a mate of the opposite sex No genetic diversity Great genetic diversity Asexual reproduction does not allow for great genetic diversity Disadvantage=can’t adapt. Sexual reproduction allows for great genetic diversity amongst offspring Genetic diversity is the raw material for evolution (the ability to successfully adapt to a changing environment)

Making Gametes (egg & sperm) What if a complex multicellular organism (like us) wants to reproduce? joining of egg + sperm Do we make egg & sperm by mitosis? No! What if we did, then….    46 + 46 92 egg sperm zygote Doesn’t work! 2005-2006

How do we make sperm & eggs? Must reduce the chromosome number 46  23 must be half the number of chromosomes Haploid (n) 23 46 zygote 23 46 egg 23 meiosis 46 23 fertilization sperm gametes

Meiosis = Reduction Division special cell division in sexually reproducing organisms reduces number of chromosomes 2n  1n diploid  haploid half makes gametes sperm, eggs 2005-2006

Double division of meiosis DNA replication 1st division of meiosis separates homologous pairs 2nd division of meiosis separates sister chromatids 2005-2006

2n = 4 single stranded Meiosis 1 Overview 1st division of meiosis separates homologous pairs of chromosomes 2n = 4 double stranded prophase1 2n = 4 double stranded metaphase1 telophase1 1n = 2 double stranded 2005-2006

Meiosis 2 Overview 2nd division of meiosis separates sister chromatids double stranded Looks like Mitosis! prophase2 1n = 2 double stranded metaphase2 1n = 2 single stranded telophase2

Steps of meiosis Meiosis 1 interphase prophase 1 metaphase 1 anaphase 1 telophase 1 Cytokenesis 1 Meiosis 2 prophase 2 metaphase 2 anaphase 2 telophase 2 Cytokenesis 2 1st division of meiosis separates homologous pairs (2n  1n) 2nd division of meiosis separates sister chromatids (1n  1n) * just like mitosis * 2005-2006

Preparing for meiosis 1st step of meiosis Duplication of DNA Why bother? meiosis evolved after mitosis convenient to use “machinery” of mitosis DNA replicated in S phase of interphase of MEIOSIS (just like in mitosis) 2n = 6 single stranded 2n = 6 double stranded M1 prophase 2005-2006

Prophase 1 Homologous chromosomes come together to form a structure called a Tetrad. There are 4 chromatids in a tetrad. When the homologous chromosomes attach, this is called Synapsis. Homologous Chromosomes A Tetrad formed by Synapsis 2005-2006

What are the advantages of sexual reproduction? Crossing over Homologous pairs swap pieces of chromosome breakage of DNA re-fusing of DNA New combinations of traits 2005-2006 synapsis

Genetic variation… Crossing over creates completely new combinations of traits in next generation 2005-2006

Metaphase 1 Pairs of homologous chromosomes (tetrads) migrate to the equatorial plane of the cell. 2005-2006

Anaphase 1 Homologous chromosomes (not chromatids) separate and move to opposite poles of the cell. 2005-2006

Telophase 1 Movement of chromosomes to the poles is completed. Cytokenesis begins 2005-2006

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End of Meiosis 1 The cell splits into two haploid cells with duplicated chromosomes. Chromosomes do not replicate (get copied) between meiosis 1 and meiosis 2. This means no duplication of chromosomes and no interphase! Why is this vitally important? 2005-2006

**Meiosis 2 is just like Mitosis!!!** Meiosis 2 in a Nutshell **Meiosis 2 is just like Mitosis!!!** Prophase II – New spindle forms 2. Metaphase II – Duplicated Chromosomes line up in the middle 3. Anaphase II – Chromosomes are pulled apart & each chromatid becomes a single chromosome and moves to an opposite side of the cell 4. Telophase II – Nuclear envelope forms, cytokenesis begins 2005-2006

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Mitosis vs. Meiosis Mitosis Meiosis 1 division daughter cells genetically identical to parent cell produces 2 cells 2n  2n produces cells for growth & repair no crossing over Meiosis 2 divisions daughter cells genetically different from parent produces 4 cells 2n  1n produces gametes crossing over

Meiosis in Males and Females In men = Spermatogenesis produces sperm in testes (gonads) In women = Oogenesis produces ova in ovaries (gonads) Both processes = 4 haploid gametes The only difference between the 2, are the size of the gametes that are produced. 2005-2006

Spermatogenesis & Oogenesis In Spermatogenesis, the cytoplasmic divisions are equal. In Oogenesis, the cytoplasmic divisions are unequal. This results in the production of 1 large haploid cell and 3 small haploid cells. The large cell becomes the ovum, and the 3 small cells become Polar Bodies. Polar bodies simply house the extra chromosomes until all 3 cells disintegrate. 2005-2006

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Sources of genetic variability Genetic variability in sexual reproduction independent assortment homologous chromosomes in Meiosis 1 crossing over between homologous chromosomes in prophase 1 random fertilization random ovum fertilized by a random sperm 2005-2006 metaphase1

Overview of meiosis I.P.M.A.T.P.M.A.T. 2n=4 interphase 1 prophase 1 metaphase 1 anaphase 1 n=2 prophase 2 metaphase 2 anaphase 2 telophase 2 2005-2006 telophase 1

What are the DISadvantages of sexual reproduction? Any Questions??