Chapter 7 Meiosis Honors Biology Cell division Asexual

Chapter 7 Meiosis Honors Biology

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

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

Budding in Yeast Fission in Amoeba Honors Biology 2005 -2006

Asexual 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 Honors Biology

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

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

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

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

2 n = 4 single stranded Meiosis 1 Overview § 1 st division of meiosis separates homologous pairs of chromosomes prophase 1 2 n = 4 double stranded metaphase 1 2 n = 4 double stranded telophase 1 Honors Biology 1 n = 2 double stranded 2005 -2006

Meiosis 2 Overview § 2 nd division of 1 n = 2 double stranded meiosis separates sister chromatids Looks like Mitosis! prophase 2 1 n = 2 double stranded metaphase 2 1 n = 2 single stranded Honors Biology telophase 2

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

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

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

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

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

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

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

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

Honors Biology 2005 -2006

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. u This means no duplication of chromosomes and no interphase! § Why is this vitally important? Honors Biology 2005 -2006

Meiosis 2 in a Nutshell **Meiosis 2 is just like Mitosis!!!** 1. 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 Honors Biology 2005 -2006

Honors Biology 2005 -2006

Honors Biology 2005 -2006

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

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 u The only difference between the 2, are the size of the gametes that are produced. Honors Biology 2005 -2006

Spermatogenesis & Oogenesis § In Spermatogenesis, the cytoplasmic divisions are equal. § In Oogenesis, the cytoplasmic divisions are unequal. u 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. w Polar bodies simply house the extra chromosomes until all 3 cells disintegrate. Honors Biology 2005 -2006

Honors Biology 2005 -2006

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

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

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