Chapter 11: MEIOSIS The Cellular Basis of Sexual Reproduction AP Biology RussellAP Biology Russell B. RhodesB. Rhodes

Introduction Terms Fertilization = combining genes from two sources of DNA Fertilization = combining genes from two sources of DNA Zygote = egg cell DNA + DNA from sperm form first cell of embryo; aka fertilized egg cell Zygote = egg cell DNA + DNA from sperm form first cell of embryo; aka fertilized egg cell Gametes = cells produced by meiosis that are haploid Gametes = cells produced by meiosis that are haploid Somatic cells = body cells Somatic cells = body cells Gametogenesis = making gametes Gametogenesis = making gametes Spermatogenesis Spermatogenesis oogenesis oogenesis

11.1 Overview: Mechanisms of Meiosis Halving of chromosome number; so each parent contributes ½ of DNA Halving of chromosome number; so each parent contributes ½ of DNA Number is restored at fertilization – not meiosis Number is restored at fertilization – not meiosis Generates variety in ways Generates variety in ways Genes from two parents Genes from two parents Gametes are not identical (daughter cells have variety) Gametes are not identical (daughter cells have variety) Eukaryotes that are at least diploid Eukaryotes that are at least diploid Replication is during one premeiotic interphase Replication is during one premeiotic interphase Paired chromatids from homologous chromosomes (4) participate in crossing over (trade regions of chromosome arms) Paired chromatids from homologous chromosomes (4) participate in crossing over (trade regions of chromosome arms)

More 11.1 Sets of chromatids move together toward the poles (don’t segregate now) Sets of chromatids move together toward the poles (don’t segregate now) Align as tetrads (sets of 4) on metaphase plane Align as tetrads (sets of 4) on metaphase plane No DNA replication between phases I and II No DNA replication between phases I and II Independent assortment Independent assortment Segregation in meiosis II Segregation in meiosis II 4 (potentially) different gametes that are haploid 4 (potentially) different gametes that are haploid

Interactions and distribution of homologous chromosomes HOMOLOGOUS CHROMOSOMES – two chromosomes in a diploid cell that have the same genes in the same order. One is the paternal chromosome (from the sperm or pollen cell) and the other is the maternal chromosome ( from the egg cell) HOMOLOGOUS CHROMOSOMES – two chromosomes in a diploid cell that have the same genes in the same order. One is the paternal chromosome (from the sperm or pollen cell) and the other is the maternal chromosome ( from the egg cell) Versions of genes on chromosomes are called ALLELES Versions of genes on chromosomes are called ALLELES Gene is region of DNA that codes for a protein (or polypeptide) Gene is region of DNA that codes for a protein (or polypeptide) Different versions of genes are Alleles; Different versions of genes are Alleles; so Gene = eye color and Alleles = brown and blue versions of proteins so Gene = eye color and Alleles = brown and blue versions of proteins Meiosis separates homologous chromosomes Meiosis separates homologous chromosomes Need to ensure that each gamete gets one of each of all the chromosomes and only one. Need to ensure that each gamete gets one of each of all the chromosomes and only one.

Meiosis – short version pg 227

Meiosis - Long version pg

Other events Nondisjunction = homologous pairs fail to split and one gamete has 2 copies of a chromosome ( trisomy 21 or Down Syndrome) Nondisjunction = homologous pairs fail to split and one gamete has 2 copies of a chromosome ( trisomy 21 or Down Syndrome) Sex chromosomes are XX and Xy Sex chromosomes are XX and Xy X can pair with X or Y and are homologous through short region X can pair with X or Y and are homologous through short region

Practice Compare and Contrast Mitosis and meiosis both… Mitosis and meiosis both… Only mitosis …. Only mitosis …. Only meiosis …. Only meiosis …. They are different because….. They are different because…..

Comparison pg 230 Make sure you can compare and contrast Mitosis and Meiosis

11.2 Overview: Mechanisms That Enhance Genetic Variability 1. crossing over 1. crossing over 2. independent assortment 2. independent assortment 3. random union of gametes 3. random union of gametes

Variation CROSSING OVER CROSSING OVER Regions of homologous chromosomes exchange so you have different combinations of alleles than either parent - still have same genes in same order Regions of homologous chromosomes exchange so you have different combinations of alleles than either parent - still have same genes in same order Recombinants Recombinants Remember to think 3-D Remember to think 3-D INDEPENDENT ASSORTMENT INDEPENDENT ASSORTMENT Of pairs of chromosomes during Meiosis I Of pairs of chromosomes during Meiosis I You are getting one of each set, but combos of maternal and paternal You are getting one of each set, but combos of maternal and paternal Segregation happens in Meiosis II ( second division without replication to make 4 haploid gamete cells) Segregation happens in Meiosis II ( second division without replication to make 4 haploid gamete cells) GAMETES GAMETES Presumably random combination of gametes from female parent and male parent (many many sperm or pollen and only one will fertilize egg) Presumably random combination of gametes from female parent and male parent (many many sperm or pollen and only one will fertilize egg) … = over 8 million ….. 8 million x 8 million = 64 trillion (Closer to 70) combos from same parents EACH offspring ….?! … = over 8 million ….. 8 million x 8 million = 64 trillion (Closer to 70) combos from same parents EACH offspring ….?! Odds of being genetically identical to a sibling that isn’t a twin Odds of being genetically identical to a sibling that isn’t a twin

11.3 Overview: The Time and Place of Meiosis in Organismal Life Cycles 3 options for how cell spends time either diploid or haploid 3 options for how cell spends time either diploid or haploid A: Animals spend most of life in diploid phase and make cells by mitosis only do meiosis to make haploid gametes A: Animals spend most of life in diploid phase and make cells by mitosis only do meiosis to make haploid gametes B: Plants and some fungi do Alternation of Generation; fertilization produces diploid sporophytes make spores that are haploid and grow by mitosis into gametophytes - these gametophytes produce eggs and pollen that fuse making the diploid sporophytes….. :-? B: Plants and some fungi do Alternation of Generation; fertilization produces diploid sporophytes make spores that are haploid and grow by mitosis into gametophytes - these gametophytes produce eggs and pollen that fuse making the diploid sporophytes….. :-? C: some fungi and protists fertilize making haploids then haploid cells make gametes directly…. C: some fungi and protists fertilize making haploids then haploid cells make gametes directly…. PAGE 234 PAGE 234

Meiotic Life Cycles