Chapter 11: MEIOSISThe Cellular Basis of Sexual

Reproduction

AP Biology Russell

B. Rhodes

Introduction Terms

• Fertilization = combining genes from two sources of DNA

• Zygote = egg cell DNA + DNA from sperm form first cell of embryo; aka fertilized egg cell

• Gametes = cells produced by meiosis that are haploid

• Somatic cells = body cells

• Gametogenesis = making gametes• Spermatogenesis• oogenesis

11.1 Overview: Mechanisms of Meiosis

• Halving of chromosome number; so each parent contributes ½ of DNA

• Number is restored at fertilization – not meiosis

• Generates variety in ways• Genes from two parents • Gametes are not identical (daughter cells have variety)

• Eukaryotes that are at least diploid

• Replication is during one premeiotic interphase

• 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)

• Align as tetrads (sets of 4) on metaphase plane

• No DNA replication between phases I and II

• Independent assortment

• Segregation in meiosis II

• 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)

• Versions of genes on chromosomes are called ALLELES• Gene is region of DNA that codes for a protein (or polypeptide)• Different versions of genes are Alleles;

• so Gene = eye color and Alleles = brown and blue versions of proteins

• Meiosis separates homologous chromosomes

• 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 228-229

Other events

• 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

• X can pair with X or Y and are homologous through short region

Practice Compare and Contrast

• Mitosis and meiosis both…

• Only mitosis ….

• Only meiosis ….

• 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

• 2. independent assortment

• 3. random union of gametes

Variation

• CROSSING OVER • Regions of homologous chromosomes exchange so you have different combinations of alleles

than either parent - still have same genes in same order• Recombinants• Remember to think 3-D

• INDEPENDENT ASSORTMENT• Of pairs of chromosomes during Meiosis I• 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)

• GAMETES• Presumably random combination of gametes from female parent and male parent (many many

sperm or pollen and only one will fertilize egg)

• …. 223 = 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

11.3 Overview: The Time and Place of Meiosis in Organismal Life Cycles

• 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

• 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….

•PAGE 234

Meiotic Life Cycles