Chapter 11: MEIOSIS
The 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
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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