Meiosis and Sexual Reproduction

1
Meiosis and Sexual Reproduction
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Sexual Reproduction
Asexual Reproduction

• Germ cells undergo meiosis and cytokinesis
• Daughter cells become gametes (sex cells)
• Gametes meet (hopefully!) at fertilization
• Produces genetic variation among offspring
• This variation in traits is the basis for
  evolutionary change

• Single parent produces offspring
• Growth and repair
• Mitosis produces daughter cells genetically
  identical to one another and to the parent
  clones

3
Chromosome Structure
Explore the chromosome viewer at this site
based on information from the Human Genome
project ( the source of this figure) http//www
.ornl.gov/sci/techresources/Human_Genome/posters/c
hromosome/index.shtml
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Homologous Chromosomes may carry different Alleles

• Each diploid cell has two of each chromosome (23
  pairs in humans)
• One chromosome in each pair from mother, other
  from father
• Paternal and maternal chromosomes may carry
  different alleles

Mom
Dad
Perhaps different forms of the same gene?
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Ploidy Chromosome Number
p. 141

• Sum total of chromosomes in a cell
• Germ cells are diploid (2n)
• Gametes are haploid (n)
• Meiosis halves chromosome number, leaving one
  complete set of 23 in each gamete
• Are these chromosomes replicated?

1-5
6-12
13-18
19-22
23 XX
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Meiosis Two Divisions

• Two successive nuclear divisions
• Meiosis I
• Meiosis II
• Four haploid nuclei are formed
• All (potentially) genetically different
• Events are similar to those in mitosis
• (prophase, metaphase, anaphase, telophase)

(Without DNA replication in between)
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Meiosis I(one homologous pair of chromosomes)
p. 141
What is the ploidy level of this nucleus? Are the
chromosomes replicated?
Each homologue in the cell pairs with its
partner,
then the partners separate
What is the ploidy level of these nuclei after
meiosis I?
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Meiosis II

• The two sister chromatids of each duplicated
  chromosome are separated from each other

two chromosomes (unduplicated)
one chromosome (duplicated)
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Meiosis I - Stages
p. 142
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Meiosis II - Stages
p. 143
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3 main reasons for genetic variability as a
result of sexual reproduction

• Crossing over
• Prophase I
• Independent Assortment (of chromosomes)
• Metaphase I
• Random Fertilization
• random re-combination of genes in fertilized
  gamete

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Crossing Over
E
p. 144
E
e
e

• All four chromatids are closely aligned
• Homologous chromosomes exchange segments
• New combinations of alleles created
• What would result if sister chromatids exchanged
  segments?

L
L
l
l
Germ cell ? synapsis
big eyes, small eyes no light
light
Big Eyes (E) small eyes (e)
Light (L) no light (l)
Bioluminescenc and eye size in viper fish. What
traits would be linked after crossing over?
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Independent Assortment

• random alignment of chromosomes during Metaphase
• number of possible combinations of chromosomes in
  a gamete is
• 2n
• n chromosome types

p. 145