Meiosis

1
Meiosis

• Chapter 10

2
Meiosis reduction divisionGametes (sperm, egg,
pollen, ovule) go through division twice
3
Sexual Reproduction

• Chromosomes are duplicated in germ (sex) cells
• Germ cells are immature reproductive cells that
  develop in male and female animals (2n, diploid)
• Germ cells undergo meiosis and cytokinesis
• Gametes are mature (1n, haploid) germ cell
  descendents (sperm, egg, pollen, ovule)
• Gametes (sperm from dad and egg from mom) meet at
  fertilization and create a fertilized egg
  (zygote)

4
Asexual Reproduction

• Single parent produces offspring
• All offspring are genetically identical to one
  another and to parent
• Fungi (yeast) and bacteria
• Binary fission (budding)

5
Sexual Reproduction

• Involves
• Meiosis
• Gamete production
• Fertilization
• Produces genetic variation among offspring

6
Homologous Chromosomes Carry Different Alleles

• Cell has two of each chromosome
• One chromosome in each pair from mother, other
  from father
• Paternal and maternal chromosomes carry different
  alleles (genes, not jeans!)

Mom
Dad
Brown eyes Blue eyes
Homologous pair
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Stepped ArtFigure 10.5Page 168
Crossing over
chiasma
8
Sexual Reproduction Shuffles Alleles

• Through sexual reproduction, offspring inherit
  new combinations of alleles, which leads to
  variations in traits
• Crossing over (chiasma) in Prophase I (Meiosis I)
• This genetic variation is the basis for
  evolutionary change and the reason you do not
  look EXACTLY like any of your relatives

9
Gamete Formation

• Gametes are haploid cells (sperm, eggs, pollen,
  ovule)
• Arise from germ cells

ovaries
anther
ovary
testes
Figure 10.2aPage 164
10
Chromosome Number

• Sum total of chromosomes in a cell
• Germ cells are diploid (2n)
• Gametes are haploid (n)
• Meiosis halves chromosome number (from 2n to 1n)

11
Meiosis Two Divisions

• Two consecutive nuclear divisions
• Meiosis I
• Meiosis II
• DNA is not duplicated between divisions
• Four haploid nuclei form

12
In-text figurePage 165
Meiosis Two Divisions
MEIOSIS I
MEIOSIS II
PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II
PROPHASE I METAPHASE I ANAPHASE I TELOPHASE I
interphase (DNA replication before meiosis I)
no interphase (no DNA replication before
meiosis II)
13
Stepped ArtFigure 9.1Page 152
Vocabulary Check

• 1 DNA molecule 1 chromatid
• 2 chromatids 1 chromosome
• Duplicated in preparation for mitosis

one chromatid
Centromere
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Meiosis I
Each homologue in the cell pairs with its partner
then the partners separate
Homologous chromosome pair
In-text figurePage 165
15
Meiosis II

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

two chromatids
one chromosome (2 chromatids)
In-text figurePage 165
16
Stepped ArtFigure 10.4(1)Page 166
Meiosis I - Overview
Crossing over (chiasma)
17
Stepped ArtFigure 10.4(2)Page 167
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Prophase I

• Each duplicated chromosome pairs with homologue
• Homologues swap segments (crossing over chiasma)
• Each chromosome becomes attached to spindle

Figure 10.4 Page 167
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Metaphase I

• Chromosomes are pushed and pulled into the middle
  of cell
• The spindle is fully formed

Figure 10.4 Page 167
20
Anaphase I

• Homologous chromosomes segregate
• The sister chromatids remain attached

Figure 10.4 Page 167
21
Telophase I

• The chromosomes arrive at opposite poles
• Usually followed by cytoplasmic division

Figure 10.4 Page 167
22
Prophase II

• Microtubules attach to the centromeres (aka
  kinetochores) of the duplicated chromosomes

Figure 10.4 Page 167
23
Metaphase II

• Duplicated chromosomes line up at the spindle
  equator, midway between the poles

Figure 10.4 Page 167
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Anaphase II

• Sister chromatids separate to become independent

Figure 10.4 Page 167
25
Telophase II

• The chromatids arrive at opposite ends of the
  cell
• A nuclear envelope forms around each set of
  chromatids
• RESULT
• 4 haploid cells

Figure 10.4 Page 167
26
Animation
Stages of meiosis animation.
Click to view animation.
27
Details of Crossing Over

• Each chromosome becomes zippered to its homologue
• All four chromatids are closely aligned
• Nonsister chromosomes exchange segments

chiasma
Figure 10.5Page 168
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Effect of Crossing Over

• After crossing over, each chromosome contains
  both maternal and paternal segments
• Creates new allele combinations in offspring
  (increased genetic variability)

29
Random Alignment

• During transition between Prophase I and
  Metaphase I, microtubules from spindle poles
  attach to centromeres of chromosomes
• Initial contacts between microtubules and
  chromosomes are random
• Random separation of homologous chromosomes is
  called independent assortment

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Random Alignment

• Either the maternal or paternal member of a
  homologous pair can end up at either pole
• The chromosomes in a gamete are a mix of
  chromosomes (independent assortment variation)
  from the two parents

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Possible Chromosome Combinations

• As a result of random alignment, the number of
  possible combinations of chromosomes in a gamete
  is
• 2n
• (n is number of chromosome types)

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Possible ChromosomeCombinations23 8
variations
1
2
3
or
or
Humans 223 8,388,608 variations
or
Figure 10.6Page 169
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Animation
Crossover animation.
Click to view animation.
34
Plant Life Cycle
multicelled sporophyte
mitosis
zygote
Diploid
meiosis
fertilization
Haploid
spores
gametes
multicelled gametophytes
mitosis
Figure 10.7Page 170
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Animal Life Cycle
multicelled body
mitosis
zygote
Diploid
meiosis
fertilization
Haploid
gametes
Figure 10.7Page 170
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Oogenesis
three polar bodies (1n)
Disintegrate
first polar body (1n)
DNA replicated
primary oocyte (2n)
oogonium (2n)
secondary oocyte (1n)
Becomes the egg
ovum (1n)
(n is number of chromosome types)
Meiosis I, Cytoplasmic Division
Meiosis II, Cytoplasmic Division
Growth
Oogenesis
Figure 10.8Page 171
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Spermatogenesis
primary spermatocyte (diploid)
spermato-gonium (diploid )
sperm (mature, haploid male gametes)
secondary spermatocytes (haploid)
spermatids (haploid)
DNA replicated
(n is number of chromosome types)
Meiosis I, Cytoplasmic Division
Meiosis II, Cytoplasmic Division
Growth
cell differentiation, sperm formation
Spermatogenesis
Figure 10.9Page 171
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Fertilization

• Male and female gametes unite and nuclei fuse
• Fusion of two haploid nuclei produces diploid
  (2n) nucleus in the zygote
• Uniting of gametes is random (2n)
• Adds to variation among offspring

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Factors Contributing to Variation among Offspring

• Crossing over during Prophase I
• Random alignment of chromosomes at Metaphase I
• Random combination of gametes at fertilization

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Mitosis Meiosis Compared

• Mitosis
• Functions
• Asexual reproduction
• Growth, repair
• Occurs in somatic cells
• Produces clones
• Two diploid (2n) cells produced

• Meiosis
• Function
• Sexual reproduction
• Occurs in germ cells
• Produces variable offspring
• Four haploid (1n) cells produced

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Prophase vs. Prophase I

• Prophase (Mitosis)
• Homologous pairs do not interact with each other
• Prophase I (Meiosis)
• Homologous pairs become zippered together and
  crossing over occurs

42
Anaphase, Anaphase I, and Anaphase II

• Anaphase I (Meiosis)
• Homologous chromosomes separate from each other
• Anaphase/Anaphase II (Mitosis/Meiosis)
• Sister chromatids of a chromosome separate from
  each other

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Results of Mitosis and Meiosis

• Mitosis
• Two diploid cells produced
• Each identical to parent
• Meiosis
• Four haploid cells produced
• Differ from parent and one another

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Figure 10.11Page 174
germ cell
germ cell
2n
each chromosome duplicated during interphase
n
Meiosis I separation of homologues
Meiosis II separation of sister chromatids
gametes
gametes
2n
diploid number restored at fertilization
zygote
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Figure 10.10Page 172-173
Summary
Mitosis
Meiosis I
Meiosis II