Meiosis Stages of Meiosis Review Comparison of Meiosis and Mitosis .
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Transcript of Meiosis
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Meiosis
Chapter 10
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10.1 Sexual Reproduction
• Chromosomes are duplicated in germ cells
• Germ cells undergo meiosis and cytoplasmic division
• Cellular descendents of germ cells become gametes
• Gametes meet at fertilization
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Asexual Reproduction
• Single parent produces offspring
• All offspring are genetically identical to one another and to parent
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Sexual Reproduction
• Involves
– Meiosis
– Gamete production
– Fertilization
• Produces genetic variation among offspring
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10.2 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
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Sexual Reproduction Shuffles Alleles
• Through sexual reproduction, offspring inherit new combinations of alleles, which leads to variations in traits
• This variation in traits is the basis for evolutionary change
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Gamete Formation
• Gametes are sex cells (sperm, eggs)
• Arise from germ cells
testes
ovaries
anther ovary
Figure 10.2aPage 164
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Chromosome Number
• Sum total of chromosomes in a cell
• Germ cells are diploid (2n)
• Gametes are haploid (n)
• Meiosis halves chromosome number
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10.3 Meiosis: Two Divisions
• Two consecutive nuclear divisions
– Meiosis I
– Meiosis II
• DNA is not duplicated between divisions
• Four haploid nuclei form
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Meiosis I
Each homologue in the cell pairs with its partner,
then the partners separate
In-text figurePage 165
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Meiosis II
• The two sister chromatids of each duplicated chromosome are separated from each other
one chromosome (duplicated)
two chromosomes (unduplicated)
In-text figurePage 165
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10.4 Meiosis I - Stages
Prophase I Metaphase I Anaphase I Telophase I
Figure 10.4 Page 167
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Prophase I
• Each duplicated chromosome pairs with homologue
• Homologues swap segments
• 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
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Anaphase I
• Homologous
chromosomes
segregate
• The sister
chromatids remain
attached
Figure 10.4 Page 167
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Telophase I
• The chromosomes
arrive at opposite
poles
• Usually followed by
cytoplasmic division
Figure 10.4 Page 167
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Prophase II
• Microtubules attach to the kinetochores of the duplicated chromosomes
Figure 10.4 Page 167
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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 chromosomes
Figure 10.4 Page 167
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Telophase II
• The chromosomes arrive at opposite ends of the cell
• A nuclear envelope forms around each set of chromosomes
• Four haploid cellsFigure 10.4 Page 167
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Crossing Over
•Each chromosome
becomes zippered to its
homologue
•All four chromatids are
closely aligned
•Nonsister chromosomes
exchange segmentsFigure 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
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Random Alignment
• During transition between prophase I and metaphase I, microtubules from spindle poles attach to kinetochores of chromosomes
• Initial contacts between microtubules and chromosomes are random
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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 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 Chromosome
Combinations
or
or
or
1 2 3
Figure 10.6Page 169
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Plant Life Cycle
multicelled sporophyte
multicelled gametophytes
zygote
gametes spores
meiosisfertilization
mitosis
mitosis
Diploid
Haploid
Figure 10.7Page 170
10.5 From Gametes to Offspring
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Animal Life Cycle
multicelled body
gametes
zygote
meiosisfertilization
mitosis
Diploid
Haploid
Figure 10.7Page 170
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Oogenesis
GrowthMeiosis I,
Cytoplasmic DivisionMeiosis II,
Cytoplasmic Division
ovum (haploid)
primary oocyte (diploid)
oogonium (diploid)
secondary oocyte
(haploid)
first polar body
(haploid)
three polar bodies
(haploid)
Figure 10.8Page 171
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Spermatogenesis
GrowthMeiosis I,
Cytoplasmic DivisionMeiosis II,
Cytoplasmic Division
cell differentiation, sperm formation
spermatids (haploid)
secondary spermatocytes
(haploid)
primary spermatocyte
(diploid)
spermato-gonium (diploid )
sperm (mature, haploid male
gametes)
Figure 10.9Page 171
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Fertilization
• Male and female gametes unite and
nuclei fuse
• Fusion of two haploid nuclei produces
diploid nucleus in the zygote
• Which two gametes unite is random
– 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• Functions
– Asexual reproduction– Growth, repair
• Occurs in somatic cells
• Produces clones
10.6 Mitosis & Meiosis Compared
Meiosis• Function
– Sexual reproduction
• Occurs in germ cells
• Produces variable offspring
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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
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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