Meiosis
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specialized cell division process
produces haploid gametes
Each gamete receives 1 member of each pair of
homologous chromosomes
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Meiosis consists of 1 round of DNA replication,
followed by 2 rounds of nuclear divisions
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These events occur in 2 stages:
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Meiosis I
Meiosis II
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Both members of
each homologous
chromosome pair are
replicated prior to
meiosis
After replication,
each chromosome
consists of 2 sister
chromatids
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Each daughter cell
receives one member
of each pair of
homologous
chromosomes
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Sister chromatids
separate into
independent
chromosomes
Each daughter cell
receives one of these
independent
chromosomes
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Meiosis reduces chromosome number by half,
producing 1n gametes (eggs and sperm)
Fusion of gametes (fertilization) combines two
chromosome sets to produce diploid (2N)
zygote
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Phases of meiosis have same names as the
phases in mitosis, followed by I or II to
distinguish the two nuclear divisions that occur
in meiosis
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Meiosis I separates homologous chromosomes
into 2 haploid daughter nuclei
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Meiosis II separates sister chromatids into 4
gametes
1. Homologous chromosomes pair up
2. Crossing over (genetic recombination) occurs
between homologues
 Enzymes facilitate exchange of DNA between
arms of adjacent chromatids, producing
chiasmata
3. Spindle microtubules assemble, nuclear envelope
breaks down, and microtubules capture
chromosomes
Duplicated homologous chromosomes are
pulled into a line perpendicular to the spindle
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Chromosomes line up as pairs of replicated
homologous chromosomes
Meiotic Anaphase I
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
Homologous
chromosome pairs
separate

Each homologous
chromosome pair moves
to a pole, pulled by
microtubules
Meiotic Telophase I
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Spindle microtubules
disappear

Cytokinesis occurs
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Nuclear envelopes may
reappear
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Chromosomes usually
remain condensed
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Meiotic Prophase II
Spindle microtubules reform and capture duplicated
chromosomes
 Each chromatid contains a kinetochore
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Meiotic Metaphase II
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Meiotic Anaphase II
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Duplicated chromosomes line up singly, perpendicular to the
spindle
Chromatids separate
Meiotic Telophase II
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Cytokinesis occurs, nuclear membranes reform, chromosomes
relax
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When Do Mitotic and Meiotic
Cell Divisions Occur in the Life
Cycles of Eukaryotes?
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Fungi and unicellular
algae
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Most of life cycle is
haploid
•
Asexual reproduction
by mitotic cell division
produces a population
of identical, haploid
cells
•
Life cycle Chlamydomonas
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Most animals
•
Most of cycle is in
diploid state
•
Haploid gametes are
formed by meiosis
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Gametes fuse to
form a diploid zygote
•
Zygote develops into
adult through mitotic
cell divisions
•
Plants
•
Includes both
multicellular diploid and
multicellular haploid body
forms
•
Multicellular diploid body
gives rise to haploid
spores, through meiosis
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Spores undergo mitosis
to produce a multicellular
haploid generation
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Eventually, certain
haploid cells
differentiate into haploid
gametes
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2 gametes fuse to form
a diploid zygote
•
Zygote grows by mitotic
cell division into a
diploid multicellular
diploid generation
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How Do Meiosis and Sexual
Reproduction Produce Genetic
Variability?
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Genetic variability among organisms is
essential in a changing environment
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Mutations produce new variation but are
relatively rare occurrences
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Randomized line up and separation of homologous
chromosomes in Meiotic Metaphase I and Anaphase I
increase variation
– # of possible combinations is 2n, where n = number of
homologous pairs
Metaphase I
Anaphase I
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Variation enhanced by genetic
recombination
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Crossing over creates chromosomes with
new allele combinations
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Combined with homologue shuffling in
Metaphase/Anaphase I, each gamete
produced in meiosis is virtually unique
•
Fusion of gametes from 2 individuals
further increases possible 2n combinations
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Gametes from 2 humans could produce
about 64 trillion different 2n combinations
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Taken together with crossing over, each
human individual is absolutely genetically
unique