Sexual Reproduction
and Meiosis
Chapter 11
1
Sexual life cycle
• Made up of meiosis and fertilization
• Diploid cells
– Somatic cells of adults have 2 sets of chromosomes
• Haploid cells
– Gametes have only 1 set of chromosomes
• Allows offspring to inherit genetic material from 2 parents
2
• Life cycles of sexually reproducing organisms
involve the alternation of haploid and diploid
stages
• Some life cycles include longer diploid phases,
some include longer haploid phases
• In most animals, diploid state dominates
– Zygote first undergoes mitosis to produce more
diploid cells
– multicellularity
– Later in the life cycle, some of these diploid cells
undergo meiosis to produce haploid gametes
– (sperm & eggs)
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Meiosis
•
•
•
DNA replicates (chromosome # doubled)
Meiosis I
– Stages:
• Prophase I – synapsis occurs
• Metaphase I – homologous chromosome pairs line up (4
chromosomes)
• Anaphase I – homologous chromosomes seperate
• Telophase I
– Results in 2 cells, but still diploid number
Meiosis II
– Stages
• Prophase II
• Metaphase II
• Anaphase II – sister chromosomes seperate
• Telophase II
– Results in 4 cells, haploid number, one of each homologous pair in every cell
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Features of Meiosis
• Meiosis includes 2 rounds of division
– Meiosis I and meiosis II
– Each has prophase, metaphase, anaphase, and
telophase stages
• Synapsis (crossing-over)
– During early prophase I
– Homologous chromosomes become closely
associated (eventually swapping DNA)
– Includes formation of synaptonemal complexes
• Formation also called tetrad or bivalents
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• First meiotic division is termed the “reduction
division”
– Results in daughter cells that contain one homologue
from each chromosome pair
• No DNA replication between meiotic divisions
• Second meiotic division does not further reduce
the number of chromosomes
– Separates the sister chromatids for each homologue
– Meiosis II is very much like mitosis (but half the DNA)
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Prophase I
•
Chromosomes coil tighter and
become visible, nuclear envelope
disappears, spindle forms
•
Each chromosome composed of 2
sister chromatids
•
Synapsis (crossing-over)
– Homologues become closely
associated
– Crossing over occurs between
nonsister chromatids
– Nonsister chromatids remain
attached at chiasmata
• Chiasmata move to the end
of the chromosome arm
before metaphase I
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Crossing over
• Occurs between nonsister chromatids
• Allows the maternal and paternal
homologues to exchange chromosomal
material (recombination)
• Alleles of genes that were formerly on
separate homologues can now be found
on the same homologue
• Chiasmata – site of crossing over
– Contact maintained until anaphase I
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Metaphase I
•
Terminal chiasmata hold homologues
together following crossing over
•
Microtubules from opposite poles attach to
each homologue (not each sister
chromatid)
•
Homologues are aligned at the metaphase
plate side-by-side
Orientation of each pair of homologues on
the spindle is random
•
•
(This sets up independent assortment)
13
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14
Many random combinations of the homologous pairing of 3 chromosomes
Anaphase I
•
Microtubules of the spindle shorten
– Chiasmata break
•
Homologues are separated from
each other and move to opposite
poles
– Sister chromatids remain
attached to each other at their
centromeres
•
Each pole has a complete haploid set
of chromosomes consisting of one
member of each homologous pair
Independent assortment of maternal
and paternal chromosomes occurs
•
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Telophase I
•
Nuclear envelope re-forms
around each daughter nucleus
•
Sister chromatids are no longer
identical because of crossing
over (prophase I)
•
Cytokinesis may or may not occur
after telophase I
•
Meiosis II occurs after an interval
of variable length
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Meiosis II
• Resembles a mitotic division
– But with a haploid rather than diploid cell
• Prophase II: nuclear envelopes dissolve and new spindle
apparatus forms
• Metaphase II: chromosomes align on metaphase plate
• Anaphase II: sister chromatids are separated from each
other
• Telophase II: nuclear envelope re-forms around 4 sets of
daughter chromosomes; cytokinesis follows
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Prophase II
Following a brief interphase,
with no S phase, meiosis II
begins.
During prophase II,
a new spindle apparatus forms
in each cell, and the nuclear
envelope breaks down.
In some species the nuclear
Envelope does not re-form in
telophase I, removing the need for
Nuclear envelope breakdown.
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Metaphase II
In metaphase II, chromosomes
consisting of sister chromatids
joined at the centromere align
along the metaphase plate in
each cell.
Now, kinetochore microtubules
from opposite poles attach to
kinetochores of sister chromatids,
as in mitosis.
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Anaphase II
When microtubules shorten
In anaphase II, sister
chromatids
are pulled to opposite poles
of the cells, as in mitosis.
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Telophase II
The nuclear membranes re-form
around four different clusters of
chromosomes.
After cytokinesis, four haploid
cells result.
No two cells are alike
due to the random alignment of
homologous pairs at
metaphase I and crossing over
during prophase I.
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Final result of Meiosis
• Four cells containing haploid sets of chromosomes
• In animals, these cells develop directly into gametes
– In Spermtogenesis: 4 sperm
– In Oogenesis: 1 egg, 3 polar bodies
• In plants, fungi, and many protists, haploid cells then divide
mitotically
– Produce greater number of gametes
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Errors in Meiosis
• Nondisjunction – failure of chromosomes to move to opposite poles
during either meiotic division
• Aneuploid gametes – gametes with missing or extra chromosomes
• Most common cause of spontaneous abortion in humans
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Meiosis vs. Mitosis
Meiosis is characterized by 4 features:
1. Synapsis and crossing over
2. Sister chromatids remain joined at their centromeres throughout
meiosis I
3. Kinetochores of sister chromatids attach to the same pole in meiosis I
4. DNA replication is suppressed between meiosis I and meiosis II
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Mitosis
Meiosis I
Metaphase I
Anaphase I
Crossovers and
sister chromatid
cohesion lock
homologues
together.
Microtubules
connect to the
kinetochores of
sister chromatids so
that homologues are
pulled toward
opposite poles.
Microtubules pull
the homologous
chromosomes
apart, but sister
chromatids are
held together at
the centromere.
Metaphase
Homologues do
not pair;
kinetochores of
sister chromatids
remain separate;
microtubules
attach to both
kinetochores on
opposite sides of
the centromere.
Anaphase
Microtubules
pull sister
chromatids
apart.
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MEIOSIS I
Prophase I
Metaphase I
Anaphase I
Telophase I
Parent cell (2n)
Paternal
homologue
Chromosome
replication
Homologous chromosomes pair;
synapsis and crossing over occur.
Paired homologous chromosomes
align on metaphase plate.
Homologous chromosomes separate;
sister chromatids remain together.
MITOSIS
Prophase
Homologous
chromosomes
Metaphase
Anaphase
Telophase
Chromosome
replication
Maternal
homologue
Homologous chromosomes
do not pair.
Two
daughter
cells
(each 2n)
Individual homologues align
on metaphase plate.
Sister chromatids separate, cytokinesis occurs, and two cells
result, each containing the original number of homologues.
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MEIOSIS II
Prophase II Metaphase II
Anaphase II
Telophase II
Four
daughter
cells
(each n)
Chromosomes align, sister chromatids separate, and four haploid cells result,
each containing half the original number of homologues.
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