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Meiosis
Is a reduction division from
diploid somatic cells (2n) to produce
haploid gametes (n).
The reduction is in the chromosome
number in each nucleus.
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Interphase
In the S-phase of the interphase
before meiosis begins, DNA replication
takes place.
Chromosomes are replicated and
these copies are attached to each
other at the centromere.
The attached chromosome and its
copy are known as sister chromatids.
Following S-phase, further growth and
preparation take place for meiosis.
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An homologous pair of chromosomes…
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An homologous pair of chromosomes…
…replicates during S-phase of interphase…
centromere
sister chromatids
…giving two pairs of sister chromatids,
each joined at the centromere.
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The homologous pair associates during prophase I,
through synapsis…
…making a bivalent.
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Prophase I
The homologous chromosomes associate
with each other, to form bivalents.
Crossing-over between non-sister
chromatids can take place. This results
in recombination of alleles and is a
source of genetic variation in gametes.
The pairs of sister chromatids are joined by
the centromere.
Non-sister chromatids are next to each
other but not joined.
This bivalent is composed of:
- One pair of homologous chromosomes
- Which have replicated to form two pairs of
sister chromatids.
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The homologous pair is separated in
anaphase I. The joined sister chromatids
are separated in anaphase II.
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Crossing-over might take place between non-sister
chromatids in prophase I…
…leading to recombination of alleles.
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Prophase I
The homologous chromosomes
associate with each other.
Crossing-over between non-sister
chromatids can take place. This results
in recombination of alleles and is a
source of genetic variation in gametes.
Crossing-over is more likely to occur between genes
which are further apart. In this example, there will be
more recombination between D and E than between C
and D.
During prophase, the nuclear membrane also breaks
down and the centrioles migrate to the poles of the cell.
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Crossing-Over
Synapsis
Homologous chromosomes
associate
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Increases genetic variation through
recombination of linked alleles.
Chiasma Formation
Neighbouring non-sister chromatids
are cut at the same point.
A Holliday junction forms as the DNA
of the cut sections attach to the
open end of the opposite non-sister
chromatid.
Recombination
As a result, alleles are
swapped between nonsister chromatids.
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Crossing-Over
Increases genetic variation through
recombination of linked alleles.
Crossing over leads to more variation in gametes.
This is the standard notation for writing genotypes of
alleles on linked genes. More of this later when we
study 10.2 Dihybrid crosses and gene linkage.
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Metaphase I
The bivalents line up at the equator.
Random orientation occurs and is a
significant source of genetic variation.
There are 2n possible orientations in
metaphase I and II. That is 223 in
humans – or 8,388,068 different
combinations in gametes!
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In anaphase I, the homologous pair is separated but
the sister chromatids remain attached.
This is the reduction division.
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Anaphase I
Spindle fibres contract.
Homologous pairs are separated and
pulled to opposing poles.
This is the reduction division.
Non-disjunction here will affect the
chromosome number of all four
gametes.
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Telophase I
New nuclei form and the cytoplasm
begins to divide by cytokinesis.
The nuclei are no longer diploid.
They each contain one pair of sister
chromatids for each of the species’
chromosomes.
If crossing-over and recombination has
occurred then the sister chromatids
will not be exact copies.
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Interphase
There is no Synthesis phase in
Interphase II.
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Prophase II
The nuclei break down.
No crossing-over occurs.
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Metaphase II
Pairs of sister chromatids align at the
equator. Spindle fibres form and
attach at the centromeres.
Random orientation again
contributes to variation in the
gametes, though not to such an
extent as in metaphase I.
This is because there is only a
difference between chromatids
where crossing-over has taken place.
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Metaphase I vs II: Genetic Variation
Lots of variation in gametes produced
• Random orientation of homologous
pairs, which may have a great diversity
in alleles present
• Therefore many possible combinations
of alleles could be pulled to each pole
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Some variation in gametes produced
• Random orientation of sister chromatids
• Variation only in regions where crossing
over has taken place in prophase I
(recombination of alleles)
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Anaphase II
Spindle fibres contract and the
centromeres are broken.
The pairs of sister chromatids are
pulled to opposing poles.
Non-disjunction here will lead to two
gametes containing the wrong
chromosome number.
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Telophase II
New haploid nuclei are formed.
Cytokinesis begins, splitting the cells.
The end result of meiosis is four
haploid gamete cells.
Fertilisation of these haploid gametes
will produce a diploid zygote.
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Outline the differences between the behaviour of
chromosomes in Mitosis and Meiosis
Mitosis
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5 marks
Meiosis
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Outline the differences between the behaviour of
chromosomes in Mitosis and Meiosis
5 marks
Mitosis
Meiosis
One division
Two divisions
Diploid cells produced
Haploid gametes produced
No crossing-over in prophase
Crossing-over in prophase I
No chiasmata formation
Chiasmata form
Homologous pairs do not associate and
line up at the equator in metaphase
Homologous pairs associate as bivalents
and lined up at the equator in metaphase I
Sister chromatids separate in anaphase
Homologous pairs separate in anaphase I
Sister chromatids separate in anaphase II
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Genetic Variation
is almost infinite as a result of meiosis.
Crossing-over in prophase I
Leads to recombination of alleles on the
chromosomes.
Random orientation in metaphase I
Huge number of maternal/paternal chromosome
combinations possible in the final gametes. There
are over 8million possible orientation in humans
(223 orientations)
Random orientation in metaphase II
Further genetic variation arises where there are
genetic differences between sister chromatids as
a result of crossing-over in prophase I.
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Genetic Variation
is almost infinite as a result of meiosis.
Crossing-over in prophase I
Leads to recombination of alleles on the
chromosomes.
Random orientation in metaphase I
Huge number of maternal/paternal chromosome
combinations possible in the final gametes. There
are over 8million possible orientation in humans
(223 orientations)
Random orientation in metaphase II
Further genetic variation arises where there are
genetic differences between sister chromatids as
a result of crossing-over in prophase I.
Even more variation!
Random fertilisation during sexual reproduction
ensures even greater variation within the population.
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Mendel’s Law of Independent Assortment
“The presence of an allele of one of the
genes in a gamete has no influence over
which allele of another gene is present.”
A and B are different genes on
different chromosomes.
A is dominant over a.
B is dominant over b.
This only holds true for unlinked genes
(genes on different chromosomes).
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Random Orientation vs Independent Assortment
“The presence of an allele of one of the
genes in a gamete has no influence over
which allele of another gene is present.”
Random Orientation refers to the
behaviour of homologous pairs of
chromosomes (metaphase I) or pairs of
sister chromatids (metaphase II) in
meiosis.
Independent assortment refers to the
behaviour of alleles of unlinked genes as
a result of gamete production (meiosis).
Due to random orientation of the chromosomes in
metaphase I, the alleles of these unlinked genes have
become independently assorted into the gametes.
Animation from Sumanas:
http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html
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Mendel and Meiosis
“The presence of an allele of one of the genes in a gamete has no
influence over which allele of another gene is present.”
Mendel deduced that characteristics were determined by the interaction
between pairs of alleles long before the details of meiosis were known.
Where Mendel states that pairs of alleles of a gene separate independently
during gamete production, we can now attribute this to random orientation of
chromosomes during metaphase I.
Mendel made this deduction when working with
pea plants. He investigated two separate traits
(colour and shape) and performed many test
crosses, recording the ratios of phenotypes
produced in subsequent generations.
It was rather fortunate that these two traits
happened to be on separate chromosomes
(unlinked genes)! Remember back then he did not
know about the contents of the nucleus.
Chromosomes and DNA were yet to be discovered.
We will use his work as an example of dihybrid
crosses in the next section.
Animation from Sumanas:
http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html
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Chromosome Collection
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Check your language. This image shows…
A. Four separate chromosomes.
B. A bivalent.
C. One pair of sister chromatids.
D. Non-disjunction.
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Check your language. This image shows…
A. Four separate chromosomes.
B. A bivalent.
C. One pair of sister chromatids.
D. Non-disjunction.
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Check your language. This image shows…
A. Two separate chromosomes.
B. A bivalent.
C. One pair of sister chromatids.
D. Crossing-over.
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Check your language. This image shows…
A. Two separate chromosomes.
B. A bivalent.
C. One pair of sister chromatids.
D. Crossing-over.
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Check your language. This image shows…
A. Two separate chromosomes.
B. A bivalent.
C. One pair of sister chromatids.
D. Homologous chromosomes.
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Check your language. This image shows…
A. Two separate chromosomes.
B. A bivalent.
C. One pair of sister chromatids.
D. Homologous chromosomes.
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Check your language. This image shows…
A. 8 separate chromosomes.
B. Two bivalents.
C. Two pairs of sister chromatids.
D. Two homologous chromosomes.
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Check your language. This image shows…
A. 8 separate chromosomes.
B. Two bivalents.
C. Two pairs of sister chromatids.
D. Two homologous chromosomes.
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Which phase of meiosis is shown? Why?
A. Interphase
B. Prophase I
C. Metaphase I
D. Metaphase II
Reason:
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Which phase of meiosis is shown? Why?
A. Interphase
B. Prophase I
C. Metaphase I
D. Metaphase II
Reason:
• Homologous pairs are aligned (at equator), so must
be metaphase.
• Crossing-over has already taken place, so must be
after prophase I.
• Homologous pairs have not yet separated, so must
be still in meiosis I (metaphase I).
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Which phase of meiosis is shown? Why?
A. Interphase
B. Prophase I
C. Metaphase I
D. Metaphase II
Reason:
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Which phase of meiosis is shown? Why?
A. Interphase
B. Prophase I
C. Metaphase I
D. Metaphase II
Reason:
• Homologous pairs have associated.
• Crossing-over has taken place.
• Homologous pairs have not aligned at the equator.
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Which phase of meiosis is shown? Why?
A. Interphase
B. Prophase I
C. Metaphase I
D. Metaphase II
Reason:
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Which phase of meiosis is shown? Why?
A. Interphase
B. Prophase I
C. Metaphase I
D. Metaphase II
Reason:
• Homologous pairs have not yet associated.
• Replication has taken place.
• Crossing-over has not yet taken place.
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