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11-4 Meiosis
11-4 Meiosis
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11-4 Meiosis
Each organism must inherit a single copy (or
allele) of every gene from each of its “parents.”
That means that gametes must be formed by a
process that separates the two alleles of each
gene so that each gamete ends up with just one
allele.
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Meiosis
Lesson 11-4
Overview
Meiosis
Chromosome Number
Review:
Chromosomes—strands of DNA and protein
inside the cell nucleus—are the carriers of
genes.
The genes are located in specific positions on
chromosomes.
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11-4 Meiosis
Chromosome Number
Chromosome Number
All organisms have
different numbers of
chromosomes.
A body cell in an adult fruit
fly has 8 chromosomes: 4
from the fruit fly's male
parent, and 4 from its
female parent.
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11-4 Meiosis
Chromosome Number
The chromosomes can be arranged in pairs.
Each of the 4 chromosomes that came from the
male parent has a corresponding chromosome
(carrying the genes for the same traits) from the
female parent.
These two sets chromosomes are homologous.
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11-4 Meiosis
Chromosome Number
A cell that contains both sets of homologous
chromosomes is said to be diploid.
The number of chromosomes in a diploid cell is
sometimes represented by the symbol 2N.
For Drosophila, the diploid number is 8, which can
be written as 2N=8.
The diploid cells of most adult organisms contain
two complete sets of inherited chromosomes and
two complete sets of genes.
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11-4 Meiosis
Chromosome Number
The gametes of sexually reproducing organisms
contain only a single set of chromosomes, and
therefore only a single set of genes.
These cells are haploid. Haploid cells are
represented by the symbol N.
For Drosophila, the haploid number is 4, which
can be written as N=4.
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11-4 Meiosis
Phases of Meiosis
Phases of Meiosis
Meiosis is a process of reduction
division in which the number of
chromosomes per cell (2N) is cut in half
through the separation of homologous
chromosomes in a diploid cell.
Gametes are produced in meiosis.
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11-4 Meiosis
Phases of Meiosis
Meiosis involves two divisions, meiosis I and
meiosis II.
At the end of meiosis I, there are 2 haploid cells.
By the end of meiosis II, the diploid cell that
entered meiosis has become 4 haploid cells.
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11-4 Meiosis
Phases of Meiosis
Meiosis I
Interphase I
Meiosis I
Prophase I
Metaphase I
Anaphase I
Telophase I
and
Cytokinesis
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11-4 Meiosis
Phases of Meiosis
Cells undergo a round of
DNA replication, forming
duplicate chromosomes.
Interphase I
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11-4 Meiosis
Phases of Meiosis
Each chromosome pairs
with its corresponding
homologous
chromosome to form a
tetrad.
MEIOSIS I
Prophase I
There are 4 chromatids in
a tetrad.
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11-4 Meiosis
Phases of Meiosis
When homologous chromosomes form tetrads in
meiosis I, they exchange portions of their
chromatids in a process called crossing over.
Crossing-over produces new combinations of
alleles.
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11-4 Meiosis
Phases of Meiosis
Spindle fibers attach to
the chromosomes.
MEIOSIS I
Metaphase I
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11-4 Meiosis
Phases of Meiosis
The fibers pull the
homologous
chromosomes toward
opposite ends of the
cell.
MEIOSIS I
Anaphase I
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11-4 Meiosis
Phases of Meiosis
Nuclear membranes form.
MEIOSIS I
Telophase I and
Cytokinesis
The cell separates into two
cells.
The two cells produced by
meiosis I have
chromosomes and alleles
that are different from each
other and from the diploid
cell that entered meiosis I.
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11-4 Meiosis
Phases of Meiosis
Meiosis I results in two haploid (N)
daughter cells, each with half the
number of chromosomes as the
original cell.
Because each pair of homologous
chromosomes was separated,
neither daughter cell has the two
complete sets of chromosomes.
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11-4 Meiosis
Phases of Meiosis
Meiosis II
The two cells produced by meiosis I now enter a
second meiotic division.
Unlike meiosis I, neither cell goes through
chromosome replication.
Remember: Each of the cell’s chromosomes has 2
chromatids (from replication that occurred back at
the beginning of meiosis.)
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11-4 Meiosis
Phases of Meiosis
Meiosis II
Telophase I and
Cytokinesis I
Meiosis II
Prophase II
Metaphase II
Anaphase II Telophase II
and
Cytokinesis
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11-4 Meiosis
Phases of Meiosis
MEIOSIS II
Prophase II
Metaphase II
A new spindle forms.
The chromosomes line
up in the center of cell.
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11-4 Meiosis
Phases of Meiosis
The sister chromatids
separate and move
toward opposite ends of
the cell.
MEIOSIS II
Anaphase II
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11-4 Meiosis
Phases of Meiosis
Meiosis II results in four
haploid (N) daughter cells.
MEIOSIS II
Telophase II and Cytokinesis
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11-4 Meiosis
Gamete Formation
Gamete Formation
In male animals, meiosis results in four equal-sized
gametes called sperm.
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11-4 Meiosis
Gamete Formation
In many female animals, only one egg results from
meiosis. The other three cells, called polar bodies,
are usually not involved in reproduction.
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Meiosis
Lesson 11-4
Overview
Meiosis
Gametes to Zygotes
Fertilization—the fusion of male and female
gametes—generates new combinations of alleles in
a zygote.
The zygote undergoes cell division by mitosis and
eventually forms a new organism.
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Meiosis
Lesson 11-4
Overview
Meiosis
Comparing Mitosis and Meiosis
In mitosis, when the two sets of genetic
material separate, each daughter cell receives
one complete set of chromosomes. In
meiosis, homologous chromosomes line up
and then move to separate daughter cells.
Mitosis does not normally change the
chromosome number of the original cell. This
is not the case for meiosis, which reduces the
chromosome number by half.
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Comparing Mitosis and Meiosis
Comparing Mitosis and Meiosis
Mitosis results in the production of two
genetically identical diploid cells. Meiosis
produces four genetically different
haploid cells.
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11-4 Meiosis
Comparing Mitosis and Meiosis
Mitosis
• Cells produced by mitosis have the same number
of chromosomes and alleles as the original cell.
• Mitosis is a single cell division.
• Mitosis allows an organism to grow and replace
cells.
• Some organisms reproduce asexually by mitosis.
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11-4 Meiosis
Comparing Mitosis and Meiosis
Meiosis
• Cells produced by meiosis have half the number
of chromosomes as the parent cell.
• Meiosis requires two rounds of cell division.
• These cells are genetically different from the
diploid cell and from each other.
• Meiosis is how sexually-reproducing organisms
produce gametes.
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11-4 Meiosis
Gene Linkage
Gene Linkage
• Thomas Hunt Morgan’s research on fruit flies
led him to the principle of linkage.
• Morgan discovered that many of the more than
50 Drosophila genes he had identified
appeared to be “linked” (inherited) together.
• They seemed to violate the principle of
independent assortment.
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Meiosis
Lesson 11-4
Overview
Meiosis
Gene Linkage
For example, Morgan used a fly with reddish-orange eyes and miniature
wings in a series of test crosses.
His results showed that the genes for those two traits were almost always
inherited together.
Only rarely did the genes separate from each other.
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11-4 Meiosis
Gene Linkage
Morgan and his associates grouped all of the
fly’s genes into four linkage groups.
Each group assorted independently from the
other groups BUT all the genes in one group
were inherited together.
As it turns out, Drosophila has four linkage
groups and four pairs of chromosomes.
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Meiosis
Lesson 11-4
Overview
Meiosis
Gene Linkage
Morgan’s findings led to two remarkable
conclusions:
•First, each chromosome is actually a group of
linked genes.
•Second, it is the chromosomes that assort
independently, not individual genes.
Alleles of different genes tend to be inherited
together when those genes are located on the
same chromosome.
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11-4 Meiosis
Gene Maps
Gene Mapping
Remember that:
• Crossing-over during meiosis sometimes
separates genes that had been on the same
chromosomes onto the homologous
chromosome.
• Crossover events occasionally separate linked
genes and produce new combinations of alleles.
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11-4 Meiosis
Gene Maps
Gene Mapping
Alfred Sturtevant, a student of Morgan, reasoned
that the farther apart two genes were on a
chromosome, the more likely it would be that a
crossover event would occur between them.
If two genes are close together, then crossovers
between them should be rare. If two genes are far
apart, then crossovers between them should be
more common.
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Meiosis
Lesson 11-4
Overview
Meiosis
Gene Mapping
By this reasoning, he could use the
frequency of crossing-over between
genes to determine their distances
from each other.
Sturtevant gathered lab data and
presented a gene map showing the
relative locations of each known
gene on one of the Drosophila
chromosomes.
Sturtevant’s method has been used
to construct gene maps ever since
this discovery.
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