meiosis and mitosis difference
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Meiosis A Source of Distinction
Why do you share some but not all characters of each parent?
What are the rules of this sharing game?
At one level, the
answers lie inmeiosis.
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Meiosis does two things -
1) Meiosis takes a cell with two copies of everychromosome (diploid) and makes cells with a
single copy of every chromosome (haploid).
This is a good idea if youre going to combine
two cells to make a new organism. This trick
is accomplished by halving chromosome
number.
In meiosis, one diploid cells produces four
haploid cells.
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Why do we need meiosis?
Meiosis is necessary to halve the number
of chromosomes going into the sex cells
Why halve the chromosomes in gametes?
At fertilization the male and female sex
cells will provide of the chromosomeseach so the offspring has genes from
both parents
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2) Meiosis scrambles the specific forms of
each gene that each sex cell (egg or sperm)receives.
This makes for a lot ofgenetic diversity. Thistrick is accomplished through independent
assortment and crossing-over.
Genetic diversity is important for the evolution
of populations and species.
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Meiosis
Parent cell
chromosome pair
Chromosomescopied
1st division - pairs split
2nd division produces
4 gamete cells with
the original no. of
chromosomes
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Meiosis mouse testes
Parent cell
4 gametes
1st division
2nd division
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The Stages of Meiosis:
aka: Reduction Division
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Meiosis I : Separates
Homologous Chromosomes
Interphase
Each of the chromosomes replicate
The result is two genetically identicalsister chromatids which remain
attached at their centromeres
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Prophase I
This is a crucial phase for mitosis.
During this phase each pair ofchromatids dont move to the equatoralone, they match up with their
homologous pair and fasten together(synapsis) in a group of four called atetrad.
Extremely IMPORTANT!!! It is during thisphase that crossing over can occur.
Crossing Over is the exchange ofsegments during synapsis.
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Metaphase I
The chromosomes line up at the
equator attached by their
centromeres to spindle fibers fromcentrioles.
Still in homologous pairs
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Anaphase I
The spindle guides the movement of the
chromosomes toward the poles
Sister chromatids remain attached
Move as a unit towards the same pole
The homologous chromosome moves
toward the opposite pole
Contrasts mitosis chromosomes appear as
individuals instead of pairs (meiosis)
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Telophase I
This is the end of the first meiotic cell
division.
The cytoplasm divides, forming two
new daughter cells.
Each of the newly formed cells has
half the number of the parent cells
chromosomes, but each chromosomeis already replicated ready for the
second meiotic cell division
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Cytokinesis
Occurs simultaneously withtelophase I Forms 2 daughter cells
Plant cells cell plate Animal cells cleavage furrows
NO FURTHER REPLICATION OF
GENETIC MATERIAL PRIOR TOTHE SECOND DIVISION OFMEIOSIS
Fi 13 Th f i i ll di i i M i i I
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Figure 13.7 The stages of meiotic cell division: Meiosis I
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Meiosis II :Separates sister chromatids
Proceeds similar to mitosis
THERE IS NO INTERPHASE II !
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Prophase II
Each of the daughter cells forms a
spindle, and the double stranded
chromosomes move toward theequator
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Metaphase II
The chromosomes are positioned
on the metaphase plate in a mitosis-
like fashion
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Anaphase II
The centromeres of sister
chromatids finally separate
The sister chromatids of each pairmove toward opposite poles
Now individual chromosomes
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Telophase II and Cytokinesis
Nuclei form at opposite poles of the
cell and cytokinesis occurs
After completion of cytokinesis thereare four daughter cells
All are haploid (n)
Figure 13 7 The stages of meiotic cell division: Meiosis II
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Figure 13.7 The stages of meiotic cell division: Meiosis II
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One Way Meiosis Makes Lots of
Different Sex Cells (Gametes)
Independent Assortment
Independent assortment produces 2n
distinct gametes, where n = the number
of unique chromosomes.
Thats a lot of diversity by this
mechanism alone.
In humans, n = 23 and 223 = 6,000,0000.
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Another Way Meiosis Makes Lots of Different
Sex Cells Crossing-Over
Crossing-over multiplies the already huge number of different gamete
types produced by independent assortment.
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The Key Difference Between Mitosis and Meiosis is
the Way Chromosomes Uniquely Pair and Align in
Meiosis
Mitosis The first (and
distinguishing)
division of meiosis
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