Meiosis Gamete Production

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MeiosisGamete Production
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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 in meiosis.
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In The Beginning Two

• Humans and many other complex multi-celled
  organisms incorporate genetic recombination in
  their reproduction
• Reproduction in which there is a re-mixing of the
  genetic material is called sexual reproduction
• Two cells, a sperm and an egg, unite to form a
  zygote, the single cell from which the organism
  develops
• Meiosis is the process of producing sperm and
  eggs (gametes)

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Fertilization Results In A Diploid Zygote
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Fertilization Results In A Diploid Zygote
Egg 1n
Haploid nucleus
Sperm 1n
Haploid nucleus
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Fertilization Results In A Diploid Zygote
Egg 1n
Haploid nucleus
Sperm 2n
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Fertilization Results In A Diploid Zygote
Egg 1n
Haploid nucleus
Sperm 1n
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From Zygote to Embryo
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Chromosomes, Chromatids and Centromeres
Chromosome arm
Two identical chromosomes
Centromere
Chromosome arm
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From Zygote to Embryo
Cleavage
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From Zygote to Embryo
Cleavage
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From Zygote to Embryo
Cleavage
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From Zygote to Embryo
Cleavage
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From Zygote to Embryo
Morula
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Gametes Are Haploid

• Gametes must have half the genetic material of a
  normal cell
• If the genetic material in the gametes was not
  halved, when they combined the zygote would have
  more genetic material than the parents
• Meiosis is specialized cell division resulting in
  cells with half the genetic material of the
  parents
• Gametes have exactly one set of chromosomes, this
  state is called haploid (1n)
• Regular cells have two sets of chromosomes, this
  state is called diploid (2n)

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Meiosis A Source of Distinction
Meiosis does two things -
1) Meiosis takes a cell with two copies of every
chromosome (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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Meiosis A Source of Distinction
The second accomplishment of meiosis
2) Meiosis scrambles the specific forms of each
gene that each sex cell (egg or sperm) receives.
This makes for a lot of genetic diversity. This
trick is accomplished through independent
assortment and crossing-over.
Genetic diversity is important for the evolution
of populations and species.
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Keys to Understanding Meiosis
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Stages Of Meiosis

• Meiosis resembles mitosis except that it is
  actually two divisions not one
• These divisions are called Meiosis I and Meiosis
  II
• Meiosis I results in haploid cells with
  chromosomes made up of two chromatids
• Meiosis II is essentially mitosis on haploid
  cells
• Stages of meiosis resemble mitosis with two
  critical differences the first in prophase I and
  the second in Metaphase I

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Oogenesis A Path of Meiosis in Humans
Woman are less busy in meiosis than men meiosis
produces only a few hundred mature eggs over a
lifetime.
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Spermatogenesis Another Path of Meiosis in
Humans
Men are busy meiosis produces roughly
250,000,000 sperm per day.
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Stages Of Meiosis - Meiosis I

• Prophase I - The beginning phase -
• DNA which was unraveled and spread all over the
  nucleus is condensed and packaged
• Homologous chromosomes (each made of two
  identical chromatids) come together and form
  tetrads (4 chromatids)
• Crossing over, in which chromatids within tetrads
  exchange genetic material, occurs
• Metaphase I - Middle stage - Tetrads line up
  along the equator of the cell

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One 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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Another Way Meiosis Makes Lots of Different Sex
Cells (Gametes) Independent Assortment
Independent assortment produces 2n distinct
gametes, where n the number of unique
chromosomes.
In humans, n 23 and 223 8,000,0000.
Thats a lot of diversity by this mechanism alone.
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Stages Of Meiosis - Meiosis I

• Anaphase I - One copy of each chromosome still
  composed of two chromatids moves to each pole of
  the cell
• Telophase I - End stage - New nuclear membranes
  are formed around the chromosomes and cytokinesis
  (cytoplasm division) occurs resulting in two
  haploid daughter cells

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Stages Of Meiosis Meiosis I
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Stages Of Meiosis - Meiosis II

• Prophase II - Cells do not typically go into
  interphase between meiosis I and II, thus
  chromosomes are already condensed
• Metaphase II - Chromosomes line up at the equator
  of the two haploid cells produced in meiosis I
• Anaphase II - Chromosomes made up of two
  chromatids split to make chromosomes with one
  chromatid which migrate to the poles of the cells
• Telophase II - Cytokinesis and reformation of the
  nuclear membrane in haploid cells each with one
  set of chromosomes made of one chromatid

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Stages Of Meiosis Meiosis II
The products of meiosis are 4 haploid cells each
with a unique set of chromosomes.
Prophase II
The products of mitosis are 2 diploid cells with
identical chromosomes.
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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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Boy or Girl? The Y Chromosome Decides
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Boy or Girl? The Y Chromosome Decides
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Crossing Over
Because of crossing over, every gamete receives a
unique set of genetic information.
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Between Independent Assortment and Crossing-Over,
No Two Gametes Are Identical.
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