Meiosis Gamete Production

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MeiosisGamete Production

• Timothy G. Standish, Ph. D.

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Chromosomes, Chromatids and Centromeres
Chromosome arm
Two identical chromosomes
Centromere
Chromosome arm
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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 2n
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 2n
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From Zygote to Embryo
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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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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 chromotids
• 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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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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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 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 I
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Stages Of Meiosis Meiosis II
The products of meiosis are 4 haploid cells each
with a unique set of chromosomes.
Telophase I
Prophase II
The products of mitosis are 2 diploid cells with
identical chromosomes.
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Crossing Over
Because of crossing over, every gamete receives a
unique set of genetic information.
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