Meiosis and Sexual Reproduction

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Meiosis and Sexual Reproduction
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Homologous Chromosomes

• Chromosomes of each pair are similar in length
  and centromere position
• Both carry genes controlling the same inherited
  characteristics

• We inherit one chromosome of each homologous
  pair from our mother and the other from our
  father(Therefore, these are NOT identical!)

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Meiosis

• A type of cell division that
• Occurs only in the sex organs
• Sperm cells in men
• Eggs in women
• Produces 4 genetically unique cells each with
  HALF the number of chromosomes as the parent cell
• Keeps the number of chromosomes in a species the
  same generation to generation

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Karyotype
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Two Types of Cells

• Body cells are called somatic cells.
• Germ cells develop into sex cells.
• Gametes are sex cells egg and sperm.
• Gametes have DNA that can be passed to offspring

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Haploid vs. Diploid

• Fertilization between egg and sperm occurs in
  sexual reproduction.
• Diploid (2n) cells have two copies of every
  chromosome.
• Body cells are diploid.
• Half the chromosomes come from each parent

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Haploid (n) cells have one copy of every
chromosome.

• Gametes are haploid.
• Gametes have 22 autosomes and 1 sex chromosome

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Meiosis makes haploid cells from diploid cells
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Some details of meiosis

• As with mitosis, meiosis begins after the
  chromosomes have been duplicated during
  Interphase
• Meiosis consists of two distinct parts
• Meiosis I
• Meiosis II

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Meiosis I

• Homologous chromosomes exchange genetic material
  (cross over) and separate during anaphase

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• Crossing over results in new combinations of
  genes

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Meiosis II

• Sister chromatids are separated much as they are
  in mitosis except the resulting cells are haploid
  and not diploid

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Key Differences between Mitosis and Meiosis

• Mitosis
• One cell division
• No pairing of homologous chromosomes
• Produces two offspring cells each with the same
  number of chromosomes as the parent cell (46)
• Produces genetically identical copies of the
  parent cell

• Meiosis
• Two cell divisions
• Homologous chromosomes pair
• Produces four offspring cells each with one set
  of chromosomeshalf the number as the parent cell
  (23)
• Creates genetically unique cells different from
  the parent cell
• This is because homologous chromosomes exchange
  genetic information

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Which process is this?

• Provides for growth, repair, and asexual
  reproduction
• Produces TWO daughter cells that are genetically
  identical to the parent cell
• Involves one division of the DNA
• Diploid parent cells produce diploid daughter
  cells
• Happens in most cells of the body

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Which process is this?

• Yields haploid daughter cells with only ONE
  member of the homologous chromosome pair
• Produces FOUR daughter cells that are genetically
  different from the parent cell
• Involves TWO divisions of the DNA
• Exchange of genetic material occurs between
  homologous chromosomes

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Gametogenesis

• All sperm and eggs produced during gametogenesis
  are genetically unique.
• What would happen if humans created gametes that
  were all genetically identical to each other?
  (Think about your siblings)
• Twinsidentical vs. fraternal (how does this
  occur?)

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Trisomy 21Downs Syndrome

• Three copies of chromosome 21

Maternal Age Incidence at Birth
20 1 in 1500
30 1 in 900
35 1 in 400
40 1 in100
45 1 in 30
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Klinefelters Syndrome

• MaleXXY
• 1 in 1000 males
• Sterile
• Tall, thin, and slightly lower IQ

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Other Disorders

• Turners SyndromeXO
• 1 in 500 female births (most are aborted before
  birth)
• Very short, infertile, broad chest and may have a
  webbed neck
• Edwards SyndromeTrisomy 18
• 1 in 3000
• Most babies die within the first month to the
  first year