Meiosis Chapter 10.2 (Pages 263

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MeiosisChapter 10.2 (Pages 263 273)
Dude, meiosis starts in five minutes . . . I
cant believe youre not condensed
yet http//www.promega.com/
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Learning Outcome

• Discuss the role of meiosis and sexual
  reproduction in producing genetic variability in
  offspring.
• Include crossing-over, randomness

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Chromosome Images
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Chromosome Images
http//hopes.stanford.edu/basics/dna/f_b11homolgs.
jpg
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Meiosis

• In complex plants and animals where sexual
  reproduction occurs, there are two types of
  cells.

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Cell Types

• Somatic Cells
• These are the cells that make up our bodies
  (muscles, bones, blood, etc.).
• Diploid
• These cells reproduce by mitosis.

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Cell Types (contd)

• Reproductive Cells
• These cells are specialized for reproducing
  offspring through sexual reproduction.
• The gametes have half the DNA (haploid, single
  stranded) compared to the regular body cells.

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Chromosomes (p 264)

• Humans have 23 pairs of chromosomes for 46
  chromosomes in each cell nucleus.
• These cells are called diploid (di means
  double).
• 23 from mom
• homologous
  pairs
• 23 from dad

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Autosomes vs Chromosomes

• Twenty-two of these pairs are called autosomes.
• The twenty-third pair are the sex chromosomes
  (determine gender more on this later).

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Autosomes

• Reproductive cells have a haploid
  (means half the number of) number of
  chromosomes.
• They contain 23 chromosomes not 23 pair
• Sperm male gamete
• Egg female gamete
• (see Fig 10.10 p 266)

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Chromosomes (contd)

• Each pair of homologous chromosomes has genes for
  the same traits, such as plant height. (not
  necessarily identical)
• These genes are arranged in the same order (see
  Fig 10.10 p 265)
• Do Problem Solving Lab 10.2 p 264

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Purpose of Meiosis

• If mom gives the baby 46 chromosomes and dad
  gives the baby 46 chromosomes we HAVE A PROBLEM!
• Junior gets 92! Oops!
• Moms and dads need a method of making special
  cells with half the DNA.

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Purpose of Meiosis (contd)

• There must be another form of cell division that
  allows offspring to have the same number of
  chromosomes as their parents.

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Meiosis has two purposes

• To make gametes with half the normal number of
  chromosomes (haploid gametes).
• To make gametes that have variations in their DNA
  code.

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The first half of Meiosis makes diploid cells
into two haploid cells(Meiosis I).
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NOTE

• The first half of Meiosis makes diploid cells
  into two haploid cells(Meiosis I).
• Second half of meiosis splits double strands to
  single strands.
• (See Fig 10.12 p 267)

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Crossing Over

• Some variation is achieved in anaphase 1, but not
  enough
• Further variation is achieved through a process
  known as Crossing Over.

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Crossing Over (contd)

• The Xmes, in a tetrad (two homologous pairs),
  pair so tightly that non sister chromatids from
  homologous pairs actually break off and exchange
  genetic material.
• This can occur at any location on a chromosome
  (random).

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Crossing Over (contd)

• It is estimated that there are 2-3 cross-overs in
  human Xmes during M1.
• What happens in one chromosome is completely
  independent from what happens in another
  chromosome.
• This is what results in so much variation

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Crossing Over (contd)
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AM I really unique?

• So how many different gametes can be produced?
• How many different offspring can be produced?

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Crossing Over (contd)

• This process results in endless possibilities
  because each pair of Xmes can line up one of 2
  ways (see p 269-270)
• (2n where n of Xme prs.)

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Crossing Over (contd)

• Peas with 7 prs. of Xmes (2n 27 128)
• Remember, any egg can be fertilized by any sperm
• 128 x 128 16 384 different possible offspring
• Human with 23 prs (2n 223 over 8 million)
• 223 x 223 about 70 trillion

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Gametogenesis

• The formation of gametes from germ cells
• Different for sperm and egg cells
• Oogenesis vs. spermatogenesis
• More about this later

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Gametogenesis
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In Summary

• Three processes guarantee variation in gamete
  formation
• Independent orientation
• Mixing of mom and dads xmes
• Crossing over
• Random fertilization
• GeneticVariation
• stage of meiosis
• Animation Unique Features of Meiosis

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Activity/Assignment

• Read p 266 270 and fill in meiosis chart.
• Play Dough Meiosis
• (MiniLab 10.2 p268 text worksheet p3)
• Finish heredity key terms

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