Meiosis

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Meiosis

• Why Arent All Brothers and Sisters Exactly
  Alike??

2
Some Vocabulary

• Gamete
• Reproductive cell (like sperm or eggs in humans)
• Gene
• Section of DNA that codes for a certain trait.
• Allele
• Form of a gene. For example, you have genes for
  eye color. Alleles for eye color include blue,
  green, hazel, and brown.
• Diploid
• Cell with 2 copies of each chromosome (46 in
  humans)
• Haploid
• Cell with 1 copy of each chromosome (23 in humans)

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Keys to Understanding Meiosis
Chromosomes are paired.
Chromosomes carry genes.
The gene forms on a pair of chromosomes may be
or different.
Identical. . .
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2 Major Accomplishments of Meiosis

• 1) Meiosis takes a cell with two copies of every
  chromosome (diploid) and makes cells with a
  single copy of every chromosome (haploid).
• 2) Meiosis shuffles the alleles of each gene that
  each cell receives. (think Genetic Variation!!)

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Making Haploid Cells

• WHY? This is a good idea if youre going to
  combine two cells to make a new organism.
• In meiosis, one diploid cells produces four
  haploid cells by dividing twice but only
  replicating DNA once.

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Shuffling Alleles

• Why is this important? genetic diversity!
• Genetic diversity is important for the evolution
  of populations and species.
• This shuffling is happens in 2 ways
• independent assortment
• crossing-over

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Independent Assortment

• Independent
• Assortment
• relies on the
• fact that the
• chromosomes
• line up in
• random ways
• when the cells
• divide.

8
Crossing Over

• Sometimes when chromosomes line up, pieces can
  cross over each other.
• Then, when they separate, the pieces switch
  places.

9

• Crossing-over multiplies the already huge number
  of different gamete types produced by independent
  assortment.
• Because of these two events, the number of
  different gametes that one person could make is
  nearly endless.
• If no 2 gametes are alike, then no 2 children
  will be alike, either.

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Females
Males
11
Fertilization-Putting It All Together