Meiosis

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Meiosis Sexual Reproduction

• Its all about the sperm and egg!

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• Remember that 1 chromosome is made when 2
  chromatids join at the centromere.
• These 2 chromatids are EXACT copies of each other
• Every human cell has 23 pairs of chromosomes
• Total of _____
• 1 set from Mom, 1 set from Dad (_______ from
  each!)
• They each give us the same chromosomes.
• Same size, shape.
• They are only different in their GENES! (DNA that
  codes for a protein)
• Different versions of a gene are called ALLELES
• Think flower color or hair color

46
23
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Chromatids vs. Homologous Chromosomes
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• Combining the 1 set from each parent gives us 23
  pairs (46 total) DIPLOID
• Diploid just means 2 of each chromosome
• All human cells except sex cells!
• HAPLOID means 1 of each chromosome
• In humans, only sex cells!

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HAPLOID
HAPLOID
DIPLOID
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Meiosis Making theSperm and Egg

• Sperm and Egg are haploid
• Meiosis has 2 phases, Meiosis I and Meiosis II
• Meiosis I Homologous chromosomes separate
• Meiosis II Sister chromatids separate

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Meiosis I
Homologous chromosomes separate
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Meiosis II
Sperm or egg
Sister chromatids separate Very similar to
Mitosis!
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Label the stages of Meiosis (interphase, I, II)
interphase
____________
Meiosis I
_____________
Meiosis II
__________
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Animations

• http//www.johnkyrk.com/meiosis.html

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Crossing Over

• Occurs during Prophase I when parts of homologous
  chromosomes overlap

Letters A/a and B/b represent alleles
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Mendels Law of Independent Assortment
Pairs assort independently of one another What
pair 1 does has no affect on pair 2 ,etc.
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Mendels Law of Random Segregation

• during gamete formation each member of the
  allelic pair separates from the other member to
  form the genetic constitution of the gamete

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Practice Questions

• What do Haploid and Diploid mean?
• How are homologous chromosomes and sister
  chromatids different?
• Describe independent assortment in your own
  words.
• Describe crossing over in your own words.
• Which phase of Meiosis (I or II) is most like
  Mitosis? Why?

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Hermaphrodites (yes, they DO exist in the human
population)
-Have both male and female genitals
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Parthogenesisspontaneous reproduction where the
egg develops into an adult without ever being
fertilized by a male.