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Meiosis. Gamete production, takes place in reproductive organs/parts in both plants and animals Gametes have half the number of chromosomes (haploid), so when they come together the zygote (fertilized egg) will be diploid

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meiosis
Meiosis
  • Gamete production, takes place in reproductive organs/parts in both plants and animals
  • Gametes have half the number of chromosomes (haploid), so when they come together the zygote (fertilized egg) will be diploid
  • If gametes weren’t haploid the offspring would have twice the correct number of chromosomes in every cell , for example:
  • Corn plant has 20 chromosomes (10 pairs) in every somatic cell, so its gametes have 10 single chromosomes
  • If it didn’t than the offspring would end up with 40 chromosomes (10 quads!)
  • 20 + 20
    • 40 + 40
      • 80 + 80

- 160 + 160

          • 320 the 5th generation would have 32 of each of the 10 chromosomes!!
meiosis cont
Meiosis, cont.
  • This is how the correct number of chromosomes is maintained from generation to generation
  • Called REDUCTION DIVISION— as cells divide, the number of chromosomes is reduced
  • Consists of 2 successive nuclear and cell divisions
  • Before Meiosis I, DNA replicates into fuzzy X’s
  • Meiosis I— homologous chromosomes pair up and then separate into 2 cells
  • Meiosis II—just like mitosis, but it happens to 2 cells simultaneously
  • Chromosomes line up at the center and sister chromatids are pulled apart
  • One “2n” (diploid) cell produces 4 “n” (haploid) cells
interesting occurrences during meiosis
Interesting occurrences during meiosis…..
  • Crossing over—when homologous chromosomes exchange chromosome fragment
  • Causes genetic recombination and provides genetic variation (good thing!)
  • Happens during Prophase I, when chromosomes are coiling up and homologous chromosomes are pairing up
  • Shown happening at both ends of chromosomes, but can happen at just one
  • This is why you are not identical to your brothers/sisters
another interesting occurrence
Another interesting occurrence….
  • Nondisjunction—homologous chromosomes do not separate properly
  • Happens during Anaphase I or II
  • End result is two gametes end up with both chromatids (of that specific chromosome) and the other 2 cells end up without a copy of that chromosome
  • So if the gamete that has both of the chromatids is fertilized then that zygote will be trisomy (3 of one chromosome) for that chromosome
  • If the gamete that is missing that chromosome is fertilized, it will be monosomy (one chromosome) for that chromosome
  • Relatively common-- 1 in 5 normal human pregnancies miscarry w/in the first 2 months due to the baby having too many or too few chromosomes