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Cell division only

for sex cells:

  • sperm
  • egg

2 stages:

  • MEIOSIS 1
  • MEIOSIS 2

Meiosis 1:

  • homologous chromosomes
  • attached together
  • point of attachment is the chiasmata
  • cross-over...so their genes are exchanged
  • chromosomes line up at equator
  • spindle fibres from centrioles, attach to centromeres
  • end of Meiosis 1, chromosomes are randomly pulled by the spindle fibres, from the centrioles, to give two daughter cells.

Meiosis 2:

  • chromosomes with 'exchanged genes' have spindle fibres
  • spindle fibres attach to the centromere
  • crhomosomes into chromatids
  • pulled randomly
  • end of Meiosis 2, four daughter cells haploid cells formed

Advantages of meiosis:

1. sperm and egg are haploid cells.

n+n=2n

-therefore genetic variation

-1/2 from father and 1/2 from mother.

2. independent assortment of chromosoems:

-crossing-over

-Meiosis 1, chromosomes are pulled randomly

-Meiosis 2, chromatids are pulled randomly

Meiosis

fig. 1

1.

2.

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