Meiosis

Sexual reproduction Biological variability

Eukaryotes, diploidy •  1-1.5 billion years ago, eukaryotic cells •  diploidy - lethal and detrimental mutations

over-ridden by dominant wild-type genes. •  Sexual reproduction and meiosis - powerful

mechanism to insure biological variability, •  shuffling the genetic deck. •  Variability fundamental in evolution •  new varieties may be more adapted and

hence survive (to reproduction) better as selection pressures change.

Humans are diploid

•  gametes (sperm and eggs) are haploid •  sperm fertilizes egg - diploid zygote •  With two homologous chromosomes... •  one from father and one is from mother. •  Both copies (alleles) of a gene might be the

same (homozygous) •  ......or they may be different (heterozygous)

human and other life cycles •  Terms: haploid (n), diploid (2n), gametes,

ova, sperm, fertilization, zygote. •  meiosis creates haploid gametes in humans, •  organisms where meiosis creates a gamete-

forming organism (alternation of generations, haploid life cycles).

•  Previously, I described how mitosis creates two identical diploid "daughter" cells;

•  Now, see (next)

In summary

•  Here, you can see that mitosis can also create haploid cells from haploid cells in protists, fungi and plants.

•  An anthropocentric view is that meiosis only makes haploid gametes, but here you can see that it can make haploid organisms, again in protists, fungi and plants.

Before we get to second half

•  two homologous chromosomes •  23 pairs for the human diploid number, 46. •  Theoretically, one could go to each gamete

with one meiotic division. •  Instead, they align, duplicate, & divide twice. •  Homologous chromosomes separate first •  Sister chromatids separate in second division

Awesome variability

•  for 2 pairs of chromosomes, 4 possible combinations of chromosomes (centromeres) in the zygote.

•  For 46 chromosomes, there are 2 to 23 power = 8.4 million.

•  The story of the inventor of chess •  Way more variabilitythan that (see

below)

Information •  Tetrad (bivalent), homologues have

duplicated and are aligned next to eachother •  Crossing-over (recombination) increases this

beyond measure (shown for one pair of chromosomes)

•  Coming attractions •  Unequal crossing over and gene duplication

as major means of protein family evolution •  in meiosis (nondisjunction) - aneuploidy

(wrong number of chromosomes) such as trisomy 21