Meiosis

Campbell and Reece Chapter 13

Sexual reproduction and meiosis give rise to biological variability, which is very fundamental in evolution since new varieties may be more adapted and hence survive (to reproduction) better.

Sexual reproduction probably evolved about 1.5 billion years ago, about when eukaryotic cells evolved, and probably gave an evolutionary advantage.

TRANSPARENCY (Fig. 13.4) human life cycle haploid (n), diploid (2n), gametes, ova, sperm, fertilization, zygote. Note that while meiosis creates haploid gametes in humans, there are many organisms where meiosis creates a gamete-forming organism (alternation of generations, many examples next semester).

TRANSPARENCY (Fig. 13.6) Meiosis. Note that there are 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, and divide twice.

TRANSPARENCY (from AHSturtevant and GWBeadle, An introduction to genetics, Dover, New York, 1939 and 1962) details two divisions, tetrad, crossing over.
TRANSPARENCY (Fig. 13.7 A and B) Your book's version of the first division.

Awesome variability TRANSPARENCY (Fig. 13.9) for 2 pairs of chromosomes, there are 4 possible outcomes. For 46 chromosomes, there are 2 to 23 power = 8.4 million.
Crossing over increases this beyond measure.(shown for one pair of chromosomes TRANSPARENCY (Fig. 13.10))
story of grains of wheat on chess board

Unequal crossing over and gene duplication as major means of
protein family evolution

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