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Understanding Meiosis: The Process of Genetic Diversity

Learn about meiosis, crossing-over, and gamete formation. Explore genetic variation, the Law of Independent Assortment, and more to grasp the significance of sex in evolution.

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Understanding Meiosis: The Process of Genetic Diversity

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  1. Do Now 2.11 (HW check: 8.13) Objectives: 1. Define crossing over Describe the process of meiosis and the cells it produces. Define the Law of Independent Assortment Task: Pass forward karyotype labs (Blue Jays only) 2. Horses: 2n = 64; Donkey: 2n = 62 Mule = offspring from male donkey & female horse Mule 2n = ??? Mule n = ???

  2. Meiosis Makes Gametes • Meiosis basics: • CHROMOSOME NUMBER REDUCED BY HALF • 2nn • Occurs only in specialized cells in gonads (testes & ovaries) • Produces sperm (spermatogenesis) and eggs (oogenesis) • Includes 2 cycles of cell division with NO INTERPHASE.

  3. A A Let’s assume we have an organism where 2n=4 a a 2n = 4 B B b b

  4. Meiosis I: 1 chromosome from each homologous pair goes to each cell Now the cells have only one set of chromosomes Haploid a a A A n = 2 n = 2 b b B B

  5. Meiosis II: The chromatids of each chromosome separate during division. What is the n of the gametes? 1n = n All 4 Are Haploid n = 2 n = 2 n = 2 n = 2 A A B B

  6. Important Notes • In meiosis I, chromosomes begin with two chromatids and END with two chromatids. • Chromosomes do not divide during meiosis I, a homologous pair of chromosomes divides. • In meiosis II, each chromosome with two chromatids divides. • The cells begin with n chromosomes that have 2 chromatids each. • The resulting cells (gametes) have n chromosomes with 1 chromatid each

  7. Are those gametes from the previous example the only outcome? NO!

  8. The homologous chromosomes can line up differently during…. 2n = 4 Metaphase I

  9. Meiosis I products: note different combination n = 2 n = 2

  10. Meiosis II: note the gametes now have a different combination of chromosomes n n

  11. The Law of Independent Assortment • Homologous chromosomes separate randomly during meiosis It does not matter which chromosome that each gamete gets, as long as it gets one of each type! In other words, when you produce gametes, each sperm or egg cell has a 50% chance of carrying the chromosome #1 you got from mom, and a 50% chance of carrying the chromosome #1 you got from dad. The chromosome #2 in a gamete has NOTHING to do with what version of #1 it has.

  12. So what’s the Point of Meiosis? • Produces haploid gametes for reproduction. • Mixes up parental chromosomes to make new combinations of genes. • END RESULT: lots of genetic variation and diversity

  13. But Wait, There’s More! • We’ve already seen how meiosis makes new combinations of genes, but there’s even more mixing (recombination) that happens! • A process called crossing-over creates NEW chromosomes UNIQUE from those you got from your parents!

  14. Crossing over: homologous chromosomes exchange genes during Prophase I. • The chromosomes in the middle are NEW combinations of genes that did not previously exist! RECOMBINATION. Tetrad

  15. Where Genetic Diversity Comes From:The Advantage of Sex!

  16. Disadvantages of Sex? • Need a mate. • Energetic costs of courtship and gamete production. • Only 50% of genetic information is passed on to each offspring.

  17. Recap • Meiosis makes haploid gametes • Meiosis I separates homologous pairs, meiosis II splits chromosomes. • Chromosomes follow the Law of Independent Assortment • Crossing over during meiosis produces new chromosomes that have new combinations of information

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