Reproduction - Sexual reproduction (crossing over / recombination, independent assortment, & Gametes))

lecture #12

meiosis I - Prophase I

in what stages do crossing over (recombination) occur in?

crossing over (recombination)

• definition: gene swapping between homologous non-sister chromatids

• steps: 

  • 1) proteins attach homologous chromosomes in a synapsis 

  • 2) attached homologous chromosomes form a tetrad 

  • 3) Non-sister chromatids overlap, forming an x-shapes chiasma

    • chiasma occurs at a locus 

• ____ can swap alleles between homologous non-sister chromatids

  • creates a new assortment of alleles along a chromosome

• produces recombinant chromosomes that combine alleles inherited from each parent

• end result: chromosomes have the same number of genes but the sister chromatids are no longer identical (daughter cell are completely unique from mother cell)

locus

• definition: location of a gene on the chromosome

alleles of a gene are always found in the same location on homologous chromosomes

Remember: alleles of a gene are always found in the same location on homologous chromosomes

No, XY are non-homologous chromosomes

can crossing over (recombination) occur between XY sex chromosomes?

Yes, XX are homologous chromosomes

can crossing over (recombination) occur between XX sex chromosomes?

meiosis I - metaphase I

in what stages do independent assortment occur in?

independent assortment 

• Definition: homologous pairs of chromosomes orient randomly

• each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs

  • possible # of combinations is 2^n (where n is the # of chromosomes in a single set)

    • for humans: n = 23

    • there are more than 8.4 million (2^23) possible combinations

Gametes

• the resulting cells formed from the completion of meiosis II

• they continue to mature and develop to reach their final functional potential (but they NEVER undergo further mitosis)