Meiosis: The Big Picture Like mitosis, all DNA copied in 2n cell to make 4n cell which then divides to make 2n cells again BUT The resulting cells are NOT identical b/c of how chromosomes line up Those 2n cells divide AGAIN to make n (haploid) cells 2n “2n” 2n 2n + Mitosis n “n” 2n “2n” n “n” n n Meiosis

Important Terminology: Sister chromatids: identical copies of same chromosome Homologous pairs of chromosomes: pairs of sister chromatids one set from mom, one set from Dad, both the same chrom. # In (cross) sexual reproduction: chromosomes come from one of two parents so chromosomes can be “paternal” or “maternal” Draw: sister chromatids of maternal chromosome 2. ... of 10. …homologous pairs of each chromosome. =maternal chrom 2 chrom 10 =paternal

Important Terminology: Sister chromatids: identical copies of same chromosome Homologous pairs of chromosomes: pairs of sister chromatids one set from mom, one set from Dad, both the same chrom. # In (cross) sexual reproduction: chromosomes come from one of two parents so chromosomes can be “paternal” or “maternal” =maternal chrom 2 =maternal chrom 10 = sister chromatids =paternal chrom 2 =paternal chrom 10 = homologous pair = sister chromatids = homologous pair

(2n) (“2n”) Meiosis involves TWO cell divisions: both resulting in reduction of chromosome copy by half Prior to Meiosis: DNA replication Draw what happens next (immediately). (2n) (“2n”) Homologous pair of chromosomes Sister chromatids (copies of a replicated chromosome)

maternal #3 paternal #3 Recall in Mitosis, homologs lined up but not side by side as a pair and therefore did not separate like they do in Meiosis paternal #3 maternal #3

Clicker: Homologous pairs of chromosomes align during meiosis Clicker: Homologous pairs of chromosomes align during meiosis. Do homologous chromosomes pair up in mitosis? Yes No 8

Clicker: Homologous pairs of chromosomes align during meiosis Clicker: Homologous pairs of chromosomes align during meiosis. Do homologous chromosomes pair up in mitosis? Yes No 9

Crossing Over between similar chromosomes during Meiosis Early in meiosis (before alignment in replicated 2n cell) Homologous chromosome pairs come together Similar regions at the molecular level attract one another. The non-sister chromatids can cross-over (swap pieces of their chromosomes) Usually at least 1 per homologous pair of replicated chromosomes Produces new allele combinations which may have different expression/regulation Produces variation beyond the 8.4 million possible with independent assortment

A Closer Look at Three Key Events in Prophase of Meiosis I. Sister chromatids Centromere Chromosomes 1. Replication, during interphase. One homolog Synaptonemal complex 2. Synapsis, during prophase I. Second homolog Non-sister chromatids 3. Crossing over, during prophase I. Figure 12.9 A Closer Look at Three Key Events in Prophase of Meiosis I. Protein complex Crossing over usually occurs at least once in each non-sister chromatid; two of four chromatids are shown crossed over here

Sexual vs Asexual Reproduction Mitosis is the mechanism of asexual reproduction in eukarya (only Genetic variation in daughter cells is by random mutation during DNA replication) Self-fertilization is 1 mechanism of sexual reproduction: one organism produced 2 different types of gametes that join and form new organism (sponges and many snails) ~More variation than reproduction by mitosis b/c self-fertilization involves meiosis to form gametes so still get variation due to independent assortment and crossing over (in addition to random mutation) Sexual reproduction between 2 organisms allows for the MOST genetic variation: random mutation, independent assortment, crossing over AND mixing of genes from two parents.

* * * * * * * * * * * * * * * * Sexual vs Asexual Reproduction But, not equal: sexual reproducing organisms have offspring with greater genetic diversity. So, if a “bad” mutation arises, it will be inherited by all offspring of asexual parents b/c no mixing of DNA between two individuals. Mitosis: * * * * * * * + 2n 4n Two 2n daughter cells * Meiosis: * * * * * * Meiosis I Meiosis II * * gametes + 2n 4n homologous pairs align