Meiosis The Big Picture Like mitosis all DNA
Meiosis: The Big Picture Like mitosis, all DNA copied in 2 n cell to make 4 n cell which then divides to make 2 n cells again BUT • The resulting cells are NOT identical b/c of how chromosomes line up • Those 2 n cells divide AGAIN to make n (haploid) cells 2 n “ 2 n” 2 n + 2 n Mitosis 2 n Meiosis “ 2 n” n “n” n n n
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 =paternal chrom 2 =paternal chrom 10 Draw: sister chromatids of maternal chromosome 2. . of 10. …homologous pairs of each chromosome.
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 =paternal chrom 2 =paternal chrom 10 = sister chromatids = homologous pair
Meiosis involves TWO cell divisions: both resulting in reduction of chromosome copy by half Prior to Meiosis: DNA replication Draw what happens next (immediately). (2 n) Homologous pair of chromosomes (“ 2 n”) Sister chromatids (copies of a replicated chromosome)
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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. Do homologous chromosomes pair up in mitosis? 1. Yes 2. No 8 8
Clicker: Homologous pairs of chromosomes align during meiosis. Do homologous chromosomes pair up in mitosis? 1. Yes 2. No 9 9
Crossing Over between similar chromosomes during Meiosis • Early in meiosis (before alignment in replicated 2 n 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
Sister chromatids Chromosomes Centromere 1. Replication, during interphase. One homolog Synaptonemal complex 2. Synapsis, during prophase I. Second homolog Non-sister chromatids Protein complex Crossing over usually occurs at least once in each non-sister chromatid; two of four chromatids are shown crossed over here 3. Crossing over, during prophase I.
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: * ** 2 n 2 n * 4 n * ** 4 n ** + * Two 2 n daughter cells Meiosis: * ** Meiosis I homologous pairs align * + * Meiosis II * gametes
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