Meiosis 1 10 17 Mitosis Genetic constancy via

Meiosis 1. 10. 17

Mitosis • Genetic constancy via replication vs. Meiosis • Genetic diversity via recombination (shuffling) • How many outfits can be made from 5 pairs of pants and 5 shirts?

Mitosis • Genetic constancy via replication vs. Meiosis • Genetic diversity via recombination (shuffling) • How many outfits can be made from 5 pairs of pants and 5 shirts? • 5 x 5 = 25 possibilities • (Imagine the # of possibilities with 25, 000+ genes!)

Meiosis • The process of cell division that reduces the chromosome # by ½

Meiosis • The process of cell division that reduces the chromosome # by ½ • Only used in sexual reproduction (not in bacteria or other asexual reproducers)

Meiosis • The process of cell division that reduces the chromosome # by ½ • Only used in sexual reproduction (not in bacteria or other asexual reproducers) • In humans: – 46 chromosomes in somatic (body) cells -> 23 chromosomes in gametes (sex cells) – 2 n -> 1 n (diploid -> haploid)

Why Sexual Reproduction?

Why Sexual Reproduction? • Why not?

Why Sexual Reproduction? • Why not? – Only ½ of population reproduces – less potential offspring – Huge energy expenditure in offspring production – Only get to pass on ½ of your genome

Why Sexual Reproduction? • Why not? – Only ½ of population reproduces – less potential offspring – Huge energy expenditure in offspring production – Only get to pass on ½ of your genome • Why?

Why Sexual Reproduction? • Why not? – Only ½ of population reproduces – less potential offspring – Huge energy expenditure in offspring production – Only get to pass on ½ of your genome • Why? – MUCH more variation (genetic diversity) – The variation provides the raw material for change, allowing species to survive changing conditions

Mitosis vs. Meiosis Mitosis Phase Meiosis • Cell grows G 1 • DNA Replication S • DNA Replication • Cell prepares for division G 2 • Cell prepares for division • Cell grows

Mitosis vs. Meiosis Mitosis • Chromosomes condense Phase • Prophase I crossing over occurs! • Individual chromosomes line up across middle of the cell • Metaphase I Meiosis • Chromosomes condense and pair with homologue to form a tetrad • Homologous chromosome pairs (tetrads) line up across middle

Mitosis vs. Meiosis Mitosis • Spindle fibers pull sister chromatids towards opposite poles • Nuclear membrane begins to re-form • Interphase Phase • Anaphase I • Telophase I Meiosis • Spindle fibers pull each homologous pair towards opposite poles • Nuclear membrane may temporarily start to form, but then the cell goes immediately into • Meiosis II

Meiosis II • Very similar to mitosis, except there is no Interphase (no DNA replication)

Sources of Variation: Independent Assortment • Maternal and paternal chromosomes align along the equator of the cell randomly, thus increasing variation

Sources of Variation: Crossing Over • The process of homologous chromosomes (one from mom and one from dad) swapping genes. • Happens randomly at multiple spots on each chromosome • Ensures genetic variation

Crossing Over

Crossing Over

Crossing Over

End Results Mitosis • • • 2 somatic cells 46 chromosomes (humans) Diploid # of chromosomes 2 n # of chromosomes Each cell is identical Meiosis

End Results of One Round Mitosis • • • 2 somatic cells 46 chromosomes (humans) Diploid # of chromosomes 2 n # of chromosomes Each cell is identical Meiosis • • • 4 gametes (sex cells) 23 chromosomes (humans) Haploid # of chromosomes 1 n # of chromosomes Each cell has a unique combination of genes