Meiosis

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

Mitosis vs. Meiosis Genetic constancy via replication 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 1/2

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) – 2n -> 1n (diploid -> haploid)

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 Cell grows DNA Replication Cell prepares for division Meiosis Cell grows DNA Replication Cell prepares for division Phase G 1 S G 2

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

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

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

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

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

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