meiosis 1.5.10. mitosis vs. meiosis genetic constancy via replication genetic diversity via...
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Meiosis
1.5.10
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 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
G1
S
G2
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 overoccurs!)
•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
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