1. Meiosis
Biology

2. Review: Mitosis Results in 2 identical daughter cells
Occurs in somatic cells
Body cells
Ex: skin, stomach, muscle
# of chromosomes in parent cell = # of chromosomes in daughter cells
Resulting cells are diploid (2N)

3. What is meiosis? Process of making gametes (sperm/egg)
Chromosomes number is reduced in half (haploid)
When gametes combine, offspring has correct # of chromosomes
Haploid + haploid = diploid

4. Example: Humans

5. Because of meiosis…

6. For the remaining slides…
Pay attention to what is happening to the DNA in each phase – not the details of what goes on in each
Red text in the remaining slides = important information – focus on this!

7. Meiosis Occurs in 2 phases: Understand homologous chromosomes!
Meiosis I
Meiosis II
Understand homologous chromosomes!
Pair of identical chromosomes containing copies of genes
One from each parent

8. Meiosis I Interphase Prophase I DNA is copied Crossing over occurs
Pieces of chromosomes break off and switch places on sister chromatids
Leads to genetic variation

9. Metaphase I Anaphase I Telophase I Cytokinesis I
Homologous chromosome line up double file at equator
Anaphase I
Homologous chromosomes separate
Telophase I
Cytokinesis I
Results in 2 haploid daughter cells containing one set of replicated chromosomes

11. Meiosis II NO interphase (why??) Prophase II Metaphase II Anaphase II
Chromosomes line up single file at equator
Anaphase II
Sister chromatids separate
Telophase II
Cytokinesis II
Results in 4 haploid daughter cells containing ONE copy of each homologous chromosome

13. Meiosis Parent cell – chromosome pair Chromosomes copied
1st division – homologous pairs split
2nd division – sister chromatids split; produces 4 gamete cells with ½ the original number of chromosomes

14. Think you are 1 in 1,000,000? Don’t sell yourself short!
Humans with 23 pairs of chromosomes, a gamete (egg or sperm) could have 223 or 8,388,604 possible combinations of chromosomes from that parent.
Any couple could have 223 × 223 or 70,368,744,177,644 (70 trillion) different possible children, based just on the number of chromosomes, not considering the actual genes on those chromosomes.
Thus, the chance of two siblings being exactly identical would be 1 in 70 trillion.
In addition, crossing over can add further variation to an individual’s genetic make-up.

15. Meiosis Animation
Meiosis Cartoon