1. Key Idea: Meiosis differs from Mitosis…Why
Section 7-1 Meiosis
Key Idea:
Meiosis differs from Mitosis…Why

2. Objectives: You will be able to:
Summarize the events that occur during meiosis
Compare to the events of mitosis

3. Meiosis Meiosis cuts the number of chromosomes in half.
Forms specialized reproductive cells
Gametes, spores
Meiosis I and II

4. Meiosis I Prophase I Metaphase I Chromosomes condense
Nuclear envelope breaks down
Crossing-over occurs
Sections of the homologous chromosomes are exchanged
Metaphase I
Homologous chromosomes move to the equator of the cell

5. Meiosis I Anaphase I Telophase I Homologous chromosomes separate
Chromosomes are moved to opposite poles
Chromatids do not separate at centromeres
Telophase I
Nuclear membrane reforms
Cytoplasm divides
Two new cells are formed

6. Meiosis II News cells undergo another division
Chromosomes do not replicate between meiosis I and II
Prophase II
New spindle forms
Metaphase II
Chromosomes line up at equator

7. Meiosis II Anaphase II Telophase II Result: Four haploid cells
Centromeres divide
Chromatids move to opposite poles
Telophase II
Nuclear envelope forms
Cytokinesis
Result: Four haploid cells

9. Independent Assortment
Each gamete receives one chromosome from each of the 23 pairs.
Random distribution
Each of the 23 pairs segregates independently
About 8 million (223) different gene combinations

10. Crossing-over Adds more recombination
Number of possible outcomes of offspring is about 64 trillion.

11. Importance of genetic variation
Increases rate of evolution
Breeding programs eventually reduce genetic variability
Evolution is both revolutionary and conservative
What does this mean?