1. Meiosis and Sexual Reproduction Ch. 11.1-11.3

2. Why Meiosis? Advantages: Dramatic increase in genetic variation
Keeps genetic material from multiplying (ploidy)
Replication done with somatic cells (body cells) would double the amount of DNA each time
Disadvantages:
Reproduction is more complex
Meiosis requires energy
Must find other half of genetic material to reproduce
Sexual Reproduction fusion of two haploid male and female cells (gametes)
Fertilization union of male and female nuclei; produces a zygote

3. The Cells of Meiosis What type of cells do meiosis?
Eukaryotes
Types of meiotic cells:
Spermatozoa (sperm) produced by male gonads (testes); contain paternal chromosomes
Ova (eggs) produces by female gonads (ovaries); contain maternal chromosomes
Source of variation though chromosomes are the same (homologous) they have different DNA for the same genes (alleles)

4. Phases of Meiosis Two Phase Groups: Meiosis I Meiosis II
Prophase I Telophase I
Homologous chromosomes separate
Meiosis II
Prophase II  Telophase II
Sister chromatids separate (like mitosis)
Both phases have cytokinesis
Leading to Meiosis I normal cell cycle (G1, S, G2)
Leading to Meiosis II short interphase but no S phase (interkinesis)

5. Meiosis I Prophase I Prometaphase I Metaphase I Anaphase I Telophase I
Chromosomes condense
Homologous pairs form tetrads through pairing (synapsis)
Crossing-over occurs
Prometaphase I
Nucleus breaks down
Spindle fibers attach to kinetochores (one for each sister chromatids)
Metaphase I
Homologous pairs line up at metaphase plate
Anaphase I
Homologous pairs separate to opposite poles
Telophase I
Nucleus reforms, spindles break down, and short interkinesis

6. Meiosis II Follows mitosis: Prophase II Prometaphase II Metaphase II
Chromosomes condense
Spindles form
Prometaphase II
Nucleus breaks down
Spindles attach to kinetochores
Metaphase II
Chromosomes move to metaphase plate
Anaphase II
Sister chromatids separate to opposite poles
Telophase II
Nucleus reforms, chromosomes decondense, spindles disappear
End result is 4 haploid cells

7. Producing Variation Where in meiosis does variation come from?
Random separation of paternal and maternal DNA
8,388,608 possible combinations with our 23 pairs
Switching of genes during cross-over
Random recombination of haploid material
Creates enough variation that no two humans ever look alike (expect to twins)
Even in the same family, the chance of offspring being genetically the same is 1:7.0x1013

8. Crossing-Over
Protein complex (synaptonemal complex) carefully exchanges alleles on homologous chromosomes
Event is random and happens in 2-3 places on every chromosome
2 (of 4) chromatids exchange alleles results in two recombinant chromosomes and two parental chromosomes

9. Producing Errors
Sometimes during Anaphase I/II or after crossing over, chromosomes get stuck together
Nondisjunction failure to separate homologous chromosomes/ sister chromatids
One cell receives extra material and one cell misses a whole chromosome
Mostly fetal in humans
Trisomy 21 (Downs Syndrome)
Produces large amounts of variation in plants

10. Alternative Life Cycles through Meiosis
Diploid Dominate Cycle
Meiosis cells used for fertilization
No mitosis for gametes
Animals
Haploid/Diploid Alternation Cycle
Sporophytes (diploid) produce spores (haploid) which under go mitosis to form gametophytes
Eventually gametophytes form sperm and eggs cells used to make sporophytes
Bushes, trees, and flowers
Haploid Dominate Cycle
Diploid only at fertilization, then cell produces haploid (+/-) spore cells
Spores produce gametophytes
Fungi and algae

11. Homework Read Ch. 12 and do Ch. 12 Vocab Ch. 11 vocab
Test Your Knowledge and Interpret the Data for Ch. 11
Test on Ch. 11, 12, and 13 next week