CH 11 - SEXUAL REPRODUCTION & MEIOSIS
I. INTRODUCTION
A. SEXUAL REPRODUCTION - VARIETY!
1. 2 parents (p. 208)
a. male/female; positive/negative
b. separate/same organism
2. Each parent has two alleles (doses) for each gene (trait)
3. Only 1 allele for each trait is passed on from each parent.
4. 1 allele is in each sperm; 1 in each egg for humans.
B. HAPLOID - n DIPLOID - 2n
1. Single # of Chromosomes Double # of Chromosomes
(from mom OR dad) (from mom & dad)
2. 1 dose of allele/chrom. 2 doses of allele/chromosome
3. Sex cells (gametes) Somatic cells (all the rest)
4. 23 in humans 46 in humans
C. MEIOSIS - Gk "meioun" to diminish
1. Reduction division
2. Produces Gametes "to marry": sperm and egg - Haploid cells
3. 2n è n (Diploid èHaploid)
4. In Mitosis it is Diploid è Diploid 2n è 2n
D. FERTILIZATION (p. 208)
1. The "fusion" or "joining" of Gametes è Zygote "pair" = SYNGAMY
2. Forms a new genetic identity (offspring)
3. Produces a diploid cell from 2 haploid cells
4. After fertilization the offspring grows by Mitosis.
II. STEPS OF MEIOSIS (an * designates an important difference from Mitosis)
*2 division sequences occur - only one in Mitosis
*Meiosis I - Homologous pairs separate
Sister chromatids (dyads) don't
Meiosis II- Sister chromatids separate
A. MEIOSIS I (p. 212 – 213) DNA REPLICATION OCCURS FIRST!
1. *Prophase I -
a. *Synapsis- homologous pairs "zip" together = tetrad. (p. 209)
b. *Crossing over - exchange of genetic information (p. 210)
2. Metaphase I- Chromosomes line up at the equator. (p. 211 bottom)
3. *Anaphase I -
a. Homologous pairs (tetrads) split. (p. 211 top)
b. The dyads (sister chromatids) don't; centromeres intact
4. Telophase I - The dyads move to the poles of the cell.
B. MEIOSIS II - *no DNA replication before this!
1. Prophase II - Just like Mitosis
2. Metaphase II - Just like Mitosis
3. Anaphase II - Just like Mitosis. The dyads separate - centromeres split (p. 213)
4. *Telophase II - 4 Haploid, genetically different cells (p. 209)
C. MITOSIS vs. MEIOSIS (p. 216-217)
MITOSIS MEIOSIS
1. Identical chromosomes Different chromosomes (crossing over)
2. Genetic constancy Genetic variation
3. Variation by mutation Variation by recombination & mutation
4. Chrom. # stays same Chromosome # is reduced by 1/2
5. Produces somatic cells Produces sex cells (Gametes)
6. 2n è2n 2n èn
D. SPERMATOGENESIS
1. 4 cells produced
2. Equal cytokinesis
3. Begins @ puberty, continues for life
E. OOGENESIS
1. 1 viable cell produced, 1-3 polar bodies - degenerate
2. Unequal cytokinesis
3. Begins before birth, stops @ Metaphase I è puberty (2,000,000)
4. 40,000 @ puberty; 300-400 ovulated during woman's reprod. life
5. Meiosis II not completed till after fertilization.
6. Women MAY produce some new eggs, most prod. before birth
F. FACTS:
1. No new genes created; existing genes "shuffled" or "rearranged" into new combos
2.10600 genetically different human gametes are possible
a. only 1010-11 humans in all history
3. 300-400 million sperm/3-5 ml semen; 100 million/day; under 20 million - sterile