1. 10/24/08
Lecture 20 – Sexual Reproduction and Meiosis 1
 There are two main forms of reproduction.
1. Asexual reproduction
 Mitosis
 Cloning.
2. Sexual reproduction
 Meiosis
 When you have a diploid number, chromosomes are in homologous pairs.
 In humans, there are 22 pairs of autosomes and on pair of sex chromosomes.
 Females have XX.
 Males have XY.
 Meiosis
 Meiosis I
 Interphase
 S phase: DNA replications itself → single-stranded to double-stranded.
 Prophase I
 Nuclear membrane and nucleolus breaks down.
 Spindle starts to form.
 Homologous pairs line up adjacent to each other.
 Chiasmata: the points where chromosomes interact with another.
Tetrad: The structure that results when two replicated homologous chromosomes pair
during prophase I of meiosis.
 Crossing over may occur.
 Metaphase I
 Tetrads line up at the equator of the cell.
 Anaphase
 One member of each homologous pair moves to opposite poles.
 No serparation at the centromere.
 Telophase I
 Cytokinesis.
 Resulting in two haploid cells (only one member of each homologous pair).
 Meiosis II (No S phrase – no replication between Meiosis I and II)
 Prophase II
 New spindle forms
 Haploid member of double-stranded chromosomes begin to organize themselves for secondary

2. division.
 Metaphase II
 Hapliods line up at the equator.
 Anaphase II
 Separation at the centromere and the single-stranded chromosomes move to the poles.
 Telophase II
 A new nuclei forms.
 Cytokinesis.
 At the end, 4 haploid cells (each being genetically distinct) are created.