Unit 3: Genetics I: Cellular reproduction describes and explains how genetic inheritance occurs asexually through mitosis and sexually through meiosis
Unit III: Essay Questions Click here to view the possible essay questions for the Unit 3 test
Lecture Notes
7.1 Meiosis forms haploid cells
A. Stages of meiosis
1) chromosomes become visible
2) nuclear envelope breaks down
3) crossing over occurs: portions of homologous chromosomes are exchanged producing genetic variation
- metaphase I: pairs of homologous chromosomes move to the equator of the cell
- anaphase I: homologous chromosomes move to opposite poles of the cell
- telophase I: chromosomes gather at the poles of the cell
- cytokinesis: cytoplasm divides
- prophase II: a new spindle forms around the chromosomes
- metaphase II: chromosomes line up at the equator
- anaphase II:
1) centromeres divide
2) chromatids (now called chromosomes) move to opposite poles of the cell
- telophase II: nuclear envelope forms around each set of chromosomes
- cytokinesis: cytoplasm divides
B. Meiosis contributes to genetic variation
- genetic variation is a key to evolution by natural selection, and meiosis produces most of the genetic variation seen in sexually reproducing populations
- independent assortment randomly separates homologous chromsomes to opposite poles in anaphase I
- in humans, with 23 pairs of homologs, independent assortment can produce about 8 million combinations of chromosomes in gametes
- crossing over exchanges portions of homologous chromosomes during prophase I, with practically unlimited possible combinations of genes
- random fertilization of gametes in humans produces up to 64 trillion possible combinations of genes
C. Gamete formation in male and female animals involves meiosis
- spermatogenesis produces 4 small haploid sperm in males at a rapid rate
- oogenesis produces a single large ovum containing much cytoplasm in females at a much slower rate
