Cell Cycle, Cell Division and Reproduction in Organisms
What the exam tests
This domain tests the cell cycle, mitosis, meiosis, and how organisms reproduce asexually and sexually. You should track chromosome and DNA amounts through division and explain how meiosis generates variation.
Core rules
- Interphase has G1 (growth), S (DNA replication), and G2 (preparation); checkpoints control progression.
- Mitosis produces two genetically identical diploid cells; prophase, metaphase, anaphase, telophase, and cytokinesis are sequential.
- Meiosis has two divisions: meiosis I separates homologous chromosomes; meiosis II separates sister chromatids. Synapsis and crossing over occur in prophase I; independent assortment occurs at metaphase I.
- Asexual reproduction (binary fission, budding, fragmentation, vegetative propagation) produces clones; sexual reproduction involves gametes and fertilization.
- In plants, cytokinesis forms a cell plate; in animals, a cleavage furrow forms.
Worked example
A diploid cell has 2n = 20. At metaphase I, homologous chromosomes are paired, so there are 10 bivalents. Each chromosome has two chromatids, giving 40 chromatids. After meiosis I, each cell has 10 chromosomes (each with two chromatids). At anaphase II, centromeres split, so each pole receives 10 chromosomes with one chromatid each. Final gametes have n = 10. Reasoning: meiosis halves chromosome number but DNA amount is halved only after meiosis II.
Traps
Do not count centromeres and chromatids interchangeably. Meiosis II resembles mitosis but starts with haploid cells. Crossing over occurs between nonsister chromatids of homologous chromosomes, not sister chromatids. In plant cells, the cell plate forms from Golgi vesicles; no cleavage furrow.