Cell Structure and Function: Cell Cycle, Cell Division and Biomolecules
This domain covers the timing and events of the cell cycle, the differences between mitosis and meiosis, and the structure and behaviour of biomolecules, especially enzymes. Numerical questions on chromosome and DNA content are common.
Cell cycle and division
- Interphase: G1 for growth and RNA/protein synthesis, S for DNA replication, G2 for spindle protein synthesis. In a typical human cell the mitotic phase occupies about one hour of a 24-hour cycle.
- Mitosis proceeds through prophase, metaphase, anaphase and telophase; plant cytokinesis builds a cell plate from Golgi vesicles.
- Meiosis I prophase substages: leptotene, zygotene (synapsis, synaptonemal complex), pachytene (crossing over with recombination nodules), diplotene (chiasmata visible), diakinesis.
- Meiosis has one round of replication followed by two divisions, so chromosome number halves at meiosis I and DNA content halves at meiosis II.
Biomolecules
- Twenty amino acids, mostly zwitterionic; peptide bonds link the carboxyl group of one to the amino group of the next.
- Protein structure levels: primary, secondary (alpha helix, beta sheet), tertiary and quaternary.
- Enzymes lower activation energy. A competitive inhibitor raises Km but leaves Vmax unchanged; a non-competitive inhibitor lowers Vmax.
- Carbohydrates, lipids and nucleic acids are covered with their monomers and bonds.
Worked example
A cell with 2n = 8 enters meiosis. After S phase it has 8 chromosomes and 4C DNA. At metaphase I there are 4 bivalents (tetrads). After meiosis I each cell has 4 chromosomes and 2C DNA. At anaphase II chromatids separate, so 8 chromosomes are temporarily present. Each gamete finally has 4 chromosomes and 1C DNA.
Traps that cost marks
- Synapsis occurs in zygotene, crossing over in pachytene.
- At anaphase I chromosomes move, not chromatids; chromatids separate only at anaphase II.
- Competitive inhibition increases Km, it does not reduce Vmax.
- Plant cytokinesis uses a cell plate, not a cleavage furrow.