Cell Structure, Function and Cell Cycle
Cell Structure, Function and Cell Cycle covers organelles, membrane models, chromosome behaviour and the arithmetic of mitosis and meiosis.
Core rules
- Cell theory; prokaryotic cells lack a nucleus and membrane-bound organelles; eukaryotic cells compartmentalise functions.
- Plasma membrane: fluid mosaic model (Singer and Nicolson); selective permeability.
- Organelles: mitochondria (ATP), chloroplasts (photosynthesis), ER (synthesis and transport), Golgi (packaging), lysosomes (hydrolytic enzymes), ribosomes (70S in prokaryotes, 80S in eukaryotes), nucleus, cytoskeleton.
- Cell cycle: G1 → S (DNA replication) → G2 → M. Mitosis maintains chromosome number; meiosis halves it and generates variation by crossing over and independent assortment.
Worked example
A cell has 2n = 24. In G1 it has 24 chromosomes and 2C DNA. After S phase, it has 24 chromosomes but 4C DNA. At mitotic metaphase: 24 chromosomes, 48 chromatids, 4C. At mitotic anaphase, sister chromatids separate, so there are 48 chromosomes, 48 chromatids and 4C. Each daughter cell after cytokinesis has 24 chromosomes and 2C. In meiosis, each gamete receives 12 chromosomes and 1C.
Traps
Chromosome number doubles at anaphase, not at S phase. DNA content doubles at S phase without a change in chromosome number. Meiosis II resembles mitosis but starts with haploid cells. Crossing over occurs at pachytene, not metaphase.