Cell Biology and Cell Division
Cell Biology and Cell Division examines how cells are built and how they reproduce. The exam tests organelle structure–function links, membrane organisation, the cell cycle, and the differences between mitosis and meiosis, including chromosome counting.
Organisation and membranes
- Cell theory (Schleiden, Schwann, Virchow) has exceptions: mature RBCs lack a nucleus, and skeletal muscle fibres are multinucleate.
- Prokaryotes have a nucleoid, 70S ribosomes, a peptidoglycan wall and no membrane-bound organelles. Eukaryotes have 80S cytoplasmic ribosomes but 70S ribosomes inside mitochondria and chloroplasts.
- The fluid mosaic model (Singer and Nicolson, 1972) describes a phospholipid bilayer with integral and peripheral proteins; cholesterol alters fluidity, and small non-polar molecules cross most easily.
Cycle and division
- Interphase = G1, S (DNA replication), G2; G0 is a quiescent, non-dividing state.
- Mitosis: prophase (condensation, spindle formation), metaphase (chromosomes on the equator, kinetochore attachment), anaphase (sister chromatids separate), telophase. Cytokinesis uses a cell plate in plants (Golgi vesicles) and a cleavage furrow in animals.
- Meiosis I separates homologous chromosomes; prophase I includes synapsis, tetrads and crossing over at chiasmata. Meiosis II separates sister chromatids. Four haploid cells result.
Worked example
A diploid cell has 2n = 8. At mitotic metaphase it has 8 chromosomes, each with 2 chromatids, so 16 chromatids. At mitotic anaphase, before cytokinesis, the same cell contains 16 chromosomes. After meiosis, each of four cells has 4 chromosomes (n = 4), and each chromosome is single-chromatid. The reasoning is that centromere number counts chromosomes, while chromatid number doubles after S phase.
Traps
Do not count chromatids as chromosomes; meiosis does not occur in gametes but in germ-line cells; plant cytokinesis is not by furrowing; G0 is not death; 70S ribosomes also occur in chloroplasts.