Cell Biology: Structure, Organelles, Cell Cycle and Division
The exam checks organelle structure–function matching, the membrane model, and chromosome behaviour stage by stage in mitosis and meiosis. Numerical questions on chromosome and DNA content across stages are frequent.
Core rules
- Cell theory: Schleiden and Schwann, extended by Virchow's omnis cellula e cellula.
- Plasma membrane - fluid mosaic model (Singer and Nicolson, 1972): phospholipid bilayer with integral and peripheral proteins, cholesterol and glycoproteins; selectively permeable.
- Organelles: mitochondria (cristae, matrix, own DNA, 70S ribosomes); chloroplast (thylakoid, granum, stroma); rough ER (ribosomes, protein synthesis) and smooth ER (lipid synthesis, detoxification); Golgi (glycosylation, cell-plate formation); lysosome (acid hydrolases); peroxisome (catalase, photorespiration); eukaryotic ribosome 80S, prokaryotic and organellar 70S.
- Cell cycle: G1 → S (DNA replication) → G2 → M. Mitosis is equational; meiosis has two divisions.
- Prophase I substages: leptotene, zygotene (synapsis, synaptonemal complex), pachytene (crossing over), diplotene (chiasmata visible), diakinesis (terminalisation).
Worked example
A root-tip cell has 2n = 24. After mitosis each daughter cell has 24 chromosomes and the same DNA content as the parent at G2. After meiosis, each of the four products has n = 12. At metaphase I there are 12 bivalents; at metaphase II there are 12 chromosomes per cell, each with two chromatids.
Traps that cost marks
Saying DNA replicates in G2 (it is S phase); halving the chromosome number at anaphase II (homologues separate at anaphase I, sister chromatids at anaphase II); counting chromatids as chromosomes; describing the synaptonemal complex as part of the spindle.