Cell Biology
This section covers cell structure and cell division, which the exam tests through organelle function, prokaryote-eukaryote comparisons, membrane transport, and chromosome arithmetic in mitosis and meiosis.
Core rules
Prokaryotes lack a membrane-bound nucleus, have circular DNA, 70S ribosomes, and a peptidoglycan cell wall. Eukaryotes have a nucleus, membrane-bound organelles, and 80S cytosolic ribosomes. Mitochondria and chloroplasts contain their own circular DNA and 70S ribosomes, reflecting their endosymbiotic origin. The plasma membrane follows the fluid mosaic model and is selectively permeable; osmosis moves water only, from lower to higher solute concentration. The cell cycle is G1, S, G2, and M. Mitosis preserves chromosome number; meiosis halves it and generates variation through crossing over and independent assortment.
Worked example
A diploid cell has 2n = 24. After mitosis, each daughter cell has 24 chromosomes. After meiosis, each gamete has 12 chromosomes. In G2, after DNA replication, the cell still has 24 chromosomes but 48 chromatids, because each chromosome now has two sister chromatids. Counting centromeres gives chromosome number; counting arms gives chromatid number. Meiosis I separates homologous chromosomes, while meiosis II separates sister chromatids.
Traps that cost marks
Do not say all eukaryotic ribosomes are 80S; mitochondrial and chloroplast ribosomes are 70S. Do not mix cell wall composition: bacteria use peptidoglycan, plants cellulose, fungi chitin. Do not claim meiosis produces identical cells. Do not treat diffusion as requiring ATP. Do not confuse chromosome number with chromatid number after S phase.