Biology - Cell Biology, Reproduction, and Genetics
Cell biology, reproduction, and genetics questions ask you to connect structures to processes and then to inheritance ratios. Expect organelle functions, membrane transport, mitosis versus meiosis, DNA replication and protein synthesis, Mendelian crosses, and mutation consequences.
Core rules
- Prokaryotes lack a nucleus and membrane-bound organelles; eukaryotes compartmentalize reactions.
- Diffusion and osmosis are passive; active transport uses ATP and can move substances against a gradient.
- Mitosis produces two genetically identical diploid cells for growth and repair. Meiosis produces four haploid gametes with crossing over and independent assortment.
- Central dogma: DNA → RNA → protein. Transcription occurs in the nucleus; translation occurs at ribosomes.
- Monohybrid cross Aa × Aa gives genotype ratio 1 AA : 2 Aa : 1 aa and phenotype ratio 3 dominant : 1 recessive.
Worked example
In pea plants, tall (T) is dominant to short (t). Cross two heterozygous tall plants, Tt × Tt. Each parent makes T and t gametes in equal proportion. A Punnett square gives TT, Tt, Tt, tt: 1 TT : 2 Tt : 1 tt. Three of four offspring are expected tall and one short, a 3:1 phenotypic ratio. The 2:1 genotypic middle class matters when interpreting test-cross data; the recessive phenotype appears only with tt.
Traps
- Saying meiosis produces identical cells; it produces genetically varied haploid cells.
- Assuming the dominant allele is more common in a population.
- Applying independent assortment to genes on the same chromosome without recombination.
- Confusing codominance with incomplete dominance.
- Forgetting that translation happens at ribosomes, not in the nucleus.