Cells, Genetics, and Evolution
This domain tests cell structure and transport, the central dogma, inheritance patterns, and the mechanism of evolution. Items ask you to identify an organelle's function, predict offspring ratios from a cross, trace information from DNA to protein, or explain how a trait spreads through a population.
Cells and transport
- Prokaryotes lack a nucleus and membrane-bound organelles; eukaryotes have both.
- Mitochondria carry out respiration; chloroplasts carry out photosynthesis; ribosomes synthesise protein; the rough ER and Golgi process and package it.
- Diffusion and osmosis are passive (high to low concentration); water moves toward the higher solute concentration. Active transport needs ATP and can move substances against the gradient.
- Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Respiration is the reverse.
- Mitosis produces two genetically identical diploid cells; meiosis produces four genetically distinct haploid gametes with crossing over and independent assortment.
Genetics
DNA is transcribed to mRNA in the nucleus and translated to protein at the ribosome; a codon of three bases specifies one amino acid. In a monohybrid cross, a dominant allele masks a recessive one in the heterozygote. A testcross (crossing with a homozygous recessive) reveals an unknown genotype.
Worked example
Two heterozygous pea plants (Aa × Aa) are crossed. Predict the offspring.
Punnett square: each parent gives A or a with equal probability, giving 1 AA : 2 Aa : 1 aa. Three of four offspring show the dominant phenotype and one shows the recessive, a 3:1 ratio; 25% are homozygous recessive and 50% are heterozygous. For 400 offspring, expect about 300 dominant and 100 recessive. Note that "dominant" describes expression, not frequency: a dominant allele can still be rare in a population.
Evolution
Natural selection requires heritable variation, differential reproductive success, and change across generations. It acts on phenotypes, not on demand; populations evolve, individuals do not. Evidence includes the fossil record, homologous structures, embryology, biogeography and molecular sequence similarity.
Traps that cost marks
- Confusing mitosis (diploid, identical) with meiosis (haploid, variable).
- Reversing the direction of osmosis, or calling diffusion active.
- Believing dominant alleles are necessarily common or "stronger."
- Saying an organism "adapted" during its lifetime when the trait is inherited.
- Mixing up transcription (DNA to RNA) with translation (RNA to protein).