Cell Biology, Genetics, and Evolution
What to know
Cells are the basic unit of life. Prokaryotes lack a nucleus and membrane-bound organelles; eukaryotes have both. Key organelles: nucleus (DNA), mitochondria (ATP production), ribosomes (protein synthesis), chloroplasts (photosynthesis in plants), cell membrane (selective transport), and cell wall (plants). Plant cells have chloroplasts and a cell wall; animal cells do not. Mitosis produces identical body cells; meiosis produces gametes with half the chromosome number and creates genetic variation through crossing over and independent assortment.
DNA is a double helix; genes are segments that code for proteins. Transcription makes mRNA from DNA; translation builds proteins at ribosomes. Mendelian genetics: dominant and recessive alleles, genotype versus phenotype, Punnett squares, and probabilities. Know monohybrid crosses and sex-linked traits.
Evolution is change in allele frequencies over generations. Mechanisms: mutation, gene flow, genetic drift, and natural selection. Evidence includes fossils, homologous structures, embryology, and DNA similarities. Natural selection acts on existing variation; individuals do not evolve - populations do. Speciation occurs when populations become reproductively isolated.
Traps
Questions confuse mitosis and meiosis, treat dominant as "more common," or say individuals adapt during their lifetime in an evolutionary sense. Watch for the misconception that evolution is goal-directed.
Rule of thumb and worked example
Cross two heterozygotes (Aa × Aa): offspring are 1 AA : 2 Aa : 1 aa, so 75% show the dominant phenotype and 25% the recessive. Use the product rule for independent traits: multiply probabilities across genes.