Biology: Cells, Division, and Signaling
Biology on the PA-CAT spans cell structure, division, signaling, and the principles that connect them. Know each organelle's job and one disease or drug that reveals it: nucleus for transcription, rough ER for secreted and membrane proteins, smooth ER for lipid synthesis and detoxification, Golgi for modification and sorting, mitochondria for oxidative ATP, lysosomes for acid hydrolysis, peroxisomes for very-long-chain fatty acids and peroxide. Prokaryotes lack membrane-bound organelles and a nucleus; their ribosomes are 70S versus eukaryotic 80S, a difference antibiotics exploit.
Cell cycle control matters: G1/S and G2/M checkpoints, cyclins and cyclin-dependent kinases, and the distinction between mitosis (two identical diploid cells) and meiosis (four haploid gametes with crossing over and independent assortment). Signal transduction questions usually ask you to order the steps: ligand, receptor, transducer, amplifier, effector, response-and to distinguish surface receptors from intracellular receptors.
Traps: confusing meiosis I (homologous separation) with meiosis II (sister chromatid separation); thinking crossing over changes chromosome number rather than creating recombinant chromatids; and assuming all signaling is fast-steroid hormones act genomically and slowly.
Worked example: a human cell with 46 chromosomes enters meiosis. After meiosis I, each daughter has 23 chromosomes, each with two chromatids. After meiosis II, each gamete has 23 chromosomes, each with one chromatid.
Rule of thumb: if a question mentions gametes or genetic diversity, think meiosis; if it mentions growth, repair, or identical copies, think mitosis.