USABO Study Guide: 2025/2026 Syllabus

USABO Study Guide: 2025/2026 Syllabus Focus Areas: Cell Biology & Biochemistry, Genetics & Molecular Biology, Plant Anatomy & Physiology, Animal Anatomy & Physiology, Ethology, Evolution & Systematics, Ecology, and Biosystematics. Cell Biology & Biochemistry 1. What is the primary mechanism by which ATP is synthesized in the mitochondrial electron transport chain? ANSWER Chemiosmosis. The energy from electron transfer is used to pump protons across the inner mitochondrial membrane, creating an electrochemical gradient. The flow of protons back through ATP synthase drives the phosphorylation of ADP to ATP. 2. During which specific phase of the cell cycle is DNA replicated? ANSWER The S phase (Synthesis phase) of interphase. 3. A researcher treats cells with a drug that disrupts the structure of microtubules. Which cellular processes will be most directly affected? ANSWER Mitosis (specifically spindle formation and chromosome separation), vesicle/organelle transport, and maintenance of cell shape. 4. How do competitive and non-competitive enzyme inhibitors differ at the molecular level? ANSWER A competitive inhibitor binds to the active site of the enzyme, competing directly with the substrate. A non-competitive inhibitor binds to an allosteric site, changing the enzyme's conformation and reducing its activity, regardless of substrate concentration. 5. What is the net yield of ATP (approximately) from the complete aerobic oxidation of one molecule of glucose in a eukaryotic cell? ANSWER Approximately 30-32 ATP molecules. The variation arises from the "cost" of shuttling NADH from glycolysis into the mitochondria (malate-aspartate shuttle yields ~32, glycerol phosphate shuttle yields ~30). 6. What is the primary function of the Golgi apparatus? ANSWER Modification, sorting, and packaging of proteins and lipids for secretion or delivery to other organelles. 7. What is the role of chaperone proteins like HSP70? ANSWER They assist in the proper folding of nascent polypeptide chains and the refolding of misfolded proteins, often using ATP. 8. Describe the signal hypothesis for the targeting of proteins to the endoplasmic reticulum. ANSWER Proteins destined for the ER, secretion, or membranes have an N-terminal signal peptide. This peptide is recognized by a Signal Recognition Particle (SRP), which pauses translation and docks the ribosome complex onto an SRP receptor on the ER membrane. Translation then resumes, and the polypeptide is translocated into the ER lumen. 9. Why is the fluidity of the cell membrane critical for function? ANSWER It allows for membrane protein movement, proper membrane assembly, vesicle formation, and cell division. Fluidity is modulated by cholesterol and the saturation of fatty acid tails. 10. What is the fundamental difference between prokaryotic and eukaryotic ribosomes? ANSWER Prokaryotic ribosomes are 70S (composed of 50S and 30S subunits), while eukaryotic ribosomes are 80S (composed of 60S and 40S subunits). This difference is the basis for many antibiotics. Genetics & Molecular Biology 11. A woman is heterozygous for an autosomal dominant disorder (Aa). She has a child with an unaffected man (aa). What is the probability their child will be affected? ANSWER 50%. The cross is Aa x aa. The possible genotypes are Aa (affected) and aa (unaffected), each with a 1/2 probability. 12. In a dihybrid cross (AaBb x AaBb) with independent assortment, what is the expected phenotypic ratio? ANSWER 9:3:3:1. 13. What is the central dogma of molecular biology? ANSWER DNA is transcribed into RNA, which is translated into protein. Information flow is DNA → RNA → Protein. 14. What is the function of DNA ligase in DNA replication? ANSWER It catalyzes the formation of a phosphodiester bond between the 3'-OH of one Okazaki fragment and the 5' phosphate of the next, sealing nicks in the sugarphosphate backbone.