DNA replication is the process by which a cell makes an exact copy of its DNA before dividing. This study note covers the semiconservative model (Meselson-Stahl), the replication fork structure, the major enzymes (helicase, primase, DNA polymerase, ligase, topoisomerase), the leading vs lagging strand distinction with Okazaki fragments, proofreading and mismatch repair, and the telomere end-replication problem.
Biology
The immune system is the body’s defense against pathogens and transformed cells. This study note covers the two branches (innate vs adaptive), the cells involved (phagocytes, NK cells, T cells, B cells), antibodies and their five classes, immunological memory and vaccination, self-tolerance mechanisms, and the failure modes (immunodeficiency, allergies, autoimmunity, sepsis, cancer immune evasion).
The cell cycle is the ordered sequence a eukaryotic cell goes through from its birth to its division. This study note covers the four phases (G1, S, G2, M), their relative durations in a typical 24-hour human cycle, the three major checkpoints (G1/S restriction point, G2/M, spindle assembly), the cyclin/CDK regulatory engine, the G0 resting state, and how cell-cycle dysregulation is the central problem in cancer.
Osmosis is the passive movement of water across a semipermeable membrane from low solute to high solute concentration. This study note covers the precise definition, why water moves (kinetic theory), tonicity vocabulary (hypotonic, hypertonic, isotonic), osmotic pressure and the van’t Hoff equation, biological roles (cell volume regulation, plant turgor, kidney function, IV fluids), and the difference between osmosis and diffusion.
Meiosis is the specialized cell division that produces 4 genetically unique haploid gametes from 1 diploid parent cell. This study note covers why meiosis exists, the two divisions (meiosis I reductional, meiosis II equational) with all phases, the three sources of genetic variation (crossing over, independent assortment, random fertilization), the meiosis vs mitosis comparison, and what happens when meiosis goes wrong (nondisjunction, Down syndrome, Klinefelter syndrome).
Glycolysis is the first stage of cellular respiration — the universal metabolic pathway that splits one glucose into two pyruvate molecules in the cytoplasm without requiring oxygen. This study note covers the 10 enzymatic steps split into investment and payoff phases, the net 2 ATP and 2 NADH per glucose, what happens to pyruvate aerobically vs anaerobically, and why glycolysis is the most universally conserved pathway in biology.
The Krebs cycle (citric acid cycle, TCA cycle) is the second stage of aerobic cellular respiration. This study note covers where the cycle sits in the bigger picture of respiration, the per-acetyl-CoA inputs and outputs (3 NADH, 1 FADH2, 1 GTP, 2 CO2), the eight enzymatic steps, why the cycle’s intermediates also feed biosynthesis, and how it is regulated by energy demand.
Natural selection is the process by which heritable traits affecting survival and reproduction become more or less common over generations. This study note covers the four conditions for natural selection, the peppered moth example, directional vs stabilizing vs disruptive selection, sexual selection, common misconceptions, and how natural selection fits into the broader modern evolutionary synthesis.
A Punnett square is a 2D grid that predicts the genotype and phenotype ratios of offspring from a known parental cross. This study note covers vocabulary (gene, allele, genotype, phenotype, dominant, recessive), how to build the basic 2×2 cross, the famous 3:1 phenotypic ratio, common cross types, the 4×4 dihybrid square and 9:3:3:1 ratio, and the limitations where the Punnett square breaks down.
The human nervous system is the body’s electrical and chemical communication network — 86 billion neurons connecting through 150 trillion synapses. This study note covers the CNS and PNS divisions, somatic vs autonomic, sympathetic vs parasympathetic, the neuron, action potentials, synapses and neurotransmitters, and glial cells.
A ribosome is the molecular machine that translates messenger RNA into protein by linking amino acids in the order specified by mRNA codons. This study note covers structure (70S vs 80S, large and small subunits), location (free vs membrane-bound), the three stages of translation, polysomes, and the antibiotics that target bacterial ribosomes.
Homeostasis is the maintenance of stable internal conditions across changing external environments. This study note covers the four components of a homeostatic loop (setpoint, sensor, control center, effector), negative vs positive feedback, examples across temperature, glucose, pH, blood pressure, and what happens when homeostasis fails.