1,800+ MCAT terms, defined.

Hydrophobic interactions are the tendency of nonpolar groups to cluster together in water, which releases ordered water molecules and increases the entropy of the water. They help drive protein folding, burying nonpolar side chains in th...

Ketone body formation (ketogenesis) is the liver's mitochondrial conversion of acetyl-CoA, mainly from fatty acid β-oxidation, into acetoacetate and β-hydroxybutyrate, plus a little acetone. It rises in fasting or low insulin, and the br...

Metabolic states are the body's fuel-handling modes set by time since the last meal: absorptive (fed), postabsorptive (fasting), and prolonged fasting. Insulin drives storage in the fed state; glucagon drives release of glucose and other...

A premature stop codon is a UAA, UAG, or UGA that appears in the reading frame before the normal end of a gene, so translation ends early and a shortened protein is made. It is usually caused by a nonsense mutation, which changes a sense...

Reverse transcription is the making of DNA from an RNA template, catalyzed by reverse transcriptase. Retroviruses such as HIV use it to copy their RNA genome into DNA that integrates into the host genome. Key point: the template is RNA a...

A ribosome is a membrane-less ribonucleoprotein machine with a small and a large subunit, each built from rRNA and proteins, that translates mRNA into protein. Prokaryotes have 70S ribosomes (30S + 50S); eukaryotes have 80S (40S + 60S)....

RNA interference (RNAi) is gene silencing after transcription: short double-stranded RNAs (siRNA or miRNA) guide the RISC complex to a complementary mRNA, which is then cleaved or blocked from translation. The testable point is that prot...

Degradation is the enzyme-catalyzed breakdown of a biomolecule, such as a protein, mRNA, or fuel molecule, into smaller units, often by hydrolysis. Most testable: many proteins are tagged with ubiquitin and destroyed by the proteasome, s...

Phosphoglycerate kinase catalyzes step 7 of glycolysis: it moves a phosphoryl group from 1,3-bisphosphoglycerate to ADP, making ATP and 3-phosphoglycerate. This is substrate-level phosphorylation, and the reaction is reversible, so it al...

Allosteric regulation is control of a protein's activity by a molecule binding at a site other than the active site, which shifts the protein's conformation. Allosteric enzymes often show sigmoidal rate versus [S] curves rather than hype...

Amino acid catabolism is the breakdown of amino acids: the amino group is removed by transamination and deamination and ends up as urea, while the carbon skeleton enters energy metabolism. Most testable: glucogenic skeletons feed glucone...

Antiparallel strands run in opposite directions. In a DNA double helix, one strand runs 5'→3' while its partner runs 3'→5', which lets the base pairs line up; β-pleated sheets can also be antiparallel. Most testable: a new strand is read...

Cell adhesion molecules (CAMs) are membrane proteins that attach a cell to neighboring cells or to the extracellular matrix. The key split: cadherins (Ca²⁺-dependent) mainly join cell to cell, while integrins join cell to matrix. Selecti...

Cholesterol metabolism is how cells make, transport, use, and remove cholesterol. It is built from acetyl-CoA, and the key testable point is that HMG-CoA reductase is the rate-limiting enzyme, lowered by high cholesterol and favored by i...

A cloning vector is a DNA molecule, usually a bacterial plasmid, that carries a foreign DNA insert into a host cell and replicates there, making many copies. Testable point: antibiotic resistance on the vector shows a cell took up a plas...

Coupled reactions pair an endergonic reaction (ΔG > 0) with an exergonic one, usually through a shared intermediate, so the overall process can proceed. The testable point: the free energy changes add, and the pair is spontaneous only if...

A disulfide linkage is a covalent S–S bond formed by oxidation of the thiol groups of two cysteine side chains. It stabilizes tertiary or quaternary structure, especially in extracellular proteins, and is broken by reducing agents such a...

Eukaryotic gene regulation is the control of which genes are expressed and how much product is made. Control is often greatest at transcription initiation, where transcription factors act at promoters and enhancers, and chromatin state m...

mRNA processing is the set of changes that convert eukaryotic pre-mRNA into mature mRNA in the nucleus: a 5′ cap, splicing out introns, and a 3′ poly-A tail. Most testable: introns are removed and exons joined, and alternative splicing l...

Hemoglobin binds up to four O₂ cooperatively: each O₂ bound makes the next bind more easily, giving a sigmoidal curve, unlike hyperbolic myoglobin. Most testable: a right shift (lower pH, more CO₂, higher temperature, more 2,3-BPG) means...

Quaternary structure is the way two or more separate folded polypeptide chains (subunits) assemble into one functional protein. It is held together mainly by noncovalent forces, sometimes with disulfide bonds. A protein made of a single...

Redox carriers are small coenzymes, mainly NAD⁺/NADH, FAD/FADH₂ and NADP⁺/NADPH, that shuttle electrons between reactions. NADH and FADH₂ carry electrons from fuel oxidation to the electron transport chain, while NADPH mostly supplies el...

Retroviruses (such as HIV) move genetic information RNA → DNA → RNA → protein: reverse transcriptase copies the RNA genome into DNA, which integrates into the host genome and is expressed by host machinery. The testable step is RNA → DNA.

In semiconservative replication, each daughter DNA double helix contains one original (parental) strand and one newly synthesized strand. The Meselson–Stahl ¹⁵N/¹⁴N density experiment supported it over the conservative and dispersive mod...

Prokaryotic transcription is the making of RNA from a DNA template by a single RNA polymerase, which a sigma factor directs to the promoter. Testable point: bacterial mRNA is not processed, is often polycistronic, and is translated while...

Zymogen activation is the conversion of an inactive enzyme precursor (a zymogen or proenzyme) into an active enzyme, typically by cutting specific peptide bonds. The cleavage is covalent and irreversible. The classic example is trypsinog...

The rate-limiting step is the slowest step in a multistep reaction or pathway, so it sets the overall rate. In metabolism it is usually an early, essentially irreversible, tightly regulated enzyme step (PFK-1 in glycolysis); speeding up...

Thymine dimers are covalent bonds between two adjacent thymine bases on the same DNA strand, formed by ultraviolet light. They distort the helix and block replication, and in humans they are mainly removed by nucleotide excision repair.

Amphoteric describes a species that can act as either an acid or a base. In biochemistry the standard example is an amino acid: its carboxyl group can donate a proton and its amino group can accept one. Its net charge therefore depends o...

A hydroxyl group is an −OH unit covalently bonded to carbon. It is polar and forms hydrogen bonds, so it usually raises water solubility and boiling point. Most testable: hydroxyl (−OH, neutral, covalent) is not hydroxide (OH⁻, a free io...

A pentose is a monosaccharide with five carbons, general formula C₅H₁₀O₅. The key examples are ribose (in RNA) and 2-deoxyribose (in DNA). The most testable link is the pentose phosphate pathway, which makes NADPH and ribose-5-phosphate...

Phosphorylated means a phosphate group (PO₃²⁻) has been covalently attached to a molecule, usually by a kinase using ATP. In proteins it typically targets the OH of serine, threonine or tyrosine and can either activate or inhibit the pro...

The amino acid sequence is the order of residues in a polypeptide, written from the N-terminus to the C-terminus; it is the protein's primary structure. It is set by the mRNA codons, and in most cases it determines how the protein folds,...

The matrix is the innermost compartment of a mitochondrion, enclosed by the inner membrane. It holds the pyruvate dehydrogenase complex and most citric acid cycle enzymes. Protons are pumped out of it during the electron transport chain,...

Phosphoglycerate mutase is the glycolytic enzyme that shifts the phosphate on glycerate from carbon 3 to carbon 2, converting 3-phosphoglycerate to 2-phosphoglycerate. It is an isomerase, so no ATP or NADH is made or used in this step, a...

Electron carriers are small molecules (NAD⁺/NADH, FAD/FADH₂, NADP⁺/NADPH) that accept electrons when a substrate is oxidized and donate them elsewhere. In catabolism NADH and FADH₂ deliver electrons to the electron transport chain, and N...

A Fischer projection is a flat drawing of a chiral molecule, such as a sugar or amino acid, with a vertical carbon chain, horizontal bonds toward the viewer and vertical bonds away. Most testable: D or L is read from the side of the OH (...

A ligand-gated channel is a membrane ion channel that opens when a specific molecule binds to it. Ions then flow passively down their electrochemical gradient. The testable point is the effect on membrane potential: Na⁺ or Ca²⁺ influx us...

A transmembrane protein is an integral membrane protein that crosses the lipid bilayer, with regions in the extracellular fluid, the membrane core, and the cytosol. Its membrane-spanning segment is usually nonpolar, most often an α-helix...

A cytochrome is a heme-containing protein whose iron cycles between Fe³⁺ and Fe²⁺ to pass electrons one at a time down the electron transport chain. Most testable: cytochrome c carries electrons from Complex III to Complex IV, which redu...

kcat is the turnover number: the number of substrate molecules one enzyme active site converts to product per second when the enzyme is saturated with substrate. It equals Vmax divided by total enzyme concentration, so unlike Vmax it doe...

DNA methylation is the addition of a methyl group (–CH₃) to a base in DNA, typically carbon 5 of cytosine in CpG sequences, by DNA methyltransferases. Methylation of a promoter usually lowers transcription without changing the base seque...

An embedded protein is an integral membrane protein, held in the phospholipid bilayer by nonpolar amino acids that contact the lipid tails. Many span the whole bilayer (transmembrane), but some sit only partly in it. It is removed from t...

Nucleotide synthesis is how cells make nucleotides, either de novo from small precursors or by salvaging free bases. Most testable: purine rings are built on PRPP (ribose-5-phosphate activated by ATP), while pyrimidine rings are built fi...

Okazaki fragments are short pieces of new DNA, each started by an RNA primer, that are made on the lagging strand during replication. DNA polymerase only extends a 3' end, so the lagging strand is built discontinuously and the pieces are...

A knockout is an organism or cell line in which a chosen gene has been deliberately disabled or deleted, so it makes no functional product. Comparing it with the wild type shows what the gene normally does. The key inference is loss of f...

A competitive inhibitor resembles the substrate and usually binds the enzyme's active site reversibly, so it competes with substrate. It raises the apparent Km but leaves Vmax unchanged, because enough substrate can outcompete the inhibi...

A soap is the sodium or potassium salt of a long-chain fatty acid, made by base hydrolysis (saponification) of triglycerides. Its charged head and nonpolar tail make it amphipathic, so it forms micelles that trap grease in a nonpolar cor...

Sphingosine is an 18-carbon amino alcohol with a long hydrocarbon chain. It is the backbone of sphingolipids, used in place of glycerol. Adding a fatty acid to its amino group by an amide bond gives ceramide, the precursor of sphingomyel...

A conjugated protein is a protein that contains a non-amino-acid component, called a prosthetic group, bound to its polypeptide chain. The most testable point is that the prosthetic group is often required for function, as with heme in h...