1,800+ MCAT terms, defined.

The capsid is the protein coat that encloses a virus's DNA or RNA genome, built from repeating viral-encoded subunits called capsomeres. Capsid plus genome is the nucleocapsid. Some viruses add a lipid envelope outside it; naked viruses...

Metaphase is the stage of mitosis, and of meiosis I and II, when chromosomes line up at the cell's equator (the metaphase plate), attached to spindle microtubules from opposite poles. Sister chromatids separate next, in anaphase. In meio...

The mitral valve (bicuspid, or left atrioventricular, valve) is the one-way valve between the left atrium and left ventricle. It opens when atrial pressure exceeds ventricular pressure and closes as the ventricle contracts, preventing ba...

An unmyelinated axon lacks an insulating myelin sheath, so the action potential regenerates at each adjacent patch of membrane (continuous conduction) instead of jumping between nodes of Ranvier. Testable point: it conducts more slowly t...

Disruptive (diversifying) selection is natural selection that favors both extreme phenotypes over the intermediate one, so a single-peaked trait distribution becomes bimodal. It raises variance, and it can contribute to speciation but do...

MHC (major histocompatibility complex) proteins are cell-surface proteins that present peptide fragments to T cells. MHC I is on nearly all nucleated cells and presents intracellular peptides to CD8⁺ T cells; MHC II is on antigen-present...

A tropic hormone acts on another endocrine gland and stimulates it to make or release its own hormone. Classic examples are the anterior pituitary's TSH, ACTH, FSH and LH. Most testable: the end hormone usually feeds back negatively, so...

Independent assortment is Mendel's second law: alleles of different genes are sorted into gametes independently, because homologous pairs orient randomly at metaphase I. It holds for genes on different chromosomes (and nearly for genes f...

A basal body is a microtubule structure at the base of a eukaryotic cilium or flagellum. It anchors the appendage and templates its growth. It has the same 9 triplet microtubule arrangement as a centriole (9+0), unlike the 9+2 doublet pa...

Clonus is a series of rhythmic, involuntary muscle contractions set off by a sudden, sustained stretch, such as a quick upward push on the foot. It typically reflects an overactive stretch reflex after loss of descending inhibition. It i...

Aerotolerant anaerobes do not use oxygen for metabolism and rely on fermentation for ATP, but they survive in the presence of O₂ and grow equally well with or without it. Unlike obligate anaerobes, they are not killed by O₂.

A chromatid is one of the two essentially identical DNA copies of a replicated chromosome. The pair, called sister chromatids, is joined at the centromere. Replication in S phase doubles the chromatids and DNA content but not the chromos...

Ependymal cells are ciliated glial cells that line the brain's ventricles and the spinal cord's central canal. Those in the choroid plexus help make cerebrospinal fluid (CSF), and cilia help circulate it. Testable point: they handle CSF,...

Leak channels are ion channels that stay open at rest, letting ions diffuse down their electrochemical gradients with no ATP. K⁺ leak channels are the main reason the resting potential is negative and close to the K⁺ equilibrium potentia...

Stabilizing selection is natural selection that favors intermediate phenotypes and acts against both extremes of a trait. The population curve gets narrower (less variance) while the mean stays roughly the same. Classic example: human bi...

The tail sheath is the protein layer around the central tail tube of a contractile-tailed bacteriophage such as T4. When the phage binds a host, the sheath contracts and drives the tube through the bacterial envelope, so the DNA is injec...

Adaptive radiation is the relatively rapid diversification of one ancestral lineage into many species, each adapted to a different ecological niche. It is typically triggered by open niches, such as a new island chain or a mass extinctio...

The founder effect is a kind of genetic drift in which a small group splits off from a larger population and starts a new one. Allele frequencies in the new population can differ sharply from the source, so rare alleles may become common...

Genetic leakage is the movement of alleles from one population or species into another, typically when reproductive isolation is incomplete and hybrids backcross with a parent group. It is a form of gene flow, and it tends to make gene p...

The tricuspid valve is the three-leaflet atrioventricular valve between the right atrium and right ventricle. It opens passively when atrial pressure exceeds ventricular pressure and closes at the start of ventricular systole, preventing...

Oogenesis is the formation of female gametes by meiosis in the ovary. Primary oocytes arrest in prophase I, and the ovulated secondary oocyte arrests in metaphase II. Meiosis II is completed only if a sperm penetrates it.

Tendons are cords of dense, collagen-rich connective tissue that attach skeletal muscle to bone and transmit the force of contraction to the skeleton. The testable point: tendons connect muscle to bone, whereas ligaments connect bone to...

Diastole is the phase of the cardiac cycle in which the heart muscle relaxes and the chambers fill with blood. In ventricular diastole the semilunar valves are closed and the AV valves open. Diastolic pressure is the lower number in a bl...

Systole is the contraction phase of the cardiac cycle, when a chamber squeezes blood onward. Ventricular systole is the usual meaning: rising ventricular pressure closes the AV valves (first heart sound), then opens the semilunar valves...

Implantation is the embedding of the blastocyst into the endometrium of the uterus about a week after fertilization. The testable point: trophoblast cells then secrete hCG, which maintains the corpus luteum and its progesterone until the...

Linked genes are loci close together on the same chromosome, so they tend to be inherited together instead of assorting independently. Crossing over can still separate them, and the recombination frequency stays below 50% and rises with...

A heme group is a porphyrin ring holding a central iron ion, the site where hemoglobin and myoglobin bind O₂ reversibly. Testable point: the iron stays Fe²⁺ when O₂ binds (oxygenation, not oxidation), and hemoglobin has four hemes, so it...

A self-antigen is a molecule made by the body's own cells that lymphocyte receptors could bind but that the immune system normally tolerates. Most self-reactive T and B cells are eliminated or silenced (negative selection in the thymus f...

The resting potential is the steady membrane voltage of an unstimulated excitable cell, about −70 mV (inside negative) in a typical neuron. It arises mainly because K⁺ leaks out through open channels while the membrane is far less permea...

Dominance is the allele relationship in which the dominant allele's phenotype appears in a heterozygote and masks the recessive allele's. In complete dominance, Aa looks like AA, so Aa × Aa gives a 3:1 phenotype ratio. Dominant does not...

Homologous chromosomes are the two members of a pair, one from each parent, with the same genes at the same loci but possibly different alleles. They pair in prophase I and separate in anaphase I of meiosis. In evolution, homologous stru...

Metabolic rate is the energy an organism uses per unit time, often measured by O₂ consumption or heat output. Per gram of tissue, smaller animals usually have higher rates than larger ones, and endotherms have higher rates than ectotherm...

Suppressor T cells, now usually called regulatory T cells (Tregs), are T cells that turn down the activity of other T and B cells after a threat is cleared and help prevent attack on self. Testable point: they dampen the immune response,...

Receptor-mediated endocytosis is a selective way for a cell to take in specific molecules: a ligand binds membrane receptors, usually in clathrin-coated pits, and the membrane pinches inward as a vesicle. It is energy-requiring and satur...

The H-zone is the central part of the A band in a sarcomere that holds only thick (myosin) filaments, with no thin-filament overlap. It narrows when the muscle contracts and can disappear in strong contraction, while the A band keeps its...

The I-band is the light region of a striated muscle sarcomere that contains only thin filaments (actin) and is bisected by the Z-line. During contraction the I-band gets shorter, while the A-band stays the same length. That contrast is t...

G2 is the second gap phase of interphase, after S phase and before mitosis. The cell grows, prepares for division, and checks at the G2/M checkpoint that replicated DNA is intact. Testable point: G2 cells have 4n DNA content but the same...

A portal system sends blood through two capillary beds in series, joined by a portal vein, before it returns to the heart. This is a biology topic, not organic chemistry. Testable point: in the hepatic portal system, most absorbed nutrie...

Granules are small cytoplasmic packets of stored material. Secretory granules are membrane-bound, form via the ER and Golgi, and release their contents by exocytosis. Granulocytes (neutrophils, eosinophils, basophils) are white blood cel...

Peristalsis is a wave of alternating smooth muscle contraction and relaxation that pushes contents along a tube, such as the esophagus and intestines. It is involuntary and mainly propulsive (one-way), unlike segmentation, which mixes wi...

The myelin sheath is a lipid-rich insulating wrapping around many axons, made by Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system. It speeds conduction because the action potential jumps b...

The acrosome reaction is the release of digestive enzymes from the acrosome at the sperm head after the sperm contacts the egg's zona pellucida. The enzymes let the sperm digest a path to the egg's plasma membrane. It is a step in fertil...

The adrenal glands sit atop the kidneys. The cortex makes steroid hormones (cortisol, aldosterone); the medulla makes the catecholamines epinephrine and norepinephrine. Testable point: the cortex is driven by hormones (ACTH, angiotensin...

The M-line is the dark line at the center of the sarcomere, in the middle of the H-zone. It anchors the thick (myosin) filaments, holding them in register. During contraction the sarcomere shortens, but the M-line stays in the middle and...

Phosphodiesterase (PDE) is an enzyme that hydrolyzes the second messengers cAMP and cGMP to AMP and GMP, ending the signal. Testable point: inhibiting PDE keeps cAMP high and prolongs the response, while in rods light-activated PDE lower...

Smooth muscle is nonstriated muscle in the walls of hollow organs and vessels, made of spindle-shaped cells that usually have one nucleus. It is mostly involuntary, under autonomic control. Testable point: it has no troponin; Ca²⁺ binds...

A virion is a complete, infectious virus particle outside a host cell: a DNA or RNA genome inside a protein capsid, sometimes with a lipid envelope. It has no metabolism of its own, so new virions are made only when a host cell's machine...

The Bowman's capsule is the cup-shaped start of the nephron that surrounds the glomerulus and collects filtrate pushed out of the blood by hydrostatic pressure. Filtration depends on size and charge, so water, ions, glucose and urea pass...

A checkpoint is a control point in the cell cycle where regulatory proteins check conditions, such as DNA damage or chromosome attachment, and can pause progression until they are met. Testable point: loss of checkpoint proteins like p53...

The antrum is the lower part of the stomach, just before the pyloric sphincter, where churning turns food into chyme and G cells secrete gastrin. Testable point: antral gastrin stimulates HCl release by parietal cells, and low luminal pH...