Biology glossary

A three-carbon molecule with two phosphates, made in the Calvin cycle when ATP adds a phosphate to 3PG. NADPH then donates electrons that turn it into G3P.

A three-carbon molecule, the first stable product of carbon fixation in the Calvin cycle. Two form each time rubisco attaches CO₂ to RuBP; ATP and NADPH then convert them to G3P.

The end of a DNA or RNA strand that has a free hydroxyl group (–OH) on the 3′ carbon of its last sugar. New nucleotides are always added at this end.

A short single-stranded tail at the tip of a chromosome, where the strand with the 3′ end extends past its partner strand.

A thick, regular coil of nucleosomes long shown in textbooks. It forms when chromatin is taken out of cells, but there is little evidence for long stretches of it in living cells.

A guanine nucleotide with an extra methyl group, attached to the front end of a eukaryotic mRNA. It helps the mRNA leave the nucleus, resist breakdown, and bind a ribosome.

The end of a DNA or RNA strand that has a free phosphate group attached to the 5′ carbon of its last sugar.

The arrangement of microtubules in cilia and flagella: nine outer doublets in a ring around two single central microtubules.

A type of protein secondary structure in which the polypeptide backbone coils into a spiral, held by regularly spaced hydrogen bonds.

One of three ATP synthase subunits that alternate with three β subunits to form a ring, the knob, around the γ stalk. The α subunits stay still and do not make ATP themselves; the β subunits do.

The central carbon atom of an amino acid, bonded to its amino group, carboxyl group, hydrogen atom, and side chain.

The form of glucose whose –OH group on carbon 1 points below the ring. Chains of it make up starch and glycogen.

An organic molecule produced during the breakdown of sugars. Cells link it with ammonium, using energy from NADH, to make the amino acid glutamate.

A citric acid cycle enzyme that removes CO₂ and electrons from α-ketoglutarate. Its step releases a large amount of energy and can limit the cycle’s rate.

A type of protein secondary structure in which stretches of polypeptide backbone lie side by side, linked by hydrogen bonds, and fold into a zigzag like a pleated fan.

One of three ATP synthase subunits that make ATP. They sit still in the knob around the γ stalk, and each 120° turn moves each one to its next conformation: bind ADP and Pᵢ, make ATP, release it.

An orange pigment, one of the carotenoids, that colors carrots and helps leaves absorb blue and blue-green light.

An enzyme, coded by lacZ, that splits lactose into glucose and galactose. It also converts a little lactose into allolactose, the molecule that switches the lac operon on.

One of the polypeptide chains that make up hemoglobin, 146 amino acids long. Changing just one of its amino acids causes sickle cell disease.

The form of glucose whose –OH group on carbon 1 points above the ring. Chains of it make up cellulose.

The long stalk-shaped subunit of ATP synthase that sits on the c ring and reaches up into the knob of α and β subunits. It spins with the ring, and each 120° turn reshapes every β subunit.

The ATP synthase subunit that links the knob of α and β subunits to the two b subunits. It is part of the side arm, which keeps the knob still while the γ stalk turns.

The symbol for the change in usable energy during a reaction: the free energy of the products minus that of the reactants. A negative value means exergonic; a positive value means endergonic.

A subunit of ATP synthase that sits at the base of the γ stalk, where both attach to the c ring. It spins along with the ring and the γ stalk as H⁺ flows through.

The spot on a ribosome where a newly arrived charged tRNA, carrying one amino acid, binds to the matching codon.

A protein subunit of ATP synthase that sits still in the membrane beside the c ring. Its two openings let H⁺ in from the intermembrane space to the ring and out from the ring into the mitochondrial matrix.

A human proto-oncogene. In one form of leukemia, a chromosomal translocation fuses it to the bcr gene, making a fused gene whose protein is overactive.

The human blood types A, B, AB, and O, set by one gene with three alleles, Iᴬ, Iᴮ, and i. Iᴬ and Iᴮ are codominant, and both are dominant to i.

The lowest possible temperature, about −273 °C, at which there would be no heat at all and atoms and molecules would stop moving.

A graph of how much light a pigment absorbs at each wavelength. Chlorophylls show high peaks in the red and violet parts of the visible spectrum and a dip in green.

A two-carbon molecule that yeast make by removing CO₂ from pyruvate during fermentation. NADH then passes electrons to it, turning it into ethanol.

A two-carbon acetyl group linked to the carrier coenzyme A. It is made from pyruvate, fatty acids, and some amino acids, and it feeds its acetyl group into the citric acid cycle.

A two-carbon group of atoms that fuels the citric acid cycle. It comes from pyruvate, fatty acids, and some amino acids and is carried in by coenzyme A.

The attachment of an acetyl group to a molecule. Adding these groups to lysines in histone tails loosens the histones’ grip on DNA, which makes transcription easier.

A neurotransmitter released by nerve cells. On skeletal muscle cells it opens a ligand-gated ion channel, while on heart muscle cells it acts through a GPCR.

An enzyme that quickly breaks down a neurotransmitter released by nerve cells, ending each signal. The nerve gas DIFP permanently blocks it, so nerve cells can’t work properly.

A substance that releases hydrogen ions (H⁺) when dissolved in water, raising the H⁺ concentration and lowering the pH. Lemon juice and stomach fluid are rich in them.

A hydrolase that works best in acidic conditions. Lysosomes hold many kinds, which together can digest nearly any large molecule.

Rain or snow made more acidic than normal by air pollution, mainly from burning coal and oil. It lowers the pH of the soil and water that plants depend on.

Describes a solution with more H⁺ than OH⁻, so its pH is below 7. Lemon juice (pH 2) and black coffee (pH 5) are examples.

Describes a chromosome whose centromere is near one end, leaving a very short p arm.

A hormone that increases the number of cells in the adrenal cortex. When its level falls, many cortex cells undergo apoptosis.

A structural protein that forms filaments inside plant and animal cells, helping give them shape. It also works with myosin to make muscles contract.

The thinnest fiber of the cytoskeleton, about 7 nm wide, made of two strands of actin twisted together. It supports the plasma membrane and drives muscle contraction and cell crawling.

A brief electrical signal that travels along a nerve cell’s membrane, produced as sodium and potassium ions rush through channels.

A graph of how fast photosynthesis runs at each wavelength of light. Its peaks match the wavelengths that chlorophylls and carotenoids absorb best, and it dips in green.

The initial input of energy that reactants need to reach the transition state, where their old bonds can break. Enzymes speed up reactions by lowering it.

A regulatory transcription factor that binds DNA and increases transcription of a gene, for example by helping RNA polymerase bind the promoter. CAP plays this role for the lac operon.

The pocket on an enzyme where its target molecules bind and the chemical reaction takes place.

Movement of a substance across a membrane from lower to higher concentration, against its gradient, through a transport protein. It requires an energy input, such as from ATP.

A heritable trait that helps an organism survive and reproduce in its environment, such as the long snout an anteater uses to reach ants in tight spots.

A protein that links two other proteins together. In the extrinsic pathway, it connects death receptors to an initiator procaspase, forming the DISC.

Describes gene effects that add together, with each contributing allele adding a small amount to the phenotype. In wheat, for example, each red allele makes the grain a bit redder.

A purine, abbreviated A, found in DNA and RNA. In DNA it pairs with thymine (T) through two hydrogen bonds; in RNA it pairs with uracil.

An enzyme in the plasma membrane that makes cAMP from ATP when the α subunit of an active G protein switches it on.

An anchoring junction that joins neighboring animal cells through cadherins linked to actin filaments inside each cell. It often forms a band around the cell.

The attraction between different kinds of molecules, such as water clinging to the walls of a plant’s water-carrying tubes or coating the lining of the digestive tract.

A protein of the extracellular matrix that sticks matrix parts to each other and to cells, using separate binding sites for each. Fibronectin and laminin are examples.

The molecule left when ATP loses one of its three phosphate groups, releasing energy. Cells use energy from food or light to turn it back into ATP.

The outer layer of the adrenal gland. ACTH keeps its cells alive; when ACTH falls, many of them undergo apoptosis.

One of two small glands that sit on top of the kidneys and release hormones, including epinephrine.

Describes conditions in which oxygen is present, or a process that uses oxygen.

The type of cellular respiration that uses oxygen as the final electron acceptor. It breaks glucose down completely to CO₂ and water, making up to 34–38 ATP per glucose.

The strength of attraction between a protein, such as an enzyme or receptor, and a molecule it binds. When it’s high, the protein binds that molecule even at low concentrations.

A gene in mice that controls a yellow pigment in growing hairs. A mutant allele with an extra, highly active promoter makes mice yellow unless DNA methylation silences that promoter.

A small amino acid whose side chain is a single methyl group. Codons beginning with GC code it.

An organic molecule in which a hydroxyl group (–OH) is attached to a carbon atom. The intoxicating ingredient in beer is one example.

A carbonyl group at the end of a carbon chain, written –CHO. It is found in the straight-chain forms of sugars and in some odor molecules.

A simple, plantlike organism, from a single cell to a large seaweed, that makes food by photosynthesis but lacks true roots, stems, and leaves. Most are protists.

Describes a solution with more OH⁻ than H⁺, so its pH is above 7; also called basic. Seawater (pH 8) and household ammonia (pH 11) are examples.

An inherited disease in which the body can’t break down homogentisic acid. The acid builds up, turning the urine black and darkening the skin and cartilage.

A small group made only of carbon and hydrogen atoms, such as a methyl or ethyl group, attached to a larger molecule.

A chemical mutagen that attaches methyl or ethyl groups to DNA bases, causing them to pair with the wrong partners when the DNA is copied.

The attachment of an alkyl group, such as a methyl (–CH₃) or ethyl (–CH₂CH₃) group, to a molecule. Alkylating DNA bases disrupts how they pair.

One of the alternative versions of a gene, differing slightly in DNA sequence. A person usually carries two alleles of each gene, one from each parent.

How common one allele of a gene is in a population, measured as its share of all the copies of that gene. A change in it over generations is microevolution.

A sugar similar to lactose that β-galactosidase makes as a side product. It is the inducer of the lac operon, binding lac repressor and pulling it off the operator.

A binding site on an enzyme outside its active site. A molecule that binds there changes the enzyme’s shape and activity, as in noncompetitive inhibition and feedback inhibition.

A positively charged particle made of two protons and two neutrons, the same as a helium nucleus. Some radioactive atoms give them off, and they were used in the gold foil experiment.

One of the polypeptide chains of hemoglobin. Each hemoglobin molecule has two of these and two beta-globin chains, and each kind is coded by its own gene.

A life cycle with both a multicellular diploid form, the sporophyte, and a multicellular haploid form, the gametophyte, each giving rise to the other. Plants and some protists have it.

An exon kept in some mRNAs made from a gene but removed from others, depending on the cell type or conditions. It often tunes the protein to a particular cell type.

The cutting and joining of RNA copied from one gene in different ways, so that the gene can produce two or more polypeptides with different amino acid sequences.

A disease in which neurons in the brain gradually die, causing memory loss and confusion that worsen over the years.

A test of whether a chemical is a mutagen. Bacteria that can’t make histidine are exposed to it; if far more colonies regain the ability than in an unexposed control, it causes mutations.

A small molecule that is the building block of proteins. Cells link about 20 kinds together, in an order set by genes, to make polypeptides.

The single-stranded 3′ end of a tRNA, where its amino acid is attached. It sits at the opposite end of the molecule from the anticodon.

A functional group made of a nitrogen atom bonded to two hydrogen atoms (–NH₂). It acts as a weak base, picking up H⁺ to become –NH₃⁺, and every building block of proteins has one.

An enzyme that uses energy from ATP to attach the correct amino acid to a tRNA, which then carries it to the ribosome for protein building.

A compound whose molecules have one nitrogen atom bonded to three hydrogen atoms. It acts as a base by taking H⁺ from water, which leaves extra hydroxide ions.

A positive ion made of one nitrogen atom bonded to four hydrogen atoms. It forms when ammonia pulls an H⁺ off a water molecule.

A single-celled protist with no fixed shape that crawls by pushing out pseudopodia.

A kind of cell crawling in which the cell pushes out pseudopodia and the rest of the cell flows after them, as in amoebas and white blood cells.

A molecule like ATP but with only one phosphate group instead of three. Phosphodiesterase makes it from cAMP, ending cAMP’s signal.

A vertebrate, such as a frog or salamander, with moist skin that usually starts life in water and can live on land as an adult.

Describes a molecule with hydrophilic (water-loving) parts in one place and hydrophobic (water-fearing) parts in another, such as a soap molecule. In water, such molecules can form micelles or bilayers.

A leucoplast that makes and stores starch, common in roots and in underground stems such as potatoes.

A chemical reaction that builds larger molecules from smaller ones, such as joining amino acids into a protein. These reactions usually require energy, so cells couple them to exergonic reactions.

Describes conditions that lack oxygen, or a process that runs without it. Deep soil, your intestines, and hard-working muscles can all be low in oxygen.

The type of cellular respiration that runs without oxygen, using another substance, such as nitrate or sulfate, as the final electron acceptor. It still uses an electron transport chain and ATP synthase.

A feature that looks or works alike in two species because each evolved it independently, not because they inherited it from a common ancestor. A hummingbird’s beak and a moth’s proboscis are examples.

The stage of cell division in which joined chromosomes separate and move to opposite spindle poles. Sister chromatids part in mitosis and in the second division of meiosis; homologs part in the first.

A homology in body structure, such as the same set of forelimb bones in humans, turtles, bats, and whales.

The study of the structures of living things, such as the parts of plants and animals and how they are arranged.

An organism or species from which later ones are descended, such as a parent, a grandparent, or an extinct species that gave rise to living ones.

A cell junction that firmly attaches an animal cell to a neighboring cell or to the extracellular matrix, using cell adhesion molecules linked to the cytoskeleton. Adherens junctions, desmosomes, hemidesmosomes, and focal adhesions are the four kinds.

A condition in which the blood carries too little oxygen, because there are too few red blood cells or too little working hemoglobin.

A change in the number of one particular chromosome, so the total is not an exact multiple of a set. Trisomy and monosomy are examples; it is usually caused by nondisjunction and usually harmful.

A multicellular organism that gets its food by eating other organisms and whose cells have no cell walls. Most can move on their own at some stage of life.

An ion with a negative charge, formed when an atom or molecule gains one or more electrons. Chloride (Cl⁻) is an example.

A drug that kills bacteria or slows their growth, used to treat bacterial infections. Most were first found in microorganisms such as fungi and soil bacteria.

The ability of a bacterium to survive a drug that would normally kill it or stop its growth. The genes behind it can spread by natural selection and by horizontal gene transfer.

A defense protein that recognizes and binds specific invaders, such as bacteria and viruses, helping the body destroy them.

The three-nucleotide sequence on a tRNA that base-pairs with a matching codon on an mRNA. It determines which amino acid the tRNA carries.

A protein that keeps ice crystals from forming in an animal’s body fluids. Unrelated fishes in icy polar seas make different ones from different genes, an example of convergent evolution.

A substance, often a protein or carbohydrate on a cell’s surface, that the immune system can recognize as foreign. The antigens on red blood cells set a person’s blood type.

A molecule that protects cells by giving a free radical the electron it lacks, without becoming dangerously reactive itself. Vitamins C and E and plant flavonoids are examples.

Describes two strands of a double helix that lie side by side but run in opposite directions, one 5′ to 3′ and its partner 3′ to 5′.

A transporter that binds two or more ions or molecules and carries them across a membrane in opposite directions, as the sodium-potassium pump does.

Describing the side of an epithelial cell that faces a lumen or the outside world, such as the side of an intestinal cell that faces the inside of the gut.

A kind of asexual reproduction in which a plant makes seeds without fertilization, so the seedlings are clones of the parent, as in dandelions and some grasses.

The orderly process by which a cell destroys itself. It breaks down its nucleus and cytoskeleton, shrinks, and falls apart into small fragments called blebs.

A protein complex that forms in the cytosol when mitochondria release cytochrome c during the intrinsic pathway. It switches on caspases, starting apoptosis.

A channel protein that lets water cross a membrane quickly by facilitated diffusion. Red blood cells and kidney cells have many.

Living in water for all or most of life, as fish and whales do.

A solution in which water is the solvent. Nearly all the chemical reactions in living things take place in one.

A five-carbon sugar, common in plants, that some bacteria can take in and break down for energy.

A single-celled prokaryote that resembles a bacterium but differs from bacteria at the molecular level. Its group forms one of life’s three domains, and many live in extreme environments such as hot springs.

One of the 20 amino acids that make up proteins. Its side chain carries a positive charge, which helps proteins such as histones bind negatively charged DNA.

A toxic element found in some pesticides, tobacco, and mining waste. It is linked to lung, skin, bladder, and other cancers.

A blood vessel that carries blood away from the heart. Its walls contain smooth muscle and stretchy elastic fibers.

The process in which people choose which domesticated plants or animals breed, based on traits they find desirable, so those traits become more common over generations. Crops and dog breeds were shaped this way.

Reproduction in which offspring come from a single parent without the fusion of gametes, so they are usually genetically identical to it. Binary fission, budding, and fragmentation are examples.

One of the 20 amino acids that make up proteins. In N-linked glycosylation, a sugar tree attaches to a nitrogen atom in its side chain.

One of the 20 amino acids found in proteins. Cells can make it from oxaloacetate, a molecule of the citric acid cycle.

A spindle microtubule that grows outward from a spindle pole, away from the chromosomes, and positions the spindle within the cell.

The pairing rule for bases in DNA. Adenine (A) pairs only with thymine (T), joined by two hydrogen bonds, and guanine (G) pairs only with cytosine (C), joined by three.

The smallest unit of an element that still has that element’s chemical properties. All matter is made of these tiny particles.

An atom’s mass on a scale where the most common carbon atom is exactly 12, measured in daltons. It roughly equals the number of protons plus neutrons; periodic table values average an element’s isotopes.

The tiny, dense center of an atom, made of protons and neutrons (hydrogen’s is a single proton). It holds nearly all of the atom’s mass and all of its positive charge.

The number of protons in the nucleus of an atom, which defines the element: hydrogen’s is 1, carbon’s 6, and oxygen’s 8. In a neutral atom it also equals the number of electrons.

A small molecule, built like an RNA nucleotide with three phosphate groups, that cells use as ready energy. Cells make it using energy from breaking down glucose and other fuels, then spend it to power their work.

The reaction in which water splits the last phosphate group off ATP, forming ADP and inorganic phosphate. It releases a lot of free energy (ΔG = −7.3 kcal/mol), which cells use to drive endergonic reactions.

A membrane protein complex that makes ATP. Hydrogen ions flowing through it spin part of the complex, and the turning changes the shape of its β subunits, which join ADP and inorganic phosphate into ATP.

A group of proteins that uses energy from ATP to slide nucleosomes, remove histones from DNA, or swap in histone variants. By making DNA more or less accessible, it helps turn genes on or off.

An enzyme that splits ATP into ADP and phosphate and uses the energy released, for example to pump ions across a membrane.

Cell signaling in which a cell secretes a signal that binds receptors on its own surface and on neighboring cells of the same type. It lets groups of cells sense how crowded they are.

A sac with a double membrane that forms around worn-out cell material, such as an old organelle. It fuses with lysosomes, whose enzymes digest the contents.

The process in which a cell breaks down its own worn-out molecules and parts in lysosomes and recycles the building blocks.

A method of locating radioactive molecules by laying a sample against X-ray film, which darkens wherever those molecules are. Calvin’s team used it to find carbon-14-labeled molecules on paper.

The inheritance pattern of a dominant allele on an autosome. Every affected person has at least one affected parent, and the trait doesn’t skip generations, as in Huntington’s disease.

The way a trait is passed on when its gene sits on an autosome. Such a pattern can be autosomal dominant or autosomal recessive.

The inheritance pattern of a recessive allele on an autosome. Affected people often have two unaffected carrier parents, and the trait can skip generations, as in cystic fibrosis.

Any chromosome that is not a sex chromosome. Each human set of chromosomes holds 22 autosomes plus one sex chromosome.

An organism that makes its own organic molecules from simple inorganic ones, such as CO₂ and water, instead of eating food. Plants, algae, and some bacteria are examples.

A plant hormone that makes cells lengthen. It builds up on the shaded side of a shoot, so that side grows faster and the shoot bends toward light.

The number of particles in one mole of any substance: 6.022 × 10²³. It equals the number of carbon atoms in exactly 12 g of carbon.

The internal core of a cilium or flagellum: microtubules in a 9 + 2 array, plus dynein motors and linking proteins.

One of two ATP synthase subunits that attach to the a subunit and extend out of the membrane. Together with the δ subunit they form the still side arm that holds the knob of α and β subunits in place.

A rod-shaped bacterium common in soil and widely studied as a model organism. It is only distantly related to E. coli.

A protein from certain archaea that pumps H⁺ across a membrane when light shines on it. Researchers used it to show that an H⁺ gradient alone can power ATP synthase.

A prokaryote, usually single-celled, belonging to one of life’s three domains. These organisms live in many environments, from soil and water to the human gut; E. coli is one example.

A molecular model that shows atoms as balls and the chemical bonds between them as sticks. Physical versions let researchers test whether atoms can fit together in three dimensions.

The series of light and dark bands a chromosome shows after staining. Each kind of chromosome has its own pattern, like a barcode, so the bands identify chromosomes and reveal changes in their structure.

An inactivated X chromosome, seen as a small, tightly packed spot at the edge of the nucleus. A cell has one fewer of these spots than it has X chromosomes.

The structure at the base of a cilium or flagellum, just inside the plasma membrane, from which its microtubules grow. It has nine microtubule triplets and resembles a centriole.

A cell at the base of an epithelium, beneath the surface cells. In the airways, mutant basal cells give rise to the most common lung cancers.

The low level of transcription a eukaryotic gene’s core promoter supports on its own, without activators. Activators bound at enhancers raise the rate above it.

A substance that lowers the hydrogen ion (H⁺) concentration of a solution, raising its pH. Some release hydroxide ions, like sodium hydroxide; others take H⁺ from water, like ammonia.

A chemical mutagen shaped so much like a normal DNA base that DNA polymerase builds it into a new strand, where it later pairs with the wrong base.

A form of excision repair that recognizes an abnormal base, removes the part of the strand containing it, and rebuilds that part from the undamaged strand.

Two nitrogenous bases on opposite DNA strands joined by hydrogen bonds: A always with T, and G always with C. Each pair joins a purine to a pyrimidine.

A point mutation in which one base pair in DNA is replaced by a different pair, such as a T–A pair becoming a G–C pair.

A thin sheet of extracellular matrix beneath an epithelium, separating it from the tissue and blood vessels below. Cancer cells that haven’t crossed it haven’t spread.

Describing the sides of an epithelial cell that face away from the lumen, toward neighboring cells and the tissue beneath, such as the side of an intestinal cell that faces the blood.

A human gene whose promoter is active in white blood cells. In one form of leukemia, its promoter and first part are fused to the proto-oncogene abl.

A graph shape that is high in the middle and low at both ends, showing that middle values are most common and extreme values are rare.

A mutation that improves a protein’s function and helps an organism survive or reproduce. Natural selection tends to make it more common over many generations.

An overgrowth of cells that stays in place and doesn’t invade nearby tissue or spread. It isn’t cancer, but further mutations can turn it into a malignant tumor.

A toxic liquid chemical found in smoke and car exhaust. It is linked to blood cancers such as leukemia.

A chemical in cigarette smoke, car exhaust, and charbroiled food. The body converts it into a compound that wedges between DNA base pairs and causes frameshift mutations.

A chemical element, number 4 in the periodic table. Each atom has four protons and two electrons in its outer shell.

A type of cell in the pancreas that makes the hormone insulin and releases it into the blood.

A negative ion made of one hydrogen, one carbon, and three oxygen atoms. It forms when carbonic acid releases H⁺, and together with carbonic acid it acts as a key buffer in blood.

DNA copying that proceeds in both directions from one origin of replication, as two replication forks move away from each other.

A way of dividing in which a cell or organelle copies its chromosome and then splits in two, as bacteria, mitochondria, and chloroplasts do.

The spot on a protein, such as a receptor or an enzyme, where a specific molecule attaches.

The system of giving each species a unique two-part scientific name: its genus followed by its specific epithet, as in Homo sapiens.

Control of a metabolic pathway by molecules that bind noncovalently to its enzymes and directly change their activity. Competitive and noncompetitive inhibitors work this way, as does feedback inhibition.

The variety of species living in an area, together with the genetic variation within those species. Higher levels help ecosystems function well and withstand disease and disasters.

The study of where living and extinct species are found on Earth. It shows that isolated islands and continents evolve their own distinct species.

The study of life and living things, from the molecules inside cells to the whole biosphere.

A small sample of tissue removed from the body to be examined, for example to check whether a growth is cancer.

All the places on Earth where life exists, including the air, bodies of water, the land, and the soil. It is the highest level of biological organization.

A pattern of inheritance in which both the egg and the pollen pass organelles, and the genes in them, to the offspring. Chloroplasts in certain plant species are inherited this way.

A pair of replicated homologs lined up side by side in prophase I of meiosis, four chromatids in all. It forms by synapsis and is where crossing over happens.

A small fragment of a cell that bulges out from the plasma membrane and breaks away as the cell comes apart during apoptosis.

The old, disproven idea that offspring are an even mix of their parents’ traits, like mixed paint, so the parents’ original versions could never reappear.

A category of blood defined by which antigens sit on the surface of a person’s red blood cells, such as type A, B, AB, or O.

A tube that carries blood through an animal’s body. Arteries, veins, and capillaries are the main kinds.

The nostril on top of a whale’s or dolphin’s head, through which it breathes at the water’s surface. It formed as the nostrils moved back from the tip of the snout.

A hard connective tissue that forms the skeleton of vertebrates. It is mostly extracellular matrix, made of collagen fibers hardened with calcium-containing minerals.

A chemical element, number 5 in the periodic table. Each atom has five protons and three electrons in its outer shell. Plants need tiny amounts of it.

A spot inside an intron to which the intron’s freed 5′ end attaches during splicing, looping the intron before it is removed.

A tumor-suppressor gene whose protein senses DNA damage and helps repair it. Inherited mutations in it raise the risk of breast cancer.

A tumor-suppressor gene whose protein senses DNA damage and helps repair it. Inherited mutations in it raise the risk of breast cancer.

A special kind of body fat whose cells are packed with mitochondria that release energy as heat, warming hibernating and young animals.

The constant, random jiggling of atoms and molecules caused by heat. It helps bring molecules into contact so they can react.

The forming of an outgrowth that pinches off from a larger structure. Vesicles bud off membranes such as the ER’s, and in asexual reproduction a new individual grows from the parent and breaks away, as in yeast.

A substance, usually a pair made of an acid and its related base, that keeps a fluid’s pH from changing much by taking up or releasing H⁺ as needed. Carbonic acid and bicarbonate do this in blood.

One of the cells that form a tight layer around each vein in a leaf. In C₄ plants, they receive malate from mesophyll cells and run the Calvin cycle surrounded by plenty of CO₂.

The ring of 10–14 c subunits that sits in the membrane part of ATP synthase. As H⁺ binds to and leaves its subunits, the ring spins, turning the γ and ε subunits attached to it.

The end of a polypeptide that carries a free carboxyl group. It holds the last amino acid added as the chain is built.

A plant that fixes carbon only through rubisco, so the first product is the three-carbon molecule 3PG. About 85% of plant species, including wheat and oak, work this way.

A plant whose first product of carbon fixation is a four-carbon molecule, oxaloacetate, made by PEP carboxylase, which ignores O₂. Corn and sugarcane are examples; CAM plants technically count too.

A cell adhesion molecule that joins one animal cell to another by binding a partner of the same kind on the neighboring cell. It needs calcium ions (Ca²⁺) to bind.

A toxic metal found in tobacco, batteries, and mining waste. It is linked to lung and other cancers.

A stimulant in coffee, tea, and energy drinks. It blocks phosphodiesterase, so cAMP lingers and signals such as epinephrine have stronger, longer-lasting effects.

A mineral element (symbol Ca) that is a building material for bones and shells. Its atoms lose two electrons to form the ion Ca²⁺.

A female cat with patches of orange and black fur. She is heterozygous for an X-linked fur-color gene, and random X-chromosome inactivation leaves each patch expressing only one of the two alleles.

The second stage of photosynthesis: a loop of enzyme-driven reactions in the stroma that uses ATP and NADPH to fix carbon from CO₂ and build it into the sugar G3P.

A plant, such as a cactus or pineapple, that opens its stomata at night, stores the CO₂ it fixes as malate, and uses it in the Calvin cycle by day, saving water.

A small molecule made from ATP by adenylyl cyclase that acts as a second messenger, carrying signals such as epinephrine’s through the cell. It switches on PKA.

A disease in which cells grow and divide out of control, forming tumors that can invade nearby tissue and spread through the body.

The stretch of DNA near the lac operon’s promoter where the activator CAP binds when cAMP is attached to it. Binding there bends the DNA and helps RNA polymerase bind the promoter.

A protein in eukaryotic cells that recognizes an mRNA’s 5′ cap and helps bring the mRNA to the small ribosomal subunit so translation can start.

The smallest kind of blood vessel, with walls only one cell thick, where materials pass between the blood and the surrounding tissues.

A very thick, jelly-like glycocalyx made by some disease-causing bacteria. It helps them escape the host’s immune system or stick to cells.

A molecule made of one or more sugar units, from a single sugar to long chains such as starch. Living things use these molecules to store energy and to build structures.

A chemical element whose atoms can each bond with up to four other atoms, forming the chains and rings that make up the backbone of most molecules in living things.

A gas made of one carbon atom bonded to two oxygen atoms. Organisms release it when they break down food, and plants take it in to make sugars by photosynthesis.

The process of attaching carbon from CO₂ gas to an organic molecule, locking it into a form that isn’t a gas. In the Calvin cycle, rubisco does this by joining CO₂ to RuBP.

A poisonous compound of carbon and oxygen, released when fuel burns without enough air. It binds hemoglobin and keeps it from carrying oxygen.

The chain or ring of bonded carbon atoms that forms the framework of an organic molecule. Functional groups attach to it.

A radioactive form of carbon. Because molecules built with it can be detected, scientists use it as a tag to follow carbon atoms through a process such as the Calvin cycle.

A weak acid formed when carbon dioxide dissolves in water. It partly breaks apart into H⁺ and bicarbonate ions, and together with bicarbonate it buffers the pH of blood.

A functional group made of a carbon atom double-bonded to an oxygen atom (C=O). It is polar and highly reactive; inside a carbon chain it is a ketone, and at the end an aldehyde.

A functional group (–COOH) made of a carbon atom double-bonded to one oxygen and bonded to an –OH. It is acidic, releasing H⁺ in water, and is found in amino acids and fatty acids.

Any agent that increases the chance of developing cancer. Most, such as UV light and chemicals in cigarette smoke, are mutagens that damage the DNA of body cells.

A cancer of epithelial cells, the cells that line body surfaces. Most lung cancers are carcinomas.

The muscle tissue of the heart. Its cells are linked by many gap junctions, so an electrical change spreads quickly from cell to cell and they contract together in one beat.

A disease of the heart muscle that weakens the heart’s ability to pump blood.

A yellow, orange, or red pigment made by plants and stored in chromoplasts. These pigments color many fruits, flowers, and autumn leaves.

A person who has one copy of a recessive disease allele in every cell but doesn’t have the disease. Such a person can still pass the allele to offspring.

A tough, flexible, fibrous tissue found in joints, the ears, and other animal body parts.

A protease that carries out apoptosis by cutting up specific proteins. It is made in an inactive form and switched on when part of it is cut away.

A chemical reaction that breaks larger molecules into smaller ones, such as splitting a protein into amino acids. These reactions are often exergonic, and they supply both building blocks and energy.

A molecule a cell breaks down, usually for energy, such as glucose.

An activator protein in bacteria that binds DNA when cAMP is attached to it. At the lac operon it binds the CAP site, bends the DNA, and helps RNA polymerase start transcription.

The way a preferred energy source, such as glucose, holds back the use of other sugars. Glucose lowers cAMP, so the activator CAP can’t bind and the lac operon runs slowly.

An enzyme in peroxisomes that breaks hydrogen peroxide down into harmless water and oxygen gas.

A substance that speeds up a chemical reaction without being used up by it. In cells, enzymes play this role for most reactions.

A subunit that carries out an enzyme’s chemical reaction. In PKA, two of them attach phosphate groups to target proteins once cAMP frees them.

An ion with a positive charge, formed when an atom or molecule loses one or more electrons. Examples include Na⁺, K⁺, and Ca²⁺.

A relationship in which one thing actually produces another. Showing it takes more than a correlation, because the effect could run the other way or a third factor could cause both.

The smallest and simplest unit that is alive: a tiny, membrane-wrapped package of molecules that can take in materials, use energy, and reproduce. Every organism is made of one or more.

The binding of cells to one another through proteins in their plasma membranes, which holds animal cells together in a multicellular body.

A protein that spans the plasma membrane, gripping a neighboring cell or the extracellular matrix on the outside and linking to the cytoskeleton on the inside. Cadherins and integrins are the main kinds.

The branch of biology that studies cells, their parts, and how cells interact.

The ways cells send, receive, and respond to signals, usually chemical messages such as hormones, from other cells or their surroundings.

The series of stages a cell goes through as it grows, copies its DNA, and divides in two. Moving through it depends on destroying specific proteins once they’ve done their job.

The process in which one cell splits to form two new cells.

A connection between neighboring cells. Some kinds, such as gap junctions, form openings that let ions and small molecules pass directly from one cell’s cytosol into the next.

A cell and all the cells descended from it by division. Cancer arises as mutations pile up, one after another, in one such family of cells.

The movement of cells to new positions in the body, as when cells of an animal embryo move into place or cells move into a damaged area. It does not happen during plant development.

A structure built from fused Golgi vesicles in the middle of a dividing plant cell during cytokinesis. It grows into a new cell wall between the two daughter cells.

A difference between opposite sides of a cell in structure or in which proteins they hold. In intestinal cells, tight junctions keep uptake proteins on one side and export proteins on the other.

A receptor in the plasma membrane with its binding site facing outside the cell. It detects signals that can’t cross the membrane, such as large or hydrophilic molecules.

The principle that all living things are made of one or more cells, that cells are the smallest units of life, and that new cells come only from existing cells.

A stiff layer outside the plasma membrane of plant, fungal, and most bacterial cells that gives them shape and protection. In plants it is made mainly of cellulose.

The contents of broken-open cells, which can build polypeptides in a test tube when mRNA is added. Researchers used one to work out the genetic code.

A protein that lets cells communicate with each other and sense their surroundings, such as the taste-sensing proteins on the tongue.

Cell signaling between cells: one cell sends a signal that a target cell detects and responds to. It lets the cells of a plant or animal coordinate their activities.

Control of metabolism in response to signals from a cell’s surroundings, such as hormones. The signals lead to changes in specific enzymes, often the addition of phosphate groups by protein kinases.

The process by which cells break down food molecules such as glucose, usually using oxygen, and capture the released energy in ATP. It gives off carbon dioxide and water.

The change a cell makes after detecting a signal, such as switching on an enzyme, turning on genes, or moving. It is the third stage of cell signaling.

A tough polysaccharide made of straight, unbranched chains of β-glucose that hydrogen-bond side by side into strong sheets. It is the main building material of plant cell walls.

The idea that genetic information usually flows one way, DNA → RNA → protein: a gene’s DNA is copied into mRNA, and the mRNA is read to build a polypeptide.

A large, fluid-filled sac that takes up 80% or more of a mature plant cell. It stores water and other materials and presses on the cell wall to keep the cell firm.

A machine that spins samples very fast, making larger or denser particles sink toward the bottom of the tube faster than smaller ones.

One of a pair of cylinder-shaped structures, set at right angles to each other, inside an animal cell’s centrosome. Plant cells lack them.

The narrowed region of a replicated chromosome where its two identical copies are held together.

The main microtubule-organizing center of a nondividing animal cell, near the nucleus. It holds a pair of centrioles and anchors the minus ends of microtubules.

A dense ionic compound that, dissolved in water and spun at high speed, forms a density gradient. Meselson and Stahl used it to separate DNA molecules by weight.

The gene that is faulty in cystic fibrosis. Its normal version codes for a transport protein that carries chloride ions out of cells across the plasma membrane.

A transmembrane protein that forms an open passageway, letting specific ions or molecules flow straight across a membrane by facilitated diffusion. Most can open and close.

A protein that binds other molecules to guide how they fold or assemble. Some keep a new protein unfolded until it is threaded into a mitochondrion; others carry histones off DNA and put them back.

A general inherited feature that varies among individuals, such as flower color or seed shape. Each version of it, such as purple or white flowers, is a trait.

The finding that double-stranded DNA contains nearly equal amounts of adenine and thymine, and of guanine and cytosine, even though the amounts differ from species to species.

A property of matter that comes in two kinds, positive and negative. Particles with opposite kinds attract each other, and particles with the same kind push each other apart.

A tRNA with its amino acid attached at its 3′ end, ready to deliver that amino acid to a ribosome.

A point in the cell cycle where the cycle can be halted if a problem, such as damaged DNA, is found. p53 acts at such a point in G₁ phase.

A protein that checks the genome for damage and stops the cell from moving past a point in the cell cycle until the problem is fixed. p53 is an example.

An attraction that holds atoms together, formed when they share or transfer electrons. Chemical reactions break some of these links and form new ones.

The potential energy stored in atoms and the chemical bonds between them. Food molecules such as glucose hold a lot of it, and breaking them down releases it.

The state reached by a reaction that runs in both directions when products form as fast as they turn back into reactants, so the amounts of each stop changing. The amounts need not be equal.

A change over time in a mixture of molecules, produced by chemical selection, so that the mixture ends up with a different chemical makeup.

A chemical that causes mutations, usually by altering bases so they pair wrongly or by distorting the double helix. Nitrous acid and benzo[a]pyrene are examples.

A trait that describes how a substance behaves in chemical reactions, such as which substances it bonds or reacts with. For an element, these traits depend mainly on its valence electrons.

A process in which substances change into different substances as bonds between atoms break and form, such as when cells break down sugars for energy.

The process by which a molecule with an advantage, such as faster self-replication, increases in number relative to other molecules in a mixture.

A way of making ATP that uses the energy stored in an electrochemical gradient, usually of hydrogen ions across a membrane, to join ADP and inorganic phosphate.

A statistical test of how far observed results differ from what a hypothesis predicts. A large value means the difference is unlikely to be random sampling error, so the hypothesis is rejected.

An X-shaped point where two homologs stay connected after crossing over, marking where they swapped segments. It holds the homologs together until anaphase I.

A tough polysaccharide whose sugar units carry nitrogen-containing groups. It forms the exoskeletons of insects and crustaceans and the cell walls of fungi.

An antibiotic that blocks bacterial translation by competing for the peptidyltransferase site on the ribosome, so peptide bonds can’t form.

The negatively charged form of chlorine, made when a chlorine atom gains an electron. It is one of the two ions in table salt, and cells move it across membranes to control salt and water balance.

A chemical element that living things use mainly in the form of chloride ions, one of the two kinds of ions in table salt.

The green pigment in chloroplasts that absorbs light energy for photosynthesis. Its abundance is why leaves are green.

One of two main kinds of chlorophyll in plants and green algae. It carries a CH₃ group at the one spot where the other kind carries CHO, which shifts the wavelengths it absorbs slightly.

One of two main kinds of chlorophyll in plants and green algae. It carries a CHO group at the one spot where the other kind carries CH₃, so it absorbs slightly different wavelengths.

An organelle in plants and algae that captures light energy and uses it to make sugars by photosynthesis. It contains the green pigment chlorophyll.

The DNA inside a chloroplast, usually one circular chromosome. It is bigger than a mitochondrion’s but still far smaller than the nuclear genome.

A sterol found in animal cell membranes and in blood. Animals use it as the starting material for steroid hormones such as estrogen and testosterone.

A small, charged, nitrogen-containing molecule attached to the phosphate group of many phospholipids, forming part of their polar head.

The only kind of cell in cartilage. These cells make all of the cartilage’s collagen fibers, glycosaminoglycans, and proteoglycans, yet in some cartilage they fill only 1–2% of its volume.

A glycosaminoglycan that is a major part of cartilage, where it helps hold water so the cartilage resists squeezing.

One of the two identical copies of a chromosome made by DNA replication. The pair stays joined at the centromere until they separate during cell division.

The material chromosomes are made of: DNA wound together with packing proteins inside the nucleus.

The moving or removing of nucleosomes, or the tighter or looser packing of a stretch of chromatin, which changes how easily the genes there can be transcribed.

A plastid that makes and stores yellow, orange, and red pigments called carotenoids, coloring many fruits, flowers, and autumn leaves.

A rearrangement in which a segment of one chromosome breaks off and attaches to a different chromosome. If the breaks fall inside genes, two genes can be fused into one.

A threadlike structure made of DNA and proteins that carries a cell’s genes. Most human cells have 46, including two sex chromosomes.

A change in which two chromosomes join into one, lowering the chromosome number. Human chromosome 2 formed this way from two smaller ancestral chromosomes.

The loss of a whole chromosome from a cell, which removes every gene it carries, including any tumor-suppressor genes.

The helix-shaped core of proteins, built by condensin II, that runs down the center of a metaphase chromosome and anchors its loops of chromatin.

One copy of each of a species’ different chromosomes, written n. A human set has 23 chromosomes, 22 autosomes plus one sex chromosome.

The distinct region of the nucleus occupied by one chromosome in a nondividing cell, kept separate from its neighbors by the nuclear matrix.

The theory that inheritance patterns are explained by how chromosomes are copied, separated in meiosis, and joined at fertilization. Genes sit on chromosomes, so alleles follow the chromosomes’ behavior.

A cell covered with cilia, hairlike projections that beat together. In the airways, ciliated cells sweep mucus and trapped particles out of the lungs.

A short, hairlike extension of a eukaryotic cell that beats to move the cell through fluid. Cells often have many, sharing the same microtubule core as flagella.

A chromosome whose DNA forms a closed loop with no ends. Most bacteria, mitochondria, and chloroplasts carry this kind.

The organ system that moves blood around the body, carrying materials to and from cells. In birds and mammals, it includes the heart and blood vessels.

The side of the Golgi apparatus nearest the ER, where cargo arriving from the ER enters first.

A stereoisomer that differs from its partner only in which side of a C=C double bond certain atoms lie on: the same side (cis) or opposite sides (trans).

One of the flattened, fluid-filled membrane sacs or tubes that make up the endoplasmic reticulum and the Golgi apparatus.

A proposed explanation of Golgi traffic in which the compartments themselves move: each new one forms at the cis face from ER vesicles, matures as it shifts toward the trans face, then breaks up into vesicles.

The six-carbon molecule formed when an acetyl group joins oxaloacetate, in the first step of each turn of the Krebs cycle.

The citric acid cycle enzyme that joins an acetyl group to oxaloacetate to form citrate. Its step releases a lot of energy and can limit the cycle’s rate.

The third stage of cellular respiration, a loop of reactions in the mitochondrial matrix. Each turn joins an acetyl group to oxaloacetate, releases 2 CO₂, makes 3 NADH, 1 FADH₂, and 1 ATP, and regenerates oxaloacetate.

An amino acid that is not used to build proteins. It is made from ornithine and converted into arginine in the last step of arginine’s pathway.

A ring-shaped protein that encircles DNA and holds DNA polymerase on the template, so it can add many nucleotides without falling off.

In classifying living things, the rank below phylum and above order. Mammalia, for example, includes all mammals.

A transmembrane protein that, together with occludin, forms the netlike strands of a tight junction, sealing the space between neighboring cells.

A groove that forms around the middle of a dividing animal cell during cytokinesis. A ring of actin filaments under the plasma membrane tightens it like a drawstring, pinching the cell in two.

The second phase of glycolysis (steps 4–5), in which six-carbon fructose-1,6-bisphosphate is split into two three-carbon molecules of glyceraldehyde-3-phosphate.

A cell or organism that is genetically identical to the one it came from, such as each cell produced when a single cell divides again and again.

The state of chromatin in which nucleosomes cover a gene’s promoter, so the transcription machinery can’t reach the DNA and the gene is hard or impossible to transcribe.

A protein that increases transcription without binding DNA itself. It binds an activator, and the pair stimulates mediator.

The part of a gene or mRNA, from the start codon through the stop codon, that spells out a polypeptide’s amino acid sequence.

The DNA strand opposite the one RNA polymerase reads. Its sequence matches the mRNA’s, except that it has T (thymine) where the mRNA has U (uracil).

An inheritance pattern in which a heterozygote fully expresses both alleles at once, as in type AB blood, whose red blood cells carry both the A and the B antigen.

A group of three nucleotides in an mRNA that stands for one amino acid or signals the end of translation. For example, CCC stands for proline.

An organic molecule that binds temporarily to an enzyme and takes part in the reaction it catalyzes, but is left unchanged when the reaction is over.

A carrier molecule that attaches to other molecules, such as fatty acids, activating them so they can take part in chemical reactions.

A nonprotein helper, usually an inorganic ion such as Fe³⁺ or Zn²⁺, that binds temporarily to an enzyme and promotes the reaction it catalyzes.

A protein that holds sister chromatids together along their length after DNA replication, until they are separated during cell division.

The attraction between molecules of the same kind, such as water molecules clinging to one another through hydrogen bonds. It helps pull water up through plants.

A strong structural protein that gives strength to tendons, cartilage, skin, and bone.

A ground tissue that gives flexible support to growing plant parts, such as the outer parts of stems and leaves. Its cells have thick secondary walls with little lignin, so they can bend and stretch.

A property of a solution that depends only on how many solute particles are dissolved, not on what kind. For example, solutes lower water’s freezing point and raise its boiling point.

A group of organisms of one kind living together. For bacteria, it is a visible spot on a dish made of cells that all grew from a single cell.

Adding artificial colors to a finished micrograph with a computer to highlight structures. Unlike staining, it doesn’t change the sample itself.

Regulation in which many factors act on one gene at once, such as activators, repressors, chromatin changes, and DNA methylation, and their combination sets how much the gene is expressed. Most eukaryotic genes are controlled this way.

A species or population from which two or more later species are descended. The more recently two species shared one, the more closely related they are.

All the populations of different species that live in the same area and interact, such as the plants, animals, algae, and microorganisms of a pond.

The division of a eukaryotic cell’s interior into separate organelles, which lets different chemical reactions run in different places at the same time.

A molecule, often shaped like the substrate, that binds reversibly to an enzyme’s active site and keeps the substrate out. It raises Kₘ but not Vₘₐₓ, and enough substrate can overcome it.

Describes two nucleic acid strands whose nitrogenous bases pair exactly, A with T and G with C, so the sequence of one strand determines the sequence of the other.

A plant tissue made of two or more cell types, but without the organization that would make it an organ. Dermal tissue and vascular tissue are examples.

A substance made of two or more different elements joined together, such as water (H₂O) or table salt (NaCl). Its properties can differ greatly from those of the elements that form it.

The amount of a solute dissolved in a given volume of solution, such as grams per liter or moles per liter (molarity).

A difference in the concentration of a substance between two regions, such as the two sides of a membrane. Left alone, the substance spreads from the crowded side to the sparse side.

A chemical reaction in which two molecules join by a covalent bond, usually giving off a small molecule. When that small molecule is water, it is a dehydration reaction.

A protein complex containing SMC proteins that compacts chromosomes for cell division by folding chromatin into loops. Cells make two forms, called I and II.

A protein complex containing SMC proteins that, during prometaphase, folds chromatin into small loops of about 80 kb nested inside larger loops.

A protein complex containing SMC proteins that starts chromosome compaction. It builds a helix-shaped protein scaffold and forms large chromatin loops, about 400 kb each, anchored to it.

The particular three-dimensional shape of a molecule, especially a protein. Many proteins work by switching between shapes; each β subunit of ATP synthase cycles through three.

A change in a protein’s shape, often triggered when it binds a molecule, that changes what the protein does, as when a transporter switches which side its pocket faces.

A route of horizontal gene transfer in which DNA passes directly from one bacterium or archaeon to another while the two cells are in contact.

A type of animal tissue that supports, binds, and protects other tissues and organs. Bone, blood, fat, and tendons are examples.

The membrane protein that builds gap junction channels in vertebrates. Six of them form a ring with a channel in the middle.

A ring of six connexin proteins in one cell’s plasma membrane with a channel through its center. Lined up with a matching ring in a neighboring cell, it forms one channel of a gap junction.

A disproven idea of DNA copying in which the original double helix stays intact and the copy is made of two entirely new strands.

An exon kept in every mature mRNA made from a gene, in every cell type. It usually codes parts needed for the protein’s basic structure and function.

Steady expression of a gene or operon whether or not conditions change. Housekeeping genes are expressed this way normally; a broken repressor or operator can make a regulated operon behave this way too.

A gene that is expressed at a fairly steady level all the time instead of being switched on and off. Such genes usually code proteins a cell always needs, such as certain enzymes of metabolism.