Biology Important Definitions

alpha (α) helix A prevalent type of secondary protein structure; a right-handed spiral.

amino acid Organic compounds containing both NH2 and COOH groups. Proteins are polymers of amino acids.

base (1) A substance that can accept a hydrogen ion in solution. (2) In nucleic acids, the purine or pyrimidine that is attached to each sugar in the backbone.

beta (β) pleated sheet Type of protein secondary structure; results from hydrogen bonding between polypeptide regions running anti-parallel to each other.

carbohydrates Organic compounds containing carbon, hydrogen, and oxygen in the ratio 1:2:1 (i.e., with the general formula CnH2nOn). Common examples are sugars, starch, and cellulose.

cellulose (sell´ you lowss) A straight-chain polymer of glucose molecules, used by plants as a structural supporting material.

chemical evolution The theory that life originated through the chemical transformation of inanimate substances.

complementary base pairing The AT (or AU), TA (or UA), CG, and GC pairing of bases in double-stranded DNA, in transcription, and between tRNA and mRNA.

condensation reaction A reaction in which two molecules become connected by a covalent bond and a molecule of water is released. (AH + BOH → AB + H2O.)

denaturation Loss of activity of an enzyme or nucleic acid molecule as a result of structural changes induced by heat or other means.

deoxyribose A five-carbon sugar found in nucleotides and DNA.

disaccharide A carbohydrate made up of two monosaccharides (simple sugars).

disulfide bridge The covalent bond between twosulfur atoms (–S—S–) linking to molecules or remote parts of the same molecule.

DNA (deoxyribonucleic acid) The fundamental hereditary material of all living organisms. In eukaryotes, stored primarily in the cell nucleus. A nucleic acid using deoxyribose rather than ribose.

double helix In DNA, the natural, right-handed coil configuration of two complementary, antiparallel strands.

ester linkage A condensation (water-releasing) reaction in which the carboxyl group of a fatty acid reacts with the hydroxyl group of an alcohol. Lipids are formed in this way.

fat A triglyceride that is solid at room temperature.

fatty acid A molecule with a long hydrocarbon tail and a carboxyl group at the other end. Found in many lipids.

functional group A characteristic combination of atoms that contribute specific properties when attached to larger molecules.

glucose The most common monosaccharide; the monomer of the polysaccharides starch, glycogen, and cellulose.

glycerol A three-carbon alcohol with three hydroxyl groups; a component of phospholipids and triglycerides.

glycogen An energy storage polysaccharide found in animals and fungi; a branched-chain polymer of glucose, similar to starch.

glycosidic linkage Bond between carbohydrate (sugar) molecules through an intervening oxygen atom (–O–).

hexose [Gk. hex: six] A sugar containing six carbon atoms.

hydrolysis A chemical reaction that breaks a bond by inserting the components of water: AB + H2O → AH + BOH.

isomers Molecules consisting of the same numbers and kinds of atoms, but differing in the bonding patterns by which the atoms are held together.

ligand Any molecule that binds to a receptor site of another (usually larger) molecule.

lipids Substances in a cell which are easily extracted by organic solvents; fats, oils, waxes, steroids, and other large organic molecules, including those which, with proteins, make up the cell membranes.

macromolecule A giant polymeric molecule. The macromolecules are proteins, polysaccharides, and nucleic acids.

monomer A small molecule, two or more of which can be combined to form oligomers (consisting of a few monomers) or polymers (consisting of many monomers).

monosaccharide A simple sugar. Oligosaccharides and polysaccharides are made up of monosaccharides.

nucleic acid A long-chain alternating polymer of deoxyribose or ribose and phosphate groups, with nitrogenous bases—adenine, thymine, uracil, guanine, or cytosine (A, T, U, G, or C)—as side chains. DNA and RNA are nucleic acids.

nucleotide The basic chemical unit in a nucleic acid. A nucleotide in RNA consists of one of four nitrogenous bases linked to ribose, which in turn is linked to phosphate. In DNA, deoxyribose is present instead of ribose.

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