A dictionary of scientific terms frequently used on the HWI Website.

Active site : The region of an enzyme to which a substrate binds and at which a chemical reaction occurs.

Alpha helix : A short, spiral-shaped section within a protein structure.

Amino acid : A chemical building block of proteins. There are 20 standard amino acids. A protein consists of a specific sequence of amino acids.

Angstrom : A unit of length used for measuring atomic dimensions. One angstrom equals 10-10 meters.

Atom : A fundamental unit of matter. It consists of a nucleus and electrons.

Base : A chemical component (the fundamental information unit) of DNA or RNA. There are four bases in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G). RNA also contains four bases, but instead of thymine, RNA contains uracil (U).

Beta sheet : A pleated section within a protein structure.

Bioinformatics : A field of research that relies on computers to store and analyze large amounts of biological data.

Cell : The basic subunit of any living organism; the simplest unit that can exist as an independent living system.

Chaperones : Proteins that help other proteins fold or escort other proteins throughout the cell.

Deoxyribose : The type of sugar in DNA.

Diffraction : The scattering of X-rays by planes of atoms in a crystal resulting in the formation of a so-called diffraction pattern.

DNA (deoxyribonucleic acid) : The substance of heredity. A long, usually double-stranded chain of nucleotides that carries genetic information necessary for all cellular functions, including the building of proteins. DNA is composed of the sugar deoxyribose, phosphate groups, and the bases adenine, thymine, guanine, and cytosine.

Enzyme : A substance, usually a protein, that speeds up, or catalyzes, a specific chemical reaction without being permanently altered or consumed. Some RNA molecules can also act as enzymes.

Eukaryote : An organism whose cells have a membrane-bound nucleus.

Gene : A unit of heredity. A segment of DNA that contains the code for a specific protein or protein subunit.

Genetics : The scientific study of genes and heredity, of how particular qualities or traits are transmitted from parents to offspring.

Genomics : The study of all of an organism's genetic material.

Genetic code : The set of triplet letters in DNA (or mRNA) that code for specific amino acids.

Hormone : A messenger molecule that helps coordinate the actions of various tissues; made in one part of the body and transported, via the bloodstream, to tissues and organs elsewhere in the body.

Inflammation : The body's characteristic reaction to infection or injury, resulting in redness, swelling, heat, and pain.

Inhibitor : A molecule that "inhibits," or blocks, the biological action of another molecule.

Kilodalton : A unit of mass equal to 1,000 daltons. A dalton is a unit used to measure the mass of atoms and molecules. One dalton equals the atomic weight of a hydrogen atom (1.66 x 10-24 grams). The sizes of protein molecules are often expressed in kilodaltons.

Lipid : A fatty, waxy, or oily molecule that will not dissolve in water; it contains hydrogen, carbon, and oxygen.

Macromolecule : A large molecule such as a protein or a nucleic acid.

Membrane : A thin covering surrounding a cell and separating it from the environment; consists of a double layer of molecules called phospholipids and has proteins embedded in it.

Messenger RNA (mRNA) : An RNA molecule that serves as an intermediate in the synthesis of protein.Messenger RNA is complementary to DNA and carries genetic information to the ribosome.

Metabolism : All enzyme-catalyzed reactions in a living organism that builds and breaks down organic molecules, producing or consuming energy in the process.

Molecule : The smallest unit of matter that retains all of the physical and chemical properties of that substance. It consists of one or more identical atoms or a group of different atoms bonded together.

Mutation : A change in a DNA sequence.

Nucleotide : A subunit of DNA or RNA that includes one base, one phosphate molecule, and one sugar molecule (deoxyribose in DNA, ribose in RNA). Thousands of nucleotides join end-to-end to create a molecule of DNA or RNA. See base, phosphate group.

Nucleus (pl. nuclei) : 1. The membrane-bounded center of a cell, which contains genetic material. 2. The center of an atom, made up of protons and neutrons.

Peptide : A small protein fragment.

Phosphate group : A chemical group found in DNA and RNA, and often attached to proteins and other biological molecules. It is composed of one phosphorous atom bound to four oxygen atoms.

Protein : A large biological molecule composed of amino acids arranged in a specific order determined by the genetic code and folded into a specific three-dimensional shape. Proteins are essential for all life processes.

Protein folding : The physical process by which, beginning from a random coil, one part of a protein molecule is laid over another part to form a characteristic and functional three-dimension structure.

Proteomics : The systematic, large-scale study of all proteins in an organism.

Receptor protein : Specific proteins found on the cell surface to which hormones or other molecules bind, triggering a specific reaction within the cell. Receptor proteins are responsible for initiating reactions as diverse as nerve impulses, changes in cell metabolism, and hormone release.

Ribose : The type of sugar found in RNA.

RNA (ribonucleic acid) : A long, usually single-stranded chain of nucleotides that has structural, genetic, and enzymatic roles. There are three major types of RNA, which are all involved in making proteins: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). RNA is composed of the sugar ribose, phosphate groups, and the bases adenine, uracil, guanine, and cytosine. Certain viruses contain RNA, instead of DNA, as their genetic material.

Sequence : The exact order of the building blocks (see nucleotides) of a particular piece of DNA or the building blocks (see amino acids) of a protein.

Side chain : The part of an amino acid that confers its identity. Side chains range from a single hydrogen atom (for glycine) to a group of 15 or more atoms.

Site of action : The place in the body where a drug exerts its effects.

Steroid : A type of molecule that has a multiple ring structure, with the rings sharing molecules of carbon.

Structural biology : A field of study dedicated to determining the detailed, three-dimensional structures of biological molecules to better understand the function of these molecules.

Structural genomics : A field of study that seeks to determine a large inventory of protein structures based on gene sequences. The eventual goal is to be able to produce approximate structural models of any protein based on its gene sequence. From these structures and models, scientists hope to learn more about the biological function of proteins.

Structure-based drug design : An approach to developing medicines that takes advantage of the detailed, three-dimensional structure of target molecules.

Substrate : A molecule that binds to an enzyme and undergoes a chemical change during the ensuing enzymatic reaction.

Target molecule (or target protein) : The molecule on which pharmaceutical researchers focus when designing a drug. Often, the target molecule is from a virus or bacterium, or is an abnormal human protein. In these cases, the researchers usually seek to design a small molecule—a drug—to bind to the target molecule and block its action.

Transcription : The first major step in protein synthesis, in which the information coded in DNA is copied (transcribed) into mRNA.

Translation : The second major step in protein synthesis, in which the information encoded in mRNA is deciphered (translated) into sequences of amino acids. This process occurs at the ribosome.

Variant : A different version of a gene, one that has a slightly different sequence of nucleotides.

X-ray crystallography : A technique used to determine the detailed, three-dimensional structure of molecules. It is based on the scattering of X-rays through a crystal of the molecule under study.