Amino acid -- The basic building block of proteins and polypeptides. It contains a basic amino group, an acidic carboxyl group, and a side chain attached to an alpha carbon atom. Amino acids link together by peptide bonds to form proteins, or function as chemical messengers and as intermediates in metabolism.

Anticodon -- A sequence of three adjacent nucleotides in tRNA that binds to a complementary codon in mRNA and designates a specific amino acid during protein synthesis.

Base pair -- Either of the two pairs--guanine and cytosine, adenine and thymine--of purine-pyrimidine bases joined by hydrogen bonds that make up DNA. In RNA, uracil replaces thymine.

Coding ratio -- The number of nucleotides in sequence required to code for amino acids. In a triplet code, the coding ratio is three.

Codon -- A sequence of three RNA or DNA nucleotides that specifies, i.e., codes for, either an amino acid or the termination of translation.

Crystallography, x-ray -- An analytical technique in which x-ray diffraction is used to obtain information about the identity or structure of a crystalline substance.

DNA, or deoxyribonucleic acid -- The primary genetic material of all cellular organisms and the DNA viruses. Located predominantly in cell nuclei, it is composed of two chains of nucleotides--deoxyribose and phosphate backbones with side chains of purine (adenine or guanine) or pyrimidine (cytosine and thymine) bases projecting inward. Hydrogen bonds link adenine to guanine, and cytosine to thymine. The two linked strands are twisted in a double helix.

Double helix -- A helix or spiral consisting of two strands in the surface of a cylinder that coil around its axis; especially the structural arrangement of DNA in space that consists of paired polynucleotide strands stabilized by cross-links between purine and pyrimidine bases.

Enzyme -- A protein molecule that catalyzes chemical reactions of other substances without itself being destroyed or altered by the reactions. Made up of a complex of amino-acids, enzymes are part of every chemical reaction in living things. They aid in digestion, the growth and building of cells, and all reactions involving transformation of energy. Inside the cell, enzymes create RNA and DNA by facilitating the reaction of ribose with adenosine. They also specify the sites for linking to build RNA along a DNA template. Each enzyme works only on one specific substance (called the substrate). Enzymes are usually designated by the suffix -ase.

Genetic code -- Linear sequences of three nucleotides, or triplets, that code for amino acids or termination codons during the process of translation at the ribosome.

Messenger RNA, or mRNA -- An RNA molecule transcribed from the DNA of a gene, and from which a protein is translated by the action of ribosomes. The basic function of the nucleotide sequence of mRNA is to determine the amino acid sequence in proteins.

Neurobiology -- The biological study of the nervous system or its components.

Nucleotide -- A unit that polymerizes into nucleic acids (DNA or RNA). Each nucleotide consists of a purine (adenine or guanine) or pyrimidine (cytosine, thymine, or uracil) base, a sugar (ribose in the case of RNA, deoxyribose in the case of DNA), and a phosphate molecule.

Oligonucleotide -- A molecule usually composed of 25 or fewer nucleotides; used as a DNA synthesis primer.

Organelle -- A specialized structure within a cell, such as a mitochondrion or chloroplast, which performs a particular function.

Peptide -- Any of various natural or synthetic compounds containing two or more amino acids linked by the carboxyl group of one amino acid to the amino group of another.

Peptide bond -- A bond joining two amino acids.

Polymer -- A molecule composed of repeated subunits, or individual polypeptide chains in a protein containing more than one polypeptide chain.

Polymerase -- An enzyme that catalyzes the synthesis of nucleic acids on preexisting nucleic acid templates, such as assembling RNA from ribonucleotides or DNA from deoxyribonucleotides.

Polynucleotide -- A DNA polymer composed of multiple nucleotides.

Polypeptide -- A chain of linked amino acids; a protein.

Polyribonucleotide -- An oligonucleotide, a short polymer of two to twenty nucleotides, consisting of a number of ribonucleotides, which are the nucleotides that contain ribose as their sugar, and which are components of RNA.

Polyuridylic acid, or Poly-U -- RNA or a segment of RNA that is composed of a polynucleotide chain consisting entirely of uracil.

Protein -- A large molecule composed of one or more chains of amino acids in a specific order. The order is determined by the base sequence of nucleotides in the gene that codes for the protein. Proteins are required for the structure, function, and regulation of the body's cells, tissues, and organs. Each protein has unique functions.

Ribonucleotide -- A subunit that polymerizes into the nucleic acid RNA. Each nucleotide consists of a nitrogenous base, a sugar (ribose), and one to three phosphate groups.

Ribosomal RNA, or rRNA -- Ribosomal RNA is the most abundant form of RNA. Together with proteins, rRNA forms the ribosomes; rRNA plays a structural role in the ribosome and also assists in the binding of messenger and transfer RNAs.

Ribosome -- A complex organelle composed of proteins and rRNA that catalyzes translation of messenger RNA into an amino acid sequence. Ribosomes consist of two non-identical subunits each consisting of a different rRNA and a different set of proteins.

RNA, or ribonucleic acid -- A single-stranded nucleic acid found in the cell nucleus and cytoplasm, which plays a key role in protein synthesis. (It also constitutes the genetic material of the RNA viruses.) It is similar to DNA but has ribose sugar, rather than deoxyribose sugar, and uracil, rather than thymine, as one of the pyrimidine bases. There are several classes of RNA molecules, including messenger RNA, transfer RNA, and ribosomal RNA, each serving a different purpose.

Transfer RNA, or tRNA -- Small RNA molecules that carry amino acids to the ribosome for polymerization into a polypeptide. During translation, the amino acid is inserted into the growing polypeptide chain when the anticodon of the tRNA pairs with a codon on the mRNA (messenger RNA) being translated.

Uracil -- A pyrimidine base that is an essential constituent of RNA, but not DNA.