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.

Assay -- The determination of the amount of a particular constituent of a mixture. Also, the analysis of a drug's potency or enzyme's purity.

Cell nucleus -- Within a cell, it is a spherical body which consists of a nucleolus or nucleoli, where ribosomal RNA is synthesized; irregular granules of chromatin, which is DNA attached to a protein base; nucleoplasm, a highly viscous liquid containing nucleotides and enzymes necessary for the replication of DNA; and a thin outer membrane.

Cesium-chloride gradient centrifugation -- A laboratory technique used to separate or purify nucleic acids in a solution. Cesium chloride, a metal salt, is combined with the nucleic acids and spun in a centrifuge until the nucleic acids stratify with the densest at the bottom and proceeding to the least dense on top; this stratification is known as a density gradient.

Culture [bacterial] -- To grow bacteria in a special medium, such as agar, which allows for their rapid reproduction. The term also refers to the colony of bacteria resulting from this process.

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

Cytoplasm -- The viscous liquid, surrounding the cell nucleus, which makes up the essential material of all plant and animal cells. It is composed of proteins, nucleic acids, lipids, inorganic salts and carbohydrates.

Cytoplasmic enzymatic constitution -- The mix of enzymes contained in the viscous liquid between the cell nucleus and the outer membrane of a cell. The mix is determined by the genome of the cell.

DNA-RNA hybridization -- A laboratory technique used to form a double strand of DNA out of a single strand of DNA and a complementary strand of RNA. The double strand of the original DNA is "unzipped" into single strands and then exposed to a complementary strand of RNA. The RNA then bonds with a strand of the DNA. Such hybridization is highly specific and, therefore, enables identification of different types and sequences of DNA and RNA. It is essential to analyzing the genome, and in the effective use of recombinant DNA technology. Also called "molecular hybridization."

DNA transcriptase -- Any of the group of enzymes that catalyze the template-directed step-by-step addition of ribonucleotides to the end of a growing RNA chain, using a single-stranded DNA template. This process is important in the flow of information from DNA to proteins.

Enzymatic adaptation, or enzymatic induction -- Increased synthesis of an enzyme by a cell or organism in response to a molecule, such as a hormone, produced by the cell or organism as the result of an exterior environmental signal.

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.

Gel electrophoresis -- The separation of the molecules of various ionic compounds dissolved in solution based on the compounds' different rates of movement through an electric field applied to the solution. Gel electrophoresis uses a gel in the form of tubes or a thin slab as a non-reacting support medium, which holds the molecules of the ionic compounds as they move in the conducting medium, in order to prevent diffusion of the compounds.

Genetic markers -- The occurrence of two or more different genetically determined phenotypes in a given population, the rarest of which cannot maintain itself simply through recurrent mutations, having a simple mode of inheritance occurring with multiple alleles. Useful in family studies and studies of the distribution of genes in populations.

Genotype -- The specific allelic composition of a cell, either of the entire cell or more commonly for a certain gene or a set of genes. Also, the genes an organism possesses.

Homology -- Similarity of nucleotide or amino acid sequences in nucleic acids, peptides, or proteins. Also: structural similarity resulting from descent from a common form.

Immune precipitation -- A method of isolating a protein from a mixture by reacting it with its homologous antibody and thereby causing the protein to settle out of the solution in solid particles.

In vitro -- An experimental situation outside a living cell or organism; biological or chemical work done in the test tube, instead of in living systems. "In vitro" is Latin for "in glass."

Isotopic labeling -- Adding a radioactive isotope, a form of a chemical element which has a different number of neutrons than protons, rendering the element unstable, into a compound to more easily track the compound's presence through a series of reactions.

Nucleotide -- A unit that polymerizes into nucleic acids (DNA or RNA). Each nucleotide consists of a nitrogenous base, a sugar (ribose in the case of RNA, deoxyribose in the case of DNA), and a phosphate molecule.

Oncogenes -- Genes with the potential to cause normal cells to become cancerous. The term may be used to describe viral or cellular genes with this potential.

Oncornaviruses -- A group of viruses consisting of a single strand of RNA which produce cancer cells in birds and mammals. From onco- (cancer), RNA, and virus.

Phenotype -- The form taken by one or a group of characteristics in a specific individual; the detectable outward manifestations of a specific genotype.

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.

Recombinant DNA, rDNA, or hybrid strings -- Genetically engineered DNA prepared in vitro by cutting up DNA molecules and splicing together specific DNA fragments, usually from more than one type of organism, e.g., inserting pieces of virus DNA into a bacterial plasmid.

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.

RNA replicase -- An enzyme that catalyzes the template-directed step-by-step addition of ribonucleotides to the end of a growing RNA chain, using a single-stranded RNA template. This enzyme is important for the transcription, and sometimes replication, of RNA in most RNA viruses.

Serial transfer experiment -- An experiment to cause RNA to replicate and mutate swiftly from a wild-type to a wholly laboratory-synthesized form and to cause the RNA to shed residues such as protein coats, stripping it down to being simply a self-replicating molecule. The experimenter takes RNA from a bacteria, bacteriophage, or RNA virus and places it in a test tube full of nucleotides and enzymes and allows the RNA to replicate. Once the rate of replication slows, a small amount of the resulting mutant strain of RNA is then transferred into another test tube of nucleotides and enzymes and allowed to replicate. The series of transfers is repeated until it becomes an entirely laboratory-synthesized mutant RNA strand which is primarily a self-replicating molecule with relatively few residues.

Substrate -- The substance acted upon and changed by an enzyme during a biochemical reaction. Enzymes, unlike most chemicals which act as catalysts (chemicals that speed or slow the rate of a reaction without being changed during the course of a reaction), are highly specific, bonding with only a few different substrates, limiting the paths a reaction may take.

Sucrose-gradient centrifugation -- A laboratory method used to purify enveloped viruses and cell organelles, the membrane-bound portions of the cell such as the cell nucleus or mitochondria, from crude cellular extracts. The sucrose, a sugar, is layered in a test tube in strata of decreasing density, called a gradient. Then a sample is placed on the top of the gradient and the tube is spun in a centrifuge. The viral or cellular particles travel through the gradient until they reach a layer which matches their density. The thus purified particles can then be removed from a given layer and studied.

Template -- In molecular biology, a molecule (as in DNA) that serves as the pattern for the manufacture of another macromolecule, e.g., messenger RNA.

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.

Virus -- Any one of a group of minute infectious agents that are not visible under a light microscope and are characterized by a lack of independent metabolism and an ability to replicate only within living host cells. Even though they are not living organisms, like living organisms they have the ability to pass genes through multiple generations and to mutate. The individual virus particle consists of a strand or strands of nucleic acid (which may be either DNA or RNA) and a protein shell. Viruses are classified into three main subgroups based on their host, bacterial viruses, plant viruses, and animal viruses. Viruses are then further classified by their origin, means of transmission, or illnesses they produce.

Yeast -- A vague term used to refer to one of the largest groupings of fungi. Yeasts are single-celled, usually rounded fungi that reproduce by budding. Some transform into a colonial stage (mold) under certain environmental conditions, while others always remain single-celled. A few yeasts cause illness in humans.