Glossary of Commonly Used Terms

A majority of these definitions were taken from DNA Technology in Forensic Science, (1992) National Research Council, Washington, D.C.: National Academy Press


Adenine: A purine base; one of the four molecules containing nitrogen present in the nucleic acids DNA and RNA; designated by letter A.

Allele: An alternative form of a gene.

Allele Frequency: The proportion of a particular allele among the chromosomes carried by individuals in a population.

Amino acid: Any of a class of 20 molecules that are combined to form proteins in living things. The sequence of amino acids in a protein and hence protein function are determined by the genetic code.

Amplification: An increase in the number of copies of a specific DNA fragment; can be in vivo or in vitro.

Autosome: A chromosome not involved in sex determination. The diploid human genome consists of 46 chromosomes, 22 pairs of autosomes, and one pair of sex chromosomes (the X and Y chromosomes).

Basepair: Two complementary nucleotides joined by hydrogen bonds; basepairing occurs between A and T and between G and C.

Base sequence: The order of nucleotide bases in a DNA molecule.

Base sequence analysis: A method, sometimes automated, for determining the base sequence.

Biotechnology: A set of biological techniques developed through basic research and now applied to research and product development.

Chromosome :The structure by which hereditary information is physically transmitted from one generation to the next.

Complementary sequences: Nucleic acid base sequences that form a double-stranded structure by matching base pairs; the complementary sequence to G-T-A-C is C-A-T-G.

Cytosine: A pyrimidine base; one of the four molecules containing nitrogen present in the nucleic acids DNA and RNA; designated by letter C.

Denaturation: the process of splitting the complementary double strands of DNA to form single strands

Deoxyribonucleic acid (DNA): The genetic material of organisms, usually double-stranded; a class of nucleic acids identified by the presence of deoxyribose, a sugar, and the four nucleobases.

DNA sequence: The relative order of base pairs, whether in a fragment of DNA, a gene, a chromosome, or an entire genome.

Double Helix: The shape that two linear strands of DNA assume when bonded together.

Electrophoresis: a technique in which molecules are separated by their velocity in an electric field

Enzyme: A protein that can speed up a specific chemical reaction without being changed or consumed in the process.

Gel: semisolid matrix (usually agarose or acrylamide) used in electrophoresis to separate molecules

Gene: the basic unit of heredity; a sequence of DNA nucleotides on a chromosome

Gene frequency: the relative occurrence of a particular allele in a population

Gene mapping: Determination of the relative positions of genes on a DNA molecule (chromosome or plasmid) and of the distance, in linkage units or physical units, between them.

Genetics: The study of the patterns of inheritance of specific traits.

Genome: All the genetic material in the chromosomes of a particular organism; its size is generally given as the total number of base pairs.

Genome projects: Research and technology development efforts aimed at mapping and sequencing some or all of the genome of an organism.

Genotype: the genetic makeup of an organism, as characterized by its physical appearance or phenotype

Guanine: a purine base; one of the four molecules containing nitrogen present in the nucleic acids DNA and RNA; designated by letter G

Heredity: the transmission of characteristics from one generation to the next

Heterozygosity: The presence of different alleles at one or more loci on homologous chromosomes.

Homologies: Similarities in DNA or protein sequences between individuals of the same linear sequences, each derived from one parent.

Homologous chromosomes: A pair of chromosomes containing the same linear gene sequences, each derived from one parent.

In vitro: Outside a living organism

Kilobase (kb): Unit of length for DNA fragments equal to 1000 nucleotides.

Linkage: The proximity of two or more markers (genes, etc.) on a chromosome; the closer together the markers are, the lower the probability that they will be separated during DNA repair or replication process, and hence the greater the probability that they will be inherited together.

Localize: Determination of the original position (locus) of a gene or other marker on a chromosome.

Locus (pl. loci): The specific physical location of a gene on a chromosome.

Marker: A gene of known location on a chromosome and phenotype that is used as a point of reference in the mapping of other loci.

Megabase (Mb): Unit of length for DNA fragments equal to one million nucleotides.

Mitosis: The process of nuclear division in cells that produces daughter cells that are genetically identical to each other and to the parent.

Multiplexing: A sequencing approach that uses several pooled samples simultaneously, greatly increasing sequencing speed.

Mutation: Any inheritable change in DNA sequence.

Nucleic acid: A nucleotide polymer that DNA and RNA are major types.

Nucleotide: A unit of nucleic acid composed of phosphate, ribose or deoxyribose, and a purine or pyrimidine base.

Nucleus: The cellular organelle in eukaryotes that contains the genetic material.

Oncogene: A gene, one or more forms of which is associated with cancer. Many oncogenes are involved, directly or indirectly, in controlling the rate of cell growth.

Physical map: A map of the locations of identifiable landmarks on DNA. Distance is measured in base pairs.

Polymerase chain reaction (PCR): An in vitro process that yields millions of copies of desired DNA through repeated cycling of a reaction involving the DNA polymerase enzyme.

Polymerase, DNA or RNA: Enzymes that catalyze the synthesis of nucleic acids on preexisting nucleic acid templates, assembling RNA from ribonucleotides or DNA from deoxyribonucleotides.

Polymorphism: Difference in DNA sequence among individuals. Genetic variations occuring in more than 1% of a population would be considered useful polymorphisms for linkage analysis.

Population: A group of individuals residing in a given area at a given time.

Primer: Short preexisting polynucleotide chain to which new deoxyribonucleotides can be added by DNA polymerase.

Probe: Single-stranded DNA or RNA of a specific base sequence, labeled either radioactively or immunology, that are used to detect the complementary base sequence by hybridization.

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 nuceotides in the gene coding for the protein. Proteins are required for the structure, function, and regulation of the body cells, tissues, organs, and each protein has unique functions.

Recombinant DNA technologies: Procedurese used to join together DNA sequences in a cell-free system. Under appropriate conditions, a recombinant DNA molecule can enter a cell and replicate there, either autonomously or after it has become integrated into a cellular chromosome.

Resolution: Degree of molecular detail on a physical map of DNA.

Restriction enzyme: A protein that recognizes specific, short nucleotide sequences and cuts DNA at the those sites. Bacteria contain over 400 such enzymes that recognize and cut over 100 DNA sequences.

Restriction fragment length polymorphism (RFLP): Variation between individuals in DNA fragment sizes cut by specific restriction enzymes; polymorphic sequences that result in RFLPs that are used as markers on both physical maps and genetic linkage maps. RFLPs are usually caused by mutation at a cutting site.

Sequencing: Determination of the order of nucleotides (base sequences) in a DNA or RNA molecule or the order of amino acids in a protein.

Sex chromosomes (x and y chromosomes): Chromosomes that are different in the two sexes and involved in sex determination.

Short tandem repeats (STR): Multiple copies of an identical DNA sequence arranged in direct succession in a particular region of a chromosome.

Southern blotting: Transfer by absorbtion of DNA fragments separated in electrophoretic gels to membrane filters for detection of specific base sequences by raiolabeled complementary probes.

Tandem repeat sequences: Multiple copies of the same base sequence on a chromosome; used as a marker in physical mapping.

Thymine: a pyrimidine base; one of the four molecules containing nitrogen present in the nucleic acids DNA and RNA; designated by letter T

Variable number tandem repeats (VNTR): repeating units of a DNA sequence which number varies between individuals