GENETICS 372 Winter 1999

W. Fangman

Definitions of Course Terms

Allele One of the different forms of a gene or DNA sequence that can exist at a single locus.

Annealing Formation of double-stranded nucleic acid from single stranded forms.

Autosome Any chromosome other than the sex chromosomes or the mitochondrial chromosome.

Carcinogen Physical or chemical agent which induces cancer.

Carrier In human genetics, an individual heterozygous for a mutant allele that generally causes disease only in the homozygous state. More generally, an individual who possesses a mutant allele but does not express it in the phenotype because of a dominant allelic partner; thus, an individual of genotype Aa is a carrier of a if there is complete dominance of A over a.

cDNA A duplex DNA where one strand is identical in sequence (except for T in place of U) and one is complementary to a particular RNA.

cis-acting locus Locus that affects the activity only of DNA sequences on the same molecule of DNA; usually implies that the locus does not code for protein.

cis configuration Two sites on the same molecule of DNA.

Clone A large number of cells or molecules identical with a single ancestral cell or molecule.

Complementarity The chemical affinity between specific nitrogenous bases as a result of their hydrogen bonding properties. The property of two nucleic acid chains having base sequences such that an antiparallel duplex can form where the adenines and thymines (or uracils) are apposed to each other, and the guanines and cytosines are apposed to each other.

Complementation The production of a wild-type phenotype when two different mutations are combined in a diploid or a heterokaryon.

Chromatid One of the two side-by-side replicas produced by chromosome duplication.

Codon A triplet of nucleotides that represents an amino acid or a termination signal.

Cytoplasmic inheritance Inheritance via genes found in cytoplasmic organelles.

Degenerate code A genetic code in which some amino acids may be encoded by more than one codon each.

Denaturation The separation of the two strands of a DNA double helix, or the severe disruption of the structure of any complex molecule without breaking the major bonds of its chains.

Domain of a protein A discrete continuous part of the amino acid sequence that can be equated with a particular function.

Dominance The expression of a trait in the heterozygous condition.

Downstream Sequences proceeding farther in the direction of transcription, for example, the coding region is downstream of the promoter.

Endonuclease An enzyme that cleaves the phosphodiester bond within a nucleotide chain.

Enzyme A protein that functions as a catalyst.

Eukaryotes Organisms (ranging from yeast to humans) which have nucleated cells.

Exon Any segment of an interrupted gene that is represented in the mature RNA product.

Exonuclease An enzyme that cleaves nucleotides one at a time from an end of a polynucleotide chain.

Familial trait Any trait that is more common in relatives of an affected individual than in the general population; could be due to genetic and/or environmental causes.

Frameshift mutations Mutations that arise by deletions or insertions that are not a multiple of 3 bp; they change the frame in which triplets are translated into protein.

Gene The fundamental physical and functional unit of heredity, which carries information from one generation to the next; a segment of DNA, composed of a transcribed region and a regulatory sequence that makes transcription possible.

Genetic code The set of correspondences between nucleotide pair triplets in DNA and amino acids in protein.

Genetic heterogeneity A similar phenotype being caused by different mutations. Most commonly used for a similar phenotype being caused by mutations in different genes. Allelic heterogeneity refers to different mutations in the same gene.

Genetic markers Alleles of genes, or DNA polymorphisms, used as experimental probes to keep track of an individual, a tissue, a cell, a nucleus, a chromosome, or a gene.

Genome The total genetic material of an organism, i.e. an organism's complete set of DNA sequences.

Genotype The genetic constitution of an individual or, more specifically, the alleles at specific genetic loci.

Haplotype A set of closely linked alleles (genes or DNA polymorphisms) inherited as a unit.

Heterozygous Having two different alleles at a given locus on a pair of homologous chromosomes.

Homologous chromosomes Chromosomes that pair with each other at meiosis.

Homozygote An individual possessing a pair of identical alleles at a given locus on a pair of homologous chromosomes.

Housekeeping gene Gene that is expressed in virtually all cells since it is fundamental to the any cell's functions.

Hydrogen bond A weak bond involving the sharing of an electron with a hydrogen atom; hydrogen bonds are important in the specificity of base pairing in nucleic acids and in the determination of protein shape.

Inbred line A group of identical pure-breeding diploid or polyploid organisms, distinguished from other individuals of the same species by some unique phenotype or genotype, that are maintained by interbreeding.

Karyotype The entire chromosome complement of an individual or cell, as seen during mitotic metaphase.

Kilobase pair or kilobase (kb) 1000 base pairs of DNA or 1000 bases of RNA.

Leader sequence The sequence at the 5' end of an mRNA that is not translated into protein.

Library A set of cloned fragments together representing the entire genome.

Ligase DNA ligase; an enzyme that can rejoin a broken phosphodiester bond in a nucleic acid; requires a 5' phosphate and a 3' OH.

Locus The site or place on a chromosome where a particular gene is located.

Marker same as Genetic marker

Melting Denaturation of DNA.

Missense mutation A single DNA base change which leads to a codon specifying a different amino acid.

mRNA (messenger RNA) An RNA molecule, transcribed from a gene, from which a protein is translated by the action of ribosomes.

Mutagen Any agent that is capable of increasing the mutation rate.

Mutant allele An allele differing from the allele found in the standard, or wild type.

Nonsense codon (also called STOP codon) Any one of three triplets (UAG, UAA, UGA) that cause termination of protein sysnthesis.

Nonsense mutation (also called STOP mutation) Any change in DNA that causes a (termination) codon to replace a codon representing an amino acid.

Nonsense suppressor (also called STOP suppressor) A gene coding for a mutant tRNA able to respond to one or more of the termination codons.

Northern blotting Procedure to transfer RNA from an agarose gel to a nylon membrane.

Null allele An allele that makes no gene product or whose product has no activity of any kind; a deletion of a gene is necessarily a null allele.

Oligonucleotides Small single-stranded segments of DNA typically 20-30 nucleotide bases in size which are synthesized in vitro.

Pedigree A "family tree," drawn with standard symbols, showing inheritance patterns for specific phenotypic characters.

Penetrance The proportion of individuals with a specific genotype who manifest that genotype at the phenotype level.

Phenotype The observed result of the interaction of the genotype with environmental factors; the observable expression of a particular gene or genes.

Plasmid Cytoplasmic, autonomously replicating extrachromosomal DNA molecule.

Point mutation A change in a single base pair.

Prokaryote An organism without a nucleus; eubacteria, archaebacteria, and blue-green algae.

Promoter A region of DNA involved in binding of RNA polymerase to initiate transcription.

Recessive allele An allele whose phenotypic effect is not expressed in a heterozygote.

Renaturation The reassociation of denatured complementary single strands of a DNA double helix.

Silent mutation Mutation in which the function of the protein product of the gene is unaltered.

Somatic cells All the cells of an organism except those of the germ line.

STOP see Nonsense.

Suppression Changes that eliminate the effects of a mutation without reversing the original change in DNA.

Suppressor mutation A mutation that counteracts the effects of another mutation. A suppressor maps at a different site than the mutation it counteracts, either within the same gene or at a more distant locus. Different suppressors act in different ways.

Temperature-sensitive mutation A class of conditional mutations; the mutant phenotype is observed in one temperature range and the wild-type phenotype is observed in another temperature range.

Template strand The strand of the DNA double helix that is copied by base pair complementarity to make an RNA. The other, non-template strand of the DNA duplex has a sequence that is identical to the synthesized RNA (except in RNA, U replaces T).

Trait Any detectable phenotypic variation of a particular inherited character.

Transcription unit The distance between sites of initiation and termination by RNA polymerase; may include more than one gene.

Trans configuration The configuration of two sites refers to their presence on two different molecules of DNA (chromosomes).

Transfection of eukaryotic cells The acquisition of new genetic markers by incorporation of added DNA.

Transformation of bacteria or yeast The acquisition of new genetic markers by incorporation of added DNA.

Transformation of eukaryotic cells Their conversion to a state of unrestrained growth in culture, resembling or identical with the tumorigenic condition; usually applied to mammalian cells.

Transgenic organism One whose genome has been modified by externally applied new DNA; a term applied to metazoans.

Vector In cloning, the plasmid, phage, or yeast chromosomal sequences used to propagate a cloned DNA segment.

Western blotting A technique in which proteins are separated by gel electrophoresis and transferred to a nylon sheet. A specific protein is then identified through its reaction with a labeled antibody.

Wild type The genotype or phenotype that is found in nature or in the standard laboratory stock for a given organism.

X-ray crystallography A technique for deducing molecular structure by aiming a beam of X rays at a crystal of the test compound and measuring the scatter of rays.