Gel electrophoresis
The process of separating charged species by subjecting them to a voltage gradient. A gel (made of agarose or polyacrylamide) provides mechanical support and prevents mixing of the molecules being separated. Electrophoresis performed in a gel matrix so that molecules of similar electric charge can be separated on the basis of size.

Gel retardation
A technique that identifies a DNA fragment that has a bound protein by virtue of its decreased mobility during gel electrophoresis.

Gel shift assay
Also known as gel retardation assay or gel mobility shift assay. See the definition under Gel retardation.

GFP
See Green Flourescent Protein.

Geminivirus
One of the two groups of DNA viruses that infect plants, the members of which have potential as cloning vectors for some species of higher plants.

Gene
The genetic unit of function. A gene may encode a polypeptide or a molecule of non-translated RNA (e.g. ribosomal RNA, transfer RNA, or a regulatory RNA).

Gene amplification
Multiple copies of a single gene within a cell.

Gene conversion
An genetic event that produces abnormal segregations by non-reciprocal recombination. Gene conversion has been extensively studied in ascomycetes, where it occurs following meiosis as the result of the formation of a mismatched base pair and subseque nt repair by the mismatch repair system.

Gene expression
The process by which a a gene product is produced. For genes that encode proteins, the gene must be transcribed into mRNA and then translated into protein. For genes that encode structural RNAs (rRNA, tRNA, etc), the gene must transcribed into RNA.

Gene fusion
A construct that joins the coding region of two open reading frames such that expression of the product results in a chimeric protein. For a more detailed description, see Operon and Gene fusions. Sometimes called a translational fusion or protein fusion.

Gene library
A random collection of DNA fragments (typically representing the entire genome of an organism) that have been inserted into a cloning vector.

Gene mapping
Determination of the relative positions of different genes on a DNA molecule.

Gene transfer agent
A phage-like agent from Rhodobacter capsulates species which can transfer DNA from the donor host to recipient cells.

Gene probe
A labelled sequence of single-stranded nucleic acid (DNA or RNA) which can be used to detect the complementary nucleic acid sequence by a hybridization techniques.

General (homologous) recombination
The common cellular mechanism for genetic recombination between DNA molecules with identical or near identical (homologous) nucleotide sequences. In enteric bacteria, homologous recombination requires the RecA protein.

Generalized transduction
The transfer of any region of the host chromosome into a recipient cell packaged in phage particles. Each phage particle only carries one region of the host chromosome, but within the population of phage every region of the host chromosome is represe nted. Generalized transduction usually results from an error resulting in packaging of host DNA into a phage particle. Typically such transducing particles only carry chromosomal DNA without any phage DNA.

Generation time
The amount of time required under specific exponential growth conditions for one cell to divide, forming two daughter cells.

Genetic code
The assignment of each of the triplet codons of mRNA to amino adds and translation stop signals. See Codon table.

Genetic engineering
The use of molecular techniques to produce DNA molecules containing new genes or new combinations of genes.

Genetic map
The relative position and distance between genes determined from recombination frequencies.

Genetic marker
A locus that has a useful phenotype which can be used for genetic selections or screens.

Genetic polymorphism
The existence of two or more genetically different classes within a population.

Genetic recombination
The process by which a fragment of DNA from one molecule (chromosome, plasmid, phage genome) is exchanged with or integrated into another molecule to produce a recombinant molecule(s).

Genome
The complete genetic content of a cell or organism, including chromosomes, plasmids, and prophages. The total genetic information of an organism. [The Oxford English Dictionary attributes the term genome to Hans Winkler who wrote in 1920 "I propose the expression Genom for the haploid chromosome set, which, together with the pertinent protoplasm, specifies the material foundations of the species ...." Source: J. Lederberg and A. McCray (2001) The Scientist 15:8]

Genomic library
A collection of clones that includes essentially all the genes of a particular organism.

Genomics
The comparative analysis of genomic DNA sequences from different organisms. Genomic analysis can provide information about the evolution of genes and can make predictions about the metabolism of an organism.

Genotype
A specific description of the genetic constitution of an organism. The genotype is defined by the allelic form of each gene in an organism, but for simplicity usually only differences from the wild-type are described.

Gentamicin
A type of aminoglycoside antibiotic. Often used to study bacterial entry into eukaryotic cells because gentamicin cannot cross the eukaryotic cell membrane, thus intracellular bacteria are resistant to gentamicin but extracellular bacteria are usually sensitive to gentamicin.

GMO
An abbreviation for "genetically modified organism". Commonly used to refer to organisms that have been developed using recombinant DNA approaches, particularly if some foreign DNA remains in the strain.

Gradient of transfer
In an Hfr mating, the decrease in the inheritance of donor markers the farther they are from the origin of transfer.

Gram-negative bacteria
A group of bacteria with a cell wall composed of an outer membrane surrounding a thin peptidoglycan layer. When stained with crystal violet then treated with organic solvents, the dye is readily removed by the organic solvent.

Gram-positive bacteria
A group of bacteria with a cell wall composed of thick peptidoglycan layer and no outer membrane. When stained with crystal violet then treated with organic solvents, the dye is retained and thus the cells appear purple colored.

Green Flourescent Protein (GFP)
An intrinsically fluorescent protein from the jellyfish Aequorea victoria. Gene fusions with GFP are commonly used for determining protein localization by fluorescence microscopy.

Growth rate
The rate of increase of cellular density per unit time.
GUS
An abbreviation for Beta-glucuronidase (encoded by the uidA gene). Commonly used for constructing operon or gene fusions in organisms with high endogenous Beta-galactosidase activity. A variety of analogs exist that make it easy to detect GUS expression on solid medium and to assay the activity of the enzyme in cells.

Gyrase
See DNA gyrase.