Gamete is either type of reproductive (germ) cell—sperm or egg—with haploid chromosome content.
Gap in DNA is the absence of one or more nucleotides in one strand of the duplex.
Gap (cistron) is the segment of DNA involved in producing a polypeptide chain; it includes regions preceding and following the coding region (leader and trailer) as well as intervening sequences (introns) between individual coding segments (exons).
Gene conversion is the alteration of one strand of a heteroduplex DNA to make it complementary with the other strand at any position(s) where there were mispaired bases.
Gene dosage gives the number of copies of a particular gene in the genome.
Gene family consists of a set of genes whose exons are related; the members were derived by duplication and variation from some ancestral gene.
Gene cluster is a group of adjacent genes that are identical or related.
Genetic code is the correspondence between triplets in DNA (or RNA) and amino acids in protein.
Genetic marker—see marker.
Genomic (chromosomal) DNA clones are sequences of the genome carried by a cloning vector.
Genotype is the genetic constitution of an organism.
Golgi apparatus consists of indivivual stacks of membranes near the endoplasmic reticulum; involed in glycosylating  proteins and sorting them for transport to different cellular locations.
G proteins are guanine nucleotide-binding trimeric proteins that reside in the plasma membrane. When bound by GDP the trimer remains intact and is inert. When the GDP bound to the αsubunit is replaced by GTP, the αsubunit is released from the βγ dimmer. One of the separated units (either the α monomer or the βγ dimmer) then activates or represses a target protein.
Gratuitous inducers resemble authentic inducers of transcription but are not substrates for the induced enzymes.
GT-AG rule describes the presence of these constant dinucleotides at the first two and last two positions of introns of nuclear genes.
Gyrase is a type II topoisomerase of E. coli with the ability to introduce negative supercoils into DNA.

Hairpin describes a double-helical region formed by base pairing between adjacent (inverted) complementary sequences in a single strand of RNA or DNA.
Haploid set of chromosomes contains one copy of each autosome and one sex chromosome; the haploid number n is characteristic of gametes of diploid organisms.
Haplotype is the particular combination of alleles in a defined region of some chromosome, in effect the genotype in miniature. Originally used to describe combinations of MHC alleles, it now may be used to describe particular combinations of RFLPs.
Hapten is a small molecule that acts as an antigen when conjugated to a protein.
Helper virus provides functions absent from a defective virus, enabling the latter to complete the infective cycle during a mixed infection.
Hemizygote is a diploid individual that has lost its copy of a particular gene  (for example, because a chromosome has been lost) and which therefore has only a single copy.
Heterochromatin describes regions of the genome that are permanently in a highly condensed condition and are not genetically expressed. May be constitutive or facultative.
Heterorduplex (hybrid) DNA is generated by hase pairing between complementary single strands derived from the different parental duplex molecules; it occurs during genetic recombination.
Helerogametic sex has the diploid chromosome constitution 2A+XY.
Heterogeneous nuclear (hn) RNA comprises transcripts of nuclear genes made by RNA polymerase II; it has a wide size distribution and low stability.
Heteromultimeric proteins consist of nonidentical subunits (coded by different genes).
Heterokaryon is a cell containing two (or more nuclei in a common cytoplasm, generate by fusing somatic cells.
Heterzygote is an idividual with different alleles at some particular locus.
Highly repetivive DNA is the first component to reassociate and is equated with satellite DNA.
Histones are conserved DNA-binding proteins of eukaryotes that form the nucleosome, the basic subunit of chromatin.
Homeobox describes the conserved sequence that is part of the coding region of D. melanogaster homeotic genes; it is also found in amphibian and mammalian genes exopressed in early embryonic development.
Homeotic genes are defined by mutations that convert on body part into another; for example, an insect leg may replace an antenna.
Homogametic sex has the diploid chromosome constitution 2A+XX.
Homologues are chromosomes carrying the same genetic loci; a diploid cell has two copies of each homologue, one derived from each parent.
Homomultimeric rotein consists of identical subunits.
Homozygote is an indivivdual with the same allele at corresponding loci on the homologus chromosomes.
Hotspot is site at which the frequency of mutation (or recombination) is very much increased.
Hox genes are clusters of mammalian genes containing homeoboxes; the individual members are related to the genes of the complex loci ANT-C and BX-C in D. melanogaster.
Housekeeping (constitutive)  genes are those (theoretically) expressed in all cells because they provide basic functions needed for sustenance of all cell types.
Hybrid-arrested translation in a technique that identifies the cDNA corresponding to an mRNA by relying on the ability to base pair with the RNA in vitro to inhibit translation.
Hybrid dysgenesis describes the inability of certain strains of D. melanogaster to interbreed, because the hybrids are sterile (although otherwise they may be phenotypically normal).
Hybrid DNA-see heteroduplex DNA.
Hybridization is the pairing of complementary RNA and DNA strands to give an RNA-DNA hybrid.
Hybridoma is a cell line produced by fusing a myeloma with a lymphocyte; it continues indefinitely to express the immunoglobulins of both parents.
Hydrolytic reaction is one in which a covalent bond is broken with the incorporation of a water molecule.
Hydropathy plot is a measure of the hydrophobicity of a protein region and therefore the likelihood that it will reside in a membrane.
Hydrophilic groups interact with water, so thath hydrophilic regions of protine or the faces of a lipid bilayer reside in an aqueous environment.
Hydrophobic groups repel water, so that they interact with one another to generate a nonaqueous environment.
Hyperchromicity is the increase in optical bensity that occurs when DNA is denatured.
DNAase I hypersensitive site is a short region of chromatin detected by its extreme sensitivity to cheavage by DNAase I and other nucleases; probably comprises an area from which nucleosome are excluded.
Hypervariable regions of an immunoglobulin are the parts of the variable region that show maximum alteration when different antibodies are compared.

Ideogram is a diagrammatic representation of the G-banding pattern of a chromosome.
Idling reaction is the production of pppGpp and ppGpp by ribosomes when an uncharged tRNA is present in the A site; triggers the stringent response.
Immortalization describes the acquisition by a eukaryotic cell line of the ability to grow through an indefinite number of divisions in culture.
Immunity in phages refers to the ability of a prophage to prevent another phage of the same type from infecting a cell. It results from the synthesis of phage repressor by the prophage genome.
Immunity in plasmids describes the ability of a plasmid to prevent another of the same type from becoming established in a cell. It results usually from interference with the ability to replicate.
Immunity in transposons refers to the ability of certain transposons to prevent others of the same type from transposing to the same DNA molecule. It results from a variety of mechanisms.
Imprinting describes a change in a gene that occurs during passage through the sperm or egg with the resule that the paternal and maternal alleles have different properties in the very early embryo. May be caused by methylation of DNA.
In situ hybridization is performed by deaturing the DNA of cells squashed on a microscope slide so that reaction is possible with an added single-stranded RNA or DNA; the added singlestranded RNA or DNA; the added preparatin is radioactively labeled and its hybridization is followed by autoradiography.
Incopmpatibility is the inability of certain bacterial plasmids to coexitst in the same cell. It is a cause of plasmid immunity.
Indircet end-labeling is a technique for examining the organization of DNA by making a cut at a specific site and isolating all fragments containing the sequence adjacent to one side of the cut; it reveals the distance from the cut to the next break(s) in DNA.
Induced mutations result from the addition of a mutagen.
Inducer is a small molecule that triggers gene transcription by binding to a regulator protein.
Induction refers to the ability of bacteria (or yeast) to synthesize certain enzymes only when their substrates are present; applied to gene expression, refers to switching on transcription as a result of interaction of the inducer with the regulator protein.
Induction of prophage describes its excision from the host genome and entry into the lytic (infective) cycle as a result of destuction of the lysogenic repressor.
Initiation factors (IF in prokaryotes, eIF in eukaryotes) are proteins that associate with the small subunit of the ribosome specifically at the stage of initiation of protein synthesis.
Insertion sequence (IS) is a small bacterial transposon that carries only the genes needed for its own transposition.