D loop is a region within mitochondrial DNA in which a short stretch of RNA is paired with one strand of DNA, displacing the orignal partner DNA strand in this region. The same term is used also to describe the displacement of a region of one strand of duplex DNA byu a single-stranded invader in the reaction catalyzed by RecA protein.
Degeneracy in the genetic code refers to the lack of an effect of many changes in the third base of the codon on the amino acid that is represented.
Deletions are generated by removal of a sequence of DNA, the regions on either side being joined together.
Denaturation of DNA or RNA describes its conversion from the double-stranded to the singlestranded state; separation of the strands is most often accomplished by heating.
Denaturation of protein describes its conversion from the physiological conformation to some other (inactive) conformation.
Derepressed state describes a gene that is turned on. It is synonymous with induced when describing the normal state of a gene; it has the same meaning as constitutive in describing the effect of mutation.
Dicentric chromosome is the product of fusing two chromosome fragments, each of which has a centromere. It is unstable and may be broken when the two centromeres are pulled to opposite poles in mitosis.
Diploid set of chromosomes contains two copies of each autosome and two sex chromosome.
Direct repeats are identical (or related) sequences present in two or more copies in the same orientation in the same molecule of DNA; they aer not necessarily adjacent.
Discontinuous replication refers to the synthesis of DNA in short (Okazaki) fragments that are later joined into a continuous strand.
Disjunction describes the movement of members of a chromosome pair to opposite poles during cell division. At mitosis and the second meiotic division, disjunction applies to sister chromatids; at first meiotic division it applies to sister chromatid pairs.
Divergence is the percent difference in nucleotide sequence between two related DNA sequences or in amino acid sequences between two proteins.
Divergent transcription refers to the intitiation of transcription at two promoters facing in the opposite direction, so that transcription proceeds away in both directions from a central region.
dna mutants of bacteria are temperature-sensitive; they cannot synthesize DNA at 42℃, but can do so at 37℃.
DNAase is an enzyme that attacks bonds in DNA.
DNA-driven bybridization involves the reaction of an excess of DNA with RNA.
DNA polymerase is an enzyme that synthesizes a daughter strand(s) of DNA (under direction from a DNA template). May be involved in repair or replication.
DNA replicase is a DNA-synthesizing enzyme required specifically for replication.
Domain of a chromosome may refer either to a discrete structural entity defined as a region within which supercoiling is independent of other domains; or to an extensive region including an expressed gene that has heightened sensitivity to degradation by the enzyme DNAase I.
Domain of a protein is a discrete continuous part of the amino acid sequence that can be equated with a particular function.
Dominant allele determines the phenotype displayed in a heterozygote with another (recessive) allele.
Donor splicing site—see left splicing junction.
Down promoter mutations decrease the frequency of initiaton of transcription.
Downstream identifies sequences proceeding farther in the direction of expression, for example, the conding region is downstream of the initiation condon.

Early development refers to the period of a phage infection before the start of DNA replication.
Extopic expression describes the expression of a gene in a tissue in which it is not usually expressed; for example, in a transgenic animal.
Elongation factors (EF in prokaryotes, eEF in eukaryotes) are proteins that associate with ribosomes cyclically, during addition of each amino acid to the polypeptide chain.
End labeling describes the addition of a radioactively labeled group to one end (5’ or 3’) of a DNA strand.
End-product inhibition describes the ability of a product of a metabolic pathway to inhibit the activity of an enzyme that catalyzes an early step in the pathway.
Endocytosis is a process by which proteins at the surface of the cell are internalized, being transported into the cell within membranous vesicles.
Endocytic vesicles are membranous particles that transport proteins through endocytosis; also known as clathrin-coated vesicles.
Endonucleases cleave bonds within a nucleic acid chain; they may be specific for RNA or for singlestranded of double-stranded DNA.
Endoplasmic reticulum is a highly convoluted sheet of membranes, extending from the outer layer of the nuclear envelope into the cytoplasm.
Enhancer element is a cis-acting sequence that increases the utilization of (some) eukaryotic promoters, and can function in either orientation and in any location (upstream or downstream) relative to the promoter.
Envelopes surround some organelles (for example , nucleus or mitochondrion) and consist of concentric membranes, each membrane consisting of the usual lipid bilayer.
Epigenetic changes influrence the phenotype without altering the genotype. They consist of changes in the properties of a cell that are inherited but that do not represent a change in genetic information.
Episome is a plasmid able to integrate into bacterial DNA.
Epistasis describes a situation in which expression of one gene wipes out the phenotypic effects of another gene.
Essential gene is one whose deletions is lethal to the organism (see also lethal locus).
Established cell lines consist of eukaryotic cells that have been adapted to indefinite growth in culture (they are said to be immortalized).
Eubacteria comprise the major line of prokaryotes.
Euchromatin comprises all of the genome in the interphase nucleus except for the heterochromatin.
Evolutionary clock is defined by the rate at which mutations accumulate in a given gene.
Excision-repair systems remove a single-stranded sequence of DNA containing damaged or mispaired bases and replace it in the duplex by synthesizing a sequence complementary to the remaining strand.
Exocytosis is the process of secreting proteins from a cell into the medium, by transport in membranous vesicles from the endoplasmic reticulum, through the Golgi, to storage vesicles, and finally (upon a regulatory signal) through the plasma membrane.
Exocytic vesicles (also secretory vesicles) are membranous particles that transport and store proteins during excytosis.
Exon is any segment of an interrupted gene that is represented in the mature RNA product.
Exonucleases cleave nucleotides one at a time from the end of a polynucleotide chain; they may be specific for either the 5’ or 3’ end of DNA or RNA.
Expression vector is a cloning vector desined so that a coding sequence inserted at a particular site will be transcribed and translated into protein.
Extranuclear genes reside outside the nucleus in organelles such as mitochondria and chloroplasts.

F factor is a bacterial sex or fertility plasmid.
F1 generation is the first generation produced by crossing two parental (homozygous) lines.
Facultative heterochromatin describes the inert state of sequences that also exist in active copies—for example, one mammalian X chromosome in females.
Fast component of a reassociation reaction is the first to reature and contains highly repetitive DNA.
Fate map is a map of an embryo showing the adult tissues that will develop from the descendants of cells that occupy particular regions of the embryo.
Figure eight describes two circles of DNA linked together by a recombination event that has not yet been completed.
Filter hybridization is performed by incubating a deatured DNA preparation immobilized on a nitrocellulose filter with a solution of radioactively labeled RNA or DNA.
Fingerprint of DNA is a pattern of polymorphic restriction fragments that differ between individual genomes.
Fingerprint of a protein is the pattern of fragments (usually resolved on a two dimensional electrophoretic gel) generated by cleavage with an enzyme such as trypsin.
Fluidity is a property of membranes; it indicates the ability of lipids to move laterally within their particular monolayer.
Focus formation describes the ability of transformed eukaryotic cells to grow in dense clusters, piled up on one another.
Focus forming unit (ffu) is a quantitative measure of forcus formation.
Foldback DNA consists of inverted repeats that have renatured by intrastrand reassociation of denatured DNA.
Foot printing is a technique for identifying the site on DNA bound by some protein by virtue of the protection of bonds in this region against attack by nucleases.
Forward mutations inatctivate a wild-type gene.
Founder effect refers to the presence in a population of many individuals all with the same chromosome (or region of a chromosome) derived from a single ancestor.
Frameshift mutations arise by deletions or insertions that are not a multiple of 3bp; they change the frame in which triplets are translated into protein.

G banding is a technique that generates a striated pattern in metaphase chromosomes that distinguishes the members of a haploid set.
G1 is the period of the eukaryotic cell cycle between the last mitosis and the start of DNA replication.
G2 is the period of the eukaryotic cell cycle between the end of DNA replication and the start of the next mitosis.