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Molecular Biology and Bioinformatics Glossary



E value

Expectation value. The number of different alignents with scores equivalent to or better than S that are expected to occur in a database search by chance. The lower the E value, the more significant the score. (NCBI)
This represents the number of sequences with a score greater-than, or equal to, X, expected absolutely by chance. The E-value connects the score ("X") of an alignment between a user-supplied sequence and a database sequence, generated by any algorithm, with how many alignments with similar or greater scores that would be expected from a search of a random sequence database of equivalent size. Since version 2.0 E-values are calculated using Hidden Markov Models, leading to more accurate estimates than before. (SMART)


A method of separating large molecules (such as DNA fragments or proteins) from a mixture of similar molecules. An electric current is passed through a medium containing the mixture, and each kind of molecule travels through the medium at a different rate, depending on its electrical charge and size. Agarose and acrylamide gels are the media commonly used for electrophoresis of proteins and nucleic acids. (ORNL)


A process using high-voltage current to make cell membranes permeable to allow the introduction of new DNA; commonly used in recombinant DNA technology.
See also: transfection (ORNL)

Embryonic stem (ES) cells

An embryonic cell that can replicate indefinitely, transform into other types of cells, and serve as a continuous source of new cells. (ORNL)


See: restriction enzyme (ORNL)


A protein that acts as a catalyst, speeding the rate at which a biochemical reaction proceeds but not altering the direction or nature of the reaction. (ORNL)


One gene interfers with or prevents the expression of another gene located at a different locus. (ORNL)

Escherichia coli

Common bacterium that has been studied intensively by geneticists because of its small genome size, normal lack of pathogenicity, and ease of growth in the laboratory. (ORNL)


The study of improving a species by artificial selection; usually refers to the selective breeding of humans. (ORNL)


Cell or organism with membrane-bound, structurally discrete nucleus and other well-developed subcellular compartments. Eukaryotes include all organisms except viruses, bacteria, and bluegreen algae.
See also: prokaryote, chromosome. (ORNL)

Evolutionarily conserved

See: conserved sequence (ORNL)

Exogenous DNA

DNA originating outside an organism that has been introducted into the organism. (ORNL)


The protein-coding DNA sequence of a gene.
See also: intron (ORNL)


An enzyme that cleaves nucleotides sequentially from free ends of a linear nucleic acid substrate. (ORNL)

Expressed gene

See: gene expression (ORNL)

Expressed sequence tag (EST)

A short strand of DNA that is a part of a cDNA molecule and can act as identifier of a gene. Used in locating and mapping genes.
See also: cDNA, sequence tagged site (ORNL)


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