Bioinformatics glossary - E

Electronic Northerns

The use of an electronic database of cDNA sequences (or probes derived from them) in order to measure the relative levels of mRNAs expressed in different cells or tissues. An example of the use of an electronic Northern might be to identify the differences in the genes expressed in prostate cancer and those in benign prostate hyperplasia, by subtracting the database of one from the other and seeing which cDNAs remain.

Electrophoresis

The use of an external electric field to separate large biomolecules on the basis of their charge by running them through acrylamide or agarose gels.

Enhancers

DNA sequences that can greatly increase the transcription rates of genes even though they may be far upstream or downstream from the promoter they stimulate.

Enzyme

A class of proteins that are capable of catalyzing chemical reactions (the making or breaking of chemical bonds). They do so by orienting their substrates into a suitable geometry in a particular location (the active site) where electrophilic or nucleophilic amino acid residues can participate in the reaction. Enzymes are protein catalyst that speeds up chemical reactions that would otherwise be prohibitively slow under physiological conditions.

Epigenomics

The study of complex expression networks or linkages both spatially (within the body) and temporally (at different times in development).

Equilibrium constant

Value that describes the equilibrium state of the reversible reaction between two molecular species.

Eukaryote

A cell or organism with a distinct membrane-bound nucleus as well as specialized membrane-based organelles (see also prokaryote).

Exon

The region of DNA within a gene that codes for a polypeptide chain or domain. Typically a mature protein is composed of several domains coded by different exons within a single gene.

Expressed Sequence Tags (ESTs)

A small sequence from an expressed gene that can be amplified by PCR. ESTs act as physical markers for cloning and full length sequencing of the cDNAs of expressed genes. Typically identified by purifying mRNAs, converting to cDNAs, and then sequencing a portion of the cDNAs.

Expression (gene or protein)

A measure of the presence, amount, and time-course of one or more gene products in a particular cell or tissue.  Expression studies are typically performed at the RNA (mRNA) or protein level in order to determine the number, type, and level of genes that may be up-regulated or down-regulated during a cellular process, in response to an external stimulus, or in sickness or disease.  Gene chips and proteomics now allow the study of expression profiles of sets of genes or even entire genomes.

Expression profile

The level and duration of expression of one or more genes, selected from a particular cell or tissue type, generally obtained by a variety of high-throughput methods, such as sample sequencing, serial analysis, or microarray-based detection.

Expression vector

A cloning vector that is engineered to allow the expression of protein from a cDNA. The expression vector provides an appropriate promoter and restriction sites that allow insertion of cDNA.