Exon Coding sequence of a gene. A segment of the gene that is transcribed to form a functional mRNA molecule together with other exons after processing.

Exon amplification A method used to amplify exons.

Exon definition The mechanism by which the splicing factors termed SR proteins (sering/arginine-rich) interact with both U1 snRNAP and U2AF during pre-mRNA splicing, span the exon and define the segment of RNA to be retained.

Exon duplication (exon repetition)  Tandem duplication of a single exon that gives rise to repetitive modules within genes. This offers two evolutionary advantages: i) for structural proteins such as collagen exon duplication results in extension of a specific structural domain, ii) for other proteins, exon duplication can increase the dosage of a specific functional domain thereby increasing protein activity, or permit posttranslational cleavage to generate multiple functional units (eg. ubiquitin genes)

Excrete To remove waste products. Also see export.

Executable file, image A file in machine code that can be interpreted by a computer processor under control of the operating system

Exon prediction  Computer programmes that predict the locations of putative exons based on the presence of open reading frames and splice consensus sequences.

Exon shuffling An evolutionary process for creating new genes (new combinations of exons) from preexisting ones. This occurs by recombination or by transposition of mobile DNA elements.

Exon sliding Shifting of the start and stop positions of the exons with respect to the surrounding introns. This may be caused by deletions, insertions or mutations of the splice sites. Exon sliding can correct exon shuffling if the participating exon is out-of-phase.

Exon trapping A technique that involves insertion of genomic clones into an intron flanked by tow exons in an expression vector and transfection of the construct into cells. If the genomic clone contains an exon, splicing will generate a mature transcript with three exons (two vector exons and the central trapped exon).If the genomic clone does not contain an exon, the transcript will contain only two exons. RNA isolated from the cells is analysed by RT-PCR for the presence of the acquired exon (exon amplification).

Exonuclease An enzyme that cleaves nucleotides from their 5’ or 3’ ends.

Exonuclease III An E.coil enzyme that removes nucleotides from the 3’-hydroxyl ends of double-stranded DNA. Also referred to as exo III, exodeoxyribonuclease III.

Exonuclease VII An exonuclease that catalyses the cleavage of short oligonucleotides from both the 3’ and 5’ ends of single-stranded DNA.

Exophthalmos Protrusion of the eye.

Exoplasmic face The face of the cell membrane that is directed away from the cytoplasm. While the exoplasmic face of the plasma membrane faces the exterior of the cell, the exoplasmic face of organelles such as the endoplasmic reticulum faces the lumen.

Exopolysaccharide  A high molecular weight polymer composed of sugar residues that is secreted by a microorganism into the surrounding environment.
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Exothermic A reaction that releases heat. The enthalpy of the products is lower than the enthalpy of the reactants.

Exotoxin A toxin released by a bacterium into the surrounding medium.

Expansins Plant cell wall proteins essential for growth. At low pH, these induce breakdown of hydrogen bonds between cellulose microfibrils. As a result, the plant cell wall becomes less rigid allowing cell growth to take place.

EXPASY (Expert Protein Analysis System) Amolecular biology server, maintained by the Swiss Institute of Bioinfomatics (SIB) that provides links, databases and software resources for the analysis of protein sequences.

Expectation maximization An algorithm for locating similar sequence patterns in a set of sequence. A guessed alignment of the sequence is initially used to produce an expected scoring matrix that represents the distribution of sequence characters in each column of alignment. This pattern is matched for each sequence and scoring matrix values are updated to maximize the alignment of matrix to the sequence. This is repeated until no further improvement is made.

Explant A plant part aseptically excised and prepared for culture in a nutrient medium.

Explant donor The source plant or mother plant from which the explants and animals for analysing their growth and development in appropriate culture media.

Exponential phase The stage of growth in which cells undergo their maximum rate of cell division. This phase follows the lag phase, and precedes the linear growth phase. Also known as logarithmic phase. See growth phases.

Export Transport of a protein out of a cell. Also to secrete, to excrete.