Eclipse period
The period of time after infection with a virus during which new virions are formed, but not released (contrast with latent period).

Electrophoresis
Separation of molecules on the basis of their net electric charge.

Electrophoresis of DNA
DNA fragments have a constant charge/length ratio due to the net negative charge of the phosphate backbone. Therefore, DNA migrates toward the (+) electrode. The rate of migration of linear DNA mainly depends on the pore size of the gel and length of the DNA. During electrophoresis DNA molecules seem to "snake" through the pores in the gel "head first". As the pore size decreases (i.e. the agarose or acrylamide concentration increases) it is harder for longer DNA molecules to orient properly to snake through the pores: smaller DNA fragments snake through the pores easier and hence migrate faster. Thus, the rate of migration of linear double stranded DNA is inversely proportional to the log of its molecular weight. By using gels with different concentrations of agarose or acrylamide, a wide range of DNA fragment sizes can be separated. The shape of the DNA molecule also affects its ability to snake through the pores in the gel. In general features that make the DNA less flexible or less compact slow the migration in a gel. The rate of migration of different forms of plasmid DNA is usually: supercoiled > linear > nicked circles. In addition, secondary structure and bends in linear DNA may affect the rate of migration.

Electroporation
A method for transferring DNA (or other small molecules) into cells by exposure to a rapid pulse of high voltage, which causes the transient formation of small pores in the cell membrane.

Elongation factor
A protein that facilitates protein synthesis.

Endonuclease
An enzyme that makes breaks in a molecule of DNA by hydrolyzing internal phosphodiester bonds. An endonuclease may be specific for either single or double stranded DNA or RNA.

Endospore
A metabolically inactive, nonreplicating form of certain bacteria, including the genera Bacillus and Clostridium. Endospores tend to be highly resistant to physical and chemical damage.

Enhancer
A cis-acting regulatory sequence that can increase transcription from an adjacent promoter.

Enrichment
A condition which increases the proportion of a particular organism or mutant strain from a mixed population. For example, a penicillin enrichment kills growing cells and allows mutants unable to grow under a particular condition to survive; however, for each round of penicillin enrichment typically only 1% of the growing cells are killed, thus each round of enrichment increases the mutant ratio about 100-fold. Compare with "selection".

Epigenetic
The inheritance of a particular trait that is not encoded in the nucleotide sequence. For example, methylation of certain DNA sequences can influence gene expression, and the methylation pattern may be maintained following DNA synthesis by preferential methylation of hemimethylated sequences.

Episome
A genetic element that can exist either as an autonomous replicating plasmid or can insert into the bacterial chromosome. This is an old term that is rarely used anymore. Such elements are simply called plasmids (for example, F-prime plasmid).

Epistasis
A condition where one gene masks the expression of another gene. For example, if there are two mutations that affect a single metabolic pathway, a mutation that disrupts the gene product which acts at an earlier step in the pathway will usually obscure the mutation that affects a later step in the pathway.

Epitope
A portion of an antigen recognized by an antibody binding site. For protein antigens the epitope is typically 5-8 amino acids. Gene fusions to epitopes can be used as a tag to recognize specific proteins.

Error prone repair
A mechanism for repair of DNA damage that often results in mutations (e.g. SOS repair).

Ethanol precipitation
Precipitation of nucleic acid molecules by ethanol plus salt. Commonly used to concentrate DNA from aqueous solutions.

Ethidium bromide
A fluorescent chemical that intercalates between base pairs in a doublestranded DNA molecule. Commonly used to detect DNA following gel electrophoresis. Ethidium bromide intercalates between the stacked bases of DNA and RNA. When excited by UV light between 254 nm (short wave) and 366 nm (long wave) it emits fluorescent light at 590 nm. Due to the decreased rotation possible when it is intercalated, the DNA-ethidium bromide complex produces about 50 times more fluorescence than free ethidium bromide. When ethidium bromide stained gels are photographed on a UV transilluminator a UV filter is required to screen out the background UV from the transilluminator. Ethidium bromide is a known carcinogen.

Eucaryote
An organism with a nuclear membrane and membrane bound organelles (e.g. mitochondria), and a mitotic spindle. This group of organisms is more correctly called Eucarya.

Excision repair
A repair system that removes nucleotides from a damaged strand of DNA and then replaces them with a new tract of DNA synthesised using the undamaged complementary strand as a template.

Exon
The sequence in a spliced gene that is retained after removal of the introns to provide the mature mRNA that is translated by the ribosome.

Exonuclease
An enzyme that digests a molecule of nucleic acid by removing successive nucleotides from the 5' or 3' end. Exonucleases may be specific for single or double stranded DNA or RNA, and may be specific for either 3' or 5' ends.

Expression vector
A cloning vector designed so that a foreign gene inserted into the vector will be expressed in the host organism. The inserted DNA is often placed under the control of a strong, regulatable promoter (e.g. the lacZ or araBAD promoter) so that expression of the gene product will only occur when desired.

Extragenic
An affect due to a second gene. For example, an extragenic suppressor is a mutation in a second gene that repairs the mutant phenotype.