- uidA
- The gene encoding Beta-glucuronidase (abbreviated GUS). Commonly used for constructing operon or gene fusions in organisms with high endogenous Beta-galactosidase activity. A variety of analogs exist that make it easy to detect GUS expression on solid medium and to assay the activity of the enzyme in cells.
- Ultraviolet (UV) radiation
- The part of the invivible electromagnetic spectrum (just below violet) with wavelengths between about 100-400 nm.
- Undefined medium
- A growth medium in which not all the components have been identified. Most "rich" media have a variety of complex, undefined componants.
- Unequal cross-over
- A recombination event that occurs between DNA molecules that are not fully aligned. For example, a crossover may occur between repeated DNA sequences resulting in the deletion or duplication the intervening DNA sequence.
- Umber
- The codon UGA. An less commonly used term for an opal codon.
- Unidirectional replication
- DNA replication that proceeds in only one direction along the DNA template.
- Unselected marker
- A genetic marker that may be co-inherited with an adjacent, selected marker but inheritance of the unselected marker is not demanded. For example, in a genetic cross you may select for one gene and screen for inheritance of an adjacent, unselected gene.
- Up promoter mutation
- A mutation that increasses expression from a promoter.
- Upstream
- A sequence located in front of a particular site relative to the direction of transcription and translation (i.e. located in the 5' direction relative to a particular site). For example, the lac promoter is located upstream of the lacZ structural gene.
- Upstream activator sequence (UAS)
- A DNA sequence some distance in front of a gene that increases transcription of the gene. Such sequences often stimulate binding of RNA polymerase to the promoter.
- Uracil-N-glycosylase
- A repair enzyme that removes uracil from DNA by cleaving the bond between the base and the sugar backbone. This enzyme ensures that DNA contains T instead of U residues.
- UV absorbance spectroscopy
- A method for measuring the concentration of a compound by determining the amount of ultraviolet radiation absorbed by a sample.
- UV reactivation
- A phenomenon in which survival of an ultraviolet-irradiated phage is greater upon infecting a host that has also been irradiated with UV than upon infecting a host that has not been irradiated with UV. The increased survival in the UV irradiated host is due to the induction of the SOS-repair system in the host.
- UvrABC
- An enzyme complex that functions as an endonuclease, cutting the DNA on both sides of DNA lesions that distort the double-helix (for example, T-T dimers). The DNA fragment is subsequently excised allowing resynthesis and repairing the DNA damage.