U

Ubiquitin:

Ubiquitin is a small protein. It marks a target protein for degradation by attaching covalently to the target. Ubiquitinated protein is degraded by proteasome.

URF:

It is an open reading frame, presumed to code for protein, but for which no product has been found.

Unequal Crossing Over:

It describes a recombination event in which the two recombining sites lie at non identical locations in the two parental DNA molecules.

Unidirectional Replication:

It refers to the movement of a single replication fork from a given origin.

Uninducible Mutants:

Mutants that cannot be induced.

Underwinding:

Underwinding of DNA is produced by negative supercoiling.

Universal Genetic Code:

The "standard" codon usage that is common to most organisms.

Up Promoter Mutations:

Mutataions that increases the frequency of initiation of transcription.

Upstream Activator Sequence:

A binding site for transcription factors, generally part of a promoter region. A UAS may be found upstream of the TATA sequence (if there is one), and its function is (like an enhancer) to increase transcription. Unlike an enhancer, it can not be positioned just anywhere or in any orientation.

Upstream/Downstream:

In an RNA, anything towards the 5' end of a reference point is "upstream" of that point. This orientation reflects the direction of both the synthesis of mRNA, and its translation - from the 5' end to the 3' end. In DNA, the situation is a bit more complicated. In the vicinity of a gene (or in a cDNA), the DNA has two strands, but one strand is virtually a duplicate of the RNA, so it's 5' and 3' ends determine upstream and downstream, respectively.

Untranscribed Region:

Genomic sequences that are not transcribed to primary transcript. Untranscribed regions may contain important regulatory elements such as promoter, enhancer, centromere, etc.

Untranslated Region (UTR):

Regions of mature RNA that do not code for proteins, including 5'-UTR and 3'-UTR. UTR contains information for regulation of translation and mRNA stability.

Uracil:

A nitrogenous base normally found in RNA but not DNA; uracil is capable of forming a base pair with adenine.