U Selenocysteine in proteins, Uracil in RNA ubiquitin A ubiquitous protein that, when attached to a second
protein, acts as a tag directing that protein for
degradation by the proteosome. uncompress A UNIX command which uncompresses .Z files. Database
files are commonly stored on servers in a compressed format
and must be uncompressed before use. unequal codon usage In protein-coding genes, the disproportionate use of one
or more of the codons in a codon family. unequal crossing over A crossing-over between two homologous sequences at
different chromosomal positions producign an unequal number
of copies of the regions involved in the resulting
chromosomes unequal sister chromatid exchange A recombination event that results in duplication of a
segment of DNA. unidentified reading frame (URF) An ORF, the product of which is not known unique DNA DNA sequences present in only one copy in the haploid
genome universal genetic code The genetic code regulating the translation of the vast
majority of genomes UNIX A computer operating system designed for use on
multiuser, multiprocessor systems. Usually running on SUN,
HP, SGI and other large multiprocessor computers. A version
of UNIX for smaller computer systems is called Linux. unprocessed pseudogene A pseudogene that has been derived through gene
duplication and the subsequent nonfunctionalization or
selencing of one of the copies. unrooted tree An evolutionary tree that specifies neither the root nor
the direction of the evolutionary path. upstream In the direction 5' of a reference point on a nucleic
acid (opposite to the direction of transcription) upstream promoter element Components of a eukaryotic promoter that lie upstream of
the position where the initiation complex is assembled. URF unidentified reading frame