Unscheduled DNA synthesis is any DNA synthesis occurring outside the S phase of the euikaryotic cell.
Up promoter mutations increase the frequency of initiation of transcription.
Upstream identifies sequences proceeding in the opposite direction from expression; for example, the bacterial promoter is upstream from the transcription unit, the initiation codon is upstream of the coding region.
URF is an open (unidentified) reading frame, presumed to code for protein, but for which no product has been found.

V gene is sequence coding for the major part of the variable (N-termainal) region of an immunoglobulin chain.
Variable region of an immunoglobulin chain is conded by the V gene and varies extensively when different chains are compared, as the result of multiple (different) genomic copies and changes introduced during construction of an active immunoglobulin.
Variegation of phenotype is produced by a change in genotype during somatic development.
Vector—see cloning vector.
Vesicles are small bodies bounded by membrane, derived by budding from one membrane, often able to fuse with another membrane.
Virion is the physical virus particle (irrespective of its ability to infect cells and reproduce).
Virulent phage mutants are unable to establish lysogeny.

Wobble hypotheis accounts for the ability of a tRNA to recognize more than one condon by unusual (non-G•C,A•T) pairing with the third base of a codon.
Writhing number is the number of times a duplx axis crosses over itself in space.

Zero time-binding DNA enters the duplex form at the start of a reassociation reaction; results from intramolecular reassociation of inverted repeats.
Zinc finger protein has a repeated motif of amino acids with characteristic spacing of cysteines that may be invoved in binding ainc; is characteristic of some proteins that bind DNA and /or RNA.
Zoo blot describes the use of Southern blotting to test the ability of a DNA probe from one species to hybridize with the DNA from the genomes of a variety of other species.
Zygote is produced by fusion of two gametes—that is, it is a fertilized egg.