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- U -

U  The uracil residue in RNA.

ultrasonication  See sonication.

ultraviolet light; ultraviolet radiation (UV)  The portion of the electromagnetic spectrum with wavelengths from about 100 to 400 nm; between ionizing radiation (X-rays) and visible light. UV is absorbed by DNA and is highly mutagenic to unicellular organisms and to the epidermal cells of multicellular organisms. UV light is used in tissue culture for its mutagenic and bactericidal properties.

undefined  A medium or substance added to medium in which not all of the constituents or their concentrations are chemically defined, such as media containing coconut milk, malt extract, casein hydrolysate, fish emulsion or other complex compounds. cf organic complex.

understock  Host plant for a grafted scion, a branch or shoot from another plant; an understock may be a fully grown tree or a stump with a living root system.

undifferentiated  In a meristematic state or resembling a meristem; lacking the specialized or differential gene expression characteristic of specialized cells.

unequal crossing over  Crossing over between repeated DNA sequences that have paired out of register, creating duplicated and deficient products.

unicellular  Describing tissues, organs or organisms consisting of a single cell.

unisexual  Describing animals and plants possessing either male or female reproductive organs, but not both.

univalent  An unpaired chromosome at meiosis.

universal donor cells  Cells that, after introduction into a recipient, will not induce an immune response that leads to their rejection.

universality  Referring to the genetic code, the codons have the same meaning, with minor exceptions, in virtually all species.

unorganized growth  In vitro formation of tissues with few differentiated cell types and no recognizable structure; many call are unorganized. cf organized growth.

upstream processing  See upstream (2).

upstream  1. In molecular biology, the stretch of DNA base pairs that lie in the 5´ direction from the site of initiation of transcription. Usually the first transcribed base is designated +1 and the upstream nucleotides are marked with minus signs, e.g., -1, -10. Also, to the 5´ side of a particular gene or sequence of nucleotides.

uracil  A pyrimidine base found in RNA but not in DNA. In DNA, uracil is replaced by thymine.

utilization of farm animal genetic resources  In AnGR: The use and development of animal genetic resources for the production of food and agriculture. (Source: FAO, 1999)

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