U (1) Uracil, a pyrimidine base in RNA; pairs with adenine (substituting for thymine in DNA). (2) Uridine, the nucleoside having uracil as it base.
UFSee ureaformaldehyde.
ultracentrifuge An instrument in which rotor speeds of 65,000 to 100,000 revolutions per minute are attained, creating forces of up to 420,000 times that of gravity (g). Preparative ultracentrifuges are used to separate and purify cells, subcellular organelles and particles, or molecules according to either size or density. Analytical ultracentrifuges are used to measure directly the position of the sample in the rotor chamber while the rotor is in motion.
unbalanced diallelic The genotype involving a multiple allelic locus in autotetraploid where two alleles are represented an unequal number of times (e.g., a1a2a2a2).
undifferentiated (1) Strictly, cells that are determined but not yet expressing cell specialization. (2) May also refer to prodetermined cells. Usage: Not a synonym for unorganized (unorganized tissues often contain various specialized cells).
uniformity index The ratio between the two fibrograph span lengths (e.g., of cotton) expressed as a percentage of the longer length.
upgrading The reprocessing of a seed lot to remove low quality seeds and/or other materials so that the remaining seed are higher in quality than the original.
upper-quartile length That length which is exceeded by 25% of the fibers, by weight, in an array test specimen (e.g., of cotton).
upstream In the 5' direction of the sense strand relative to a given reference point. Compare downstream.
URF Uninterrupted reading frame. See open reading frame.
utilized metabolizable energy The amount of metabolizable energy (ME) grown as forage that is eaten by grazing animals. Usage: May be expressed either per animal on a daily basis (megajoules per head, MJ head-1) or per unit area over a given time period (gigajoules per hectare, GJ ha-1). It is a measure of output from a forage system based on the estimated energy requirement of animals and the energy value of forage. It is calculated as the sum of ME requirements for maintenance and production of the classes of livestock involved, less the ME value of all feed supplements provided from outside the area and augmented by the ME value of any conserved herbage made on the area but unused. Abbr.: UME.
UTP Uridine 5'-triphosphate.