U (1) uracil; (2) uranium.
ubidecarenone Coenzyme Q10.
ubiquinone /yoo-BEEK-kwin-n&$333;n/ n. Coenzyme Q10.
UDP Uridine diphosphate. MOLECULAR STRUCTURE
ulna /ƏL-nə/ n. The larger of the two bones of the forearm; it lies on the side opposite the thumb. PICTURE1 |
ulnocarpal /ƏL-nō-CARP-əl/ adj. Relating to the ulna and carpus.
ulocace /oo-LAWK-ə-see/ n. Ulcerating inflammation of the gums.
ulorrhea /oo-lor-REE-ə/ n. Bleeding from the gums.
ultraviolet (UV) light /ƏL-trə-VIE-lət/ n. Rays beyond the visible spectrum, emitted by ionized gases and hot bodies such as the sun, with wavelengths between 1800 and 3900 Å. UV-B (wavelength 3200 Å-2800 Å) and UV-C (wavelength 2800 Å-1000 Å) radiation both damage DNA and are therefore detrimental to most organisms. Sunburn is a reaction to DNA damage caused by UV light.
umbel /ƏM-bəl/ n. An inflorescence in which the flower stalks (pedicels) radiate from a common point. PICTURE OF CARROT UMBEL
Online Biology Dictionary (UMBILI-)
umbilicus /əm-BILL-ik-əs/ n. The navel.
umbilical cord /əm-BILL-ik-əl/ n. The hoselike structure connecting the placenta to the embryo or fetus; contains the umbilical artery and vein. It is severed at birth. The navel (or umbilicus) marks the point of its former attachment. PICTURE
UMP Uridine monophosphate. MOLECULAR STRUCTURE
undulipodium (pl undulipodia) /ƏN-dyə-lə-PODE-ee-əm/ n. A movable, whiplike extension of the plasma membrane of certain eukaryotic cells, such as spermatozoa. An undulipodium contains cytoplasm and a ropelike structure composed of microtubules known as an axoneme. Where the undulipodium attaches to the cell there is a basal body called the kinetosome which is connected to the axoneme with motor proteins. Undulipodia function in moving the cell, the organism to which the cell belongs, or fluid in contact with the cell. The flagella of prokaryotes are similar in appearance and function to the undulipodia of eukaryotes, but differ from them with respect to structure.
ungulate /ƏN-gyə-lət/ n. A hoofed mammal
Online Biology Dictionary (UNI-)
unicellular /yoon-ə-SEL-yə-ler/ adj. Single-celled (said of an organism, such as a bacterium).
uninucleate /yoon-ə-N(Y)OOK-lee-ət/ adj. Having a single nucleus.
uniparental inheritance /yoon-ə-pə-RENT-əl/ n. See: cytoplasmic trait.
unsaturated fatty acid /ən-SATCH-yur-ayt-ed/ adj. A fatty acid with at least one double bond joining the carbons in its hydrocarbon tail. PICTURE 1 | PICTURE 2
Online Biology Dictionary (URA-)
uracil /YER-ə-sill/ n. One of the four nucleotide bases, a pyrimidine, found in RNA (another pyrimidine, thymine, replaces uracil in DNA). PICTURE
uranium (U) /yer-RAIN-ee-əm/ n. A weakly radioactive chemical element; atomic weight 238.02891; atomic number 92. There are three isotopes: U-238, U-235, and U-234. Uranium decays slowly by emitting an alpha particle. The half-life of U-238 is about 4.47 billion years and that of U-235 is 704 million years, which make them useful for dating geological strata (the methods used for this purpose are uranium-thorium dating, uranium-lead dating, and uranium-uranium dating). PERIODIC TABLE
urea /yer-EE-ə/ n. The diamide of carbonic acid. A soluble nitrogenous waste; found in blood and lymph. It is the primary nitrogenous component of urine in mammals and amphibians. MOLECULAR STRUCTURE
urea cycle See: ornithine cycle.
ureter /YER-ə-ter or yer-EE-ter/ n. Either of the tubes leading from the kidneys to the bladder. The internal surface (lumen) of the human ureter is star-shaped (see figure).
urethra /yə-REE-thrə or yer-EE-thra/ n. The tube leading urine from the bladder to the exterior of the body.
uric acid /YER-ik/ n. The ultimate product of nucleic acid catabolism; main nitrogenous component of urine in birds and reptiles.
uridine /YER-ə-deen/ n. A nucleoside, uracil attached to a ribose ring.
urinary /YER-in-air-ee/ adj. Of, pertaining to, containing, or producing urine.
urinary bladder /YER-in-air-ee BLAH-der/ n. The muscular reservoir for urine, which it collects from the ureters, and discharges from the body through the urethra. MORE INFORMATION
urine /YER-in/ n. The waste fluid produced by the kidneys and stored in the urinary bladder prior to excretion.
urogenital /yer-ō-JEN-ə-təl/ adj. Of or pertaining to the reproductive and urinary organs.
urology /yer-AWL-ə-jee/ n. The study and treatment of disorders of the urogenital organs.
Ursus /ER-səs/ n. The genus to which the brown bear, Ursus arctos, and the cave bear, Ursus spelaeus belong. Some authorities would include all extant bears other than sloth bears in this genus. MORE INFORMATION
uterus /YOO-ter-əs, YOO-trəs/ n. The organ of gestation, present only in females, commonly known as the womb. It holds and nourishes the developing embryo and fetus.
uterine /YOO-ter-ən, -in/ adj. adj. Of, pertaining to, or in the uterus.
UTP Uridine triphosphate. MOLECULAR STRUCTURE
uvea /OO-vee-ə/ n. The choroid, ciliary body, and iris taken together. DIAGRAM
Online Biology Dictionary (V)
v (1) volt; (2) vision.
vaccinate /VAKS-ə-nate/ v. To treat with a vaccine.
vaccination /vaks-sə-NAY-shən, -sin-/ n. Introduction of an inactive or weakened pathogen into the body in order to stimulate an immune reaction that will allow the body to respond quickly to the pathogen if real infection ever occurs.
vaccine /VAK-seen/ n. A preparation eliciting an immune response when injected into the body. Vaccines contain dead or weakened pathogens.
vacuole /VAK-yoo-ōl/ n. In plant and animal cells, organelles that remove waste and store food. PICTURE
vagina /və-JINE-ə/ n. (1) a mucomembranous, thin-walled passage forming the birth canal; the passageway leading out from the uterus to the exterior of the body; the birth canal; (2) any sheathlike part.
vaginal /VAHJ-jin-əl/ adj. Of or pertaining to the vagina.
valine (V or val) /VAL-een/ n. An essential amino acid. CODONS | MOLECULAR STRUCTURE
variance (s2 also sample variance) n. A measure of the variability of a sample. The variance of a sample, x1, x2, ..., xn, is defined by the formula
s2 = [(x1 - m)2 + (x2 - m)2 + ... + (xn - m)2]/(N-1),
where m is the sample mean. The variance is the square of the standard deviation.
varves In geological formations, annual alternating layers caused by seasonal variation. MORE INFORMATION
Online Biology Dictionary (VAS)
vas (pl vasa) n. A duct or tube within a living organism.
vascular /VASK-yə-ler/ adj. Containing or composed of conductive tubes, arteries, or veins — vascularized /VASK-yə-ler-īzd/ adj.
vascular cylinder /VASK-yə-ler/ n. The central core of the vascular tissue in a plant root. It is surrounded by parenchymal ground tissue.
vascular plants /VASK-yə-ler/ n. Plants with vascular tissue (i.e., all plants other than mosses and their relatives).
vascular tissue /VASK-yə-ler/ n. Plant tissue, composed of phloem and xylem, that functions in fluid conduction.
vas deferens /vas DEF-er-əns/ n. The sperm-bearing tube leading from the epididymis to the urethra. PICTURE
vasectomy /və-SEK-tə-mee/ n. An operation severing the vas deferens for the purpose of contraception.
vector /VEK-ter/ n. (1) an animal that transmits disease from an infected individual to some other, uninfected individual (2) cloning vector.
vegetative reproduction /VEG-ə-TATE-iv/ n. Reproduction by fragmentation or budding. MORE INFORMATION
vein A thin-walled vessel carrying blood toward the heart. ILLUSTRATION OF VENOUS SYSTEM
venom /VEN-əm/ n. A toxin produced by an animal; usually injected by a bite or sting — venomous /VEN-əm-əs/ adj. poisonous, having a poisonous bite or sting.
ventral /VEHN-trəl/ adj. (1) of or pertaining to the belly; (2) in humans: toward the front; (3) in animals: toward the underside or belly.
ventrolateral /VEHN-trəl/ adj. (1) in humans: toward, on, or in the anterior and sides; (2) in animals: toward, on, or in the belly and sides.
venules /VEHN-yools/ n. The small vessels that connect veins with capillaries.
Online Biology Dictionary (VERMI-)
vermiform appendix /VERM-ə-form/ n. A blind, fingerlike tube of unknown function attached to the lower end of the human cecum. PICTURE
vernalization /vern-əl-eye-ZAY-shən/ n. Cooling seeds to cause them to germinate early.
Online Biology Dictionary (VERTEBR-)
vertebrae (also vertebras, sing vertebra) /VERT-ə-bree or -bray, sing: VERT-ə-brə/ n. The bones of which the spinal column is composed. CRANIAL VIEW OF HUMAN VERTEBRA | LATERAL VIEW OF HUMAN VERTEBRA
vertebral canal /VERT-ə-brəl/ n. The spinal cavity.
vertebrate /VERT-ə-brət/ n. A chordate animal with a backbone; a member of the subphylum Vertebrata (includes mammals, birds, reptiles, amphibians, and fishes).
vesicle /VES-ə-kəl/ n. (1) a small cavity with thin walls; (2) a thin, usually fluid-filled, sac; (3) a blister; (4) a hollow, membrane-bound body that encloses and transports macromolecules into and out of eukaryotic cells, and between organelles within such cells.
vesicular /və-SICK-yə-ler/ adj. adj. Pertaining to a vesicle.
vestibule /VEST-ə-byool/ n. n. Anatomy: An enlarged region at the entrance of a passage, canal, organ or cavity — vestibular /vest-TIB-yə-ler/
vestibule of the ear /VEST-ə-byool/ n. In human beings: the middle part of the internal ear. The vestibule lies between the cochlea and the semicircular canals.
Online Biology Dictionary (VES-)
vessel /VES-əl/ n. A fluid-conducting tube or duct within a living organism.
vessel elements Wide cells that join end-to-end to form water-carrying tubes in angiosperms.
veterinary science /VET-er-in-air-ee, Brit: VET-in-ree/ n. Animal medicine.
Vibrio /VIB-ree-&$333;/ n. A genus of bacteria including the causative agents of cholera (Vibrio comma) and brucellosis (Vibrio fetus).
Online Biology Dictionary (VILLI)
villi (sing villus) /VIL-ee, VIL-əs/ n. Short, filamentous processes that occur on certain membranous surfaces such as the interior of the intestines or the surface of the chorion. PICTURE
villiform /VIL-ə-form/ adj. Shaped like a villus.
villosity /vil-LOSS-ə-tee/ n. (1) the condition of having villi; (2) a localized region covered with villi.
Online Biology Dictionary (VIRAL)
viral /VIRE-əl/ adj. Of, or pertaining to, viruses.
viroids /Vire-oids/ n. Tiny RNA plant pathogens, usually about 300 nucleotides long; thousands of times smaller than the smallest virus.
virology /vire-AWL-ə-jee/ n. The study of viruses.
virus /VIRE-əs/ n. An ultramicroscopic, noncellular parasite that can reproduce only within a host cell (DIAGRAM OF VIRAL REPLICATON). Viruses consist of nucleic acid covered by protein; some animal viruses are also surrounded by membrane. Inside the infected cell, the virus uses the translational system of the host to produce progeny virus. Viruses are tiny. They only carry enough genetic material to encode a few proteins. Many viruses, such as poliovirus and rhinovirus, carry the bare minimum — just enough to specify their own structure and get synthesis started once they get inside cells.
viscera /VISS-er-ə/ n. (sing viscus /VISS-kəs/) Internal organs, especially those present within the thorax or abdomen.
visceral /VISS-er-əl/ adj. Of or pertaining to the visera.
visceroptosis /viss-er-awp-TŌ-səs/ n. Downward displacement of a bodily organ.
vitamin /VITE-ə-mən/ n. Any of a wide variety of chemical substances required by the body's metabolism, but that cannot be synthesized by the body.
vitreous /VIT-ree-əs/ adj. (1) glasslike, glassy, transparent; (2) pertaining to the vitreous body.
vitreous body /VIT-ree-əs/ n. The gelatinous, transparent mass that fills the eyeball. It is enclosed in the hyaline membrane. MORE INFORMATION
vitreous chamber /VIT-ree-əs/ n. The portion of the ocular cavity lying behind the lens.
Online Biology Dictionary (VIVI-)
vivisect /VIV-e-sect/ v. To dissect a living animal — n. vivisection
viviparous /vye-VIP-er-əs/ adj. Giving live birth as the ordinary mode of reproduction.
VLDL Very low-density lipoprotein.
vola /VŌ-lə/ n. The palm of the hand or sole of the foot — vola manus palm of the hand — vola pedis sole of the foot.
vomit /VAH-mit, -mət/ (1) (v.) the ejection of stomach contents through the mouth and nose; (2) (n.) material ejected from the stomach through the mouth and nose; vomitus.
vomitus /VAH-mə-təs/ n. Material ejected from the stomach through the mouth and nose.
Online Biology Dictionary (W)
W chromosome n. See: sex chromosomes.
water bears n. Common name for tardigrades.
western blot n. A technique used to identify and locate proteins based on their ability to bind to specific antibodies.
whip scorpion n. Any arachnid belonging to the order Thelyphonida. MORE INFORMATION
whip spider n. Any arachnid belonging to the order Amblypygi. MORE INFORMATION
white blood cell n. Cells present in blood, also known as leukocytes, that play a role in the immune system. MORE INFORMATION
wild type n. In genetics, the type that normally occurs.
wood n. A hard, fibrous tissue found in many plants. It is composed of cellulose fibers (which resists tension) and lignin (which withstands compression).
woody Composed of, or being plant tissue composed of, cells containing large amounts of lignin.
Online Biology Dictionary (X)
X chromosome n. See: sex chromosomes.
xanthochroia /ZAN-tho-KRŌ-yə/ n. Yellowing of the skin.
xanthoderma /ZAN-tho-DERM-ə/ n. Yellowing of the skin.
xanthous /ZAN-thəs/ adj. Yellow.
Online Biology Dictionary (XENO-)
xenograft /ZEE-nō-graft/ n. Tissue or organs from an individual of one type of organism transplanted into or grafted onto an individual of some other type treated as a distinct species. For example, pig heart valves are often transplanted into humans.
xenon (Xe) /ZEE-nawn or ZEN-awn/ n. Chemical element; A noble gas present in the atmosphere; atomic weight 131.30, atomic number 54; colorless, heavy, odorless,largely inert. PERIODIC TABLE
Online Biology Dictionary (XER-)
xeric /ZEAR-ik/ adj. Dry; this term is normally used only in describing environments.
xerophyte /ZEAR-ō-fīt/ n. A plant capable of living under dry (xeric) conditions — xerophytic /zear-ō-FIT-ick/
xiphoid process /ZĪ-foid/ n. An extension of the sternum protruding caudally between the two halves of the rib cage. PICTURE1 | PICTURE2 | PICTURE3
xiphocostal /ZĪ-fō-CAWST-əl/ adj. Concerning the ribs and xiphoid process.
XMP Xanthosine monophosphate.
x-ray diffraction /də-FRAK-shən/ n. A technique used to study the molecular structure of substances. In this method, x-rays are passed through the substance under analysis and then, on the basis of the resulting patterns of diffraction, inferences are made about its structure.
xylem /ZĪ-ləm/ n. The vascular tissue in plants that carries water and minerals upward from the roots. Compare: phloem.
Online Biology Dictionary (Y)
Y chromosome See: sex chromosomes.
YAC /YAK/ n. yeast artificial chromosome.
yeast artificial chromosome (YAC) n. A vector, constructed from yeast DNA, used to clone large DNA fragments. See also: cloning vector, cosmid.
YADH Yeast alcohol dehydrogenase.
yakalo /YAK-ə-lō/ n. A hybrid between yak (Bos grunniens) and American bison (Bison bison).
yolk /yōk/ n. The contents of an ovum, especially the nutritive portion thereof.
yolk /yōk/ n. The contents of an ovum, especially the nutritive portion thereof.
Online Biology Dictionary (Z)
Z chromosome n. See: sex chromosomes.
zebrass /ZEE-brass/ n. A hybrid resulting from hybridization of an ass with a zebra. PICTURE
zebroid /ZEE-broid/ n. (also zorse) A hybrid resulting from hybridization of an horse with a zebra. PICTURE
zinc finger n. See: finger protein.
zone of differentiation /diff-er-rehn-shee-AY-shən/ n. In plant roots, the region where new cells develop into distinct cell types.
zone of elongation /el-lawng-GAY-shən/ n. In plant roots, the region where new cells grow and lengthen before differentiating.
zonula ciliaris Zonule of Zinn.
zonule of Zinn /ZŌN-yool/ n. Ligament supporting the lens of the eye (syn. zonula ciliaris).
zonulitis /zōn-yə-LĪT-əs/ n. Inflammation of the zonule of Zinn.
Online Biology Dictionary (ZOO-)
zoochore /ZŌ-ə-kore/ n. Any plant spread by animals.
zoochorous /ZŌ-ə-KORE-əs/ adj. Being (a plant) of the sort spread by animals.
zoochory /ZŌ-ə-kor-ee/ n. The geographic dispersion of plants by animals.
zoogeography /zō-ə-jee-AWG-grə-phee/ n. The branch of biogeography concerned with the geographic distribution of animals.
zoology /zō-AWL-ə-jee/ n. The scientific study of animals — zoologist /zō-AWL-ə-jist, zoo-AWL-/ n. — zoological /zō-ə-LODGE-ik-əl, zoo-ə-/
zoonosis /zō-AWN-ə-səs/ n. An infectious disease transmissible from a non-human animal to humans — zoonotic /zō-ə-NAWT-ik/
zoophagous /zō-AWF-ə-gəs/ adj. Eating animals as a diet.
zoophyte /ZŌ-ə-fĪt/ n. A plant with animal-like characteristics.
zooplankton /zō-ə-PLANK-tən/ n. Plankton composed of animal life. Compare: phytoplankton
zoosterol /zō-AWST-ə-rawl/ n. Any animal sterol.
zorse n. A hybrid resulting from hybridization of a horse with a zebra. PICTURE
Online Biology Dictionary (ZYGO)
zygodactyl /ZĪ-gō-DAK-əl/ n. Said of birds: Having two digits of the foot directed forward and two backward, as in parrots.
zygodactyly /ZĪ-gō-DAK-til-ee/ n. (1) congenital fusion of fingers; (2) of, pertaining, to or being zygodactyl.
zygoma (also zygomatic arch or zygomatic process) /zī-GŌ-mə/ The bridge of bone connecting the malar and temporal bones and forming the lateral region of the orbit. PICTURE
zygomatic /ZĪ-gō-MAT-ick/ adj. Of or pertaining to the zygoma.
Zygomycetes /ZĪ-gə-mī-SEED-eez/ n. A class of fungi including the bread molds. Its members produce zygospores.
zygospore /ZĪ-gə-spor/ n. A structure in fungi that forms from a diploid zygote that results from fusion of two hyphae of different mating types. The zygospore forms sporangia, in which meiosis takes place and spores form.
zygote /ZĪ-gōt/ n. A fertilized egg, produced by the union of a male and female gamete. A zygote goes on to develop into a multicellular organism by repeated cell division (mitosis).
Online Biology Dictionary (ZYMO-)
zymogene /ZĪ-mō-jeen/ n. A microorganism causing fermentation; a zymophyte.
zymogenic /zī-mō-JEN-ik/ adj. The scientific study of fermentation.
zymology /zī-MALL-ə-gee/ n. Producing fermentation.
Zymonema /zī-mō-NEEM-ə/ n. A former name of the fungus genus Histoplasma. The fungus Histoplasma capsulatum is the causative agent of histoplasmosis.
zymophyte /ZĪ-mō-fīt/ n. A microorganism causing fermentation; a zymogene.
zymotic /zī-MAWT-ick/ adj. Of or relating to fermentation.