Search for a word or term

Word / termDefinition


A small molecule that latches onto damaged or mutated proteins in the cell. Ubiquitin acts as a flag indicating that these proteins can be destroyed by proteosomes.

Ubiquitin-proteasome system (UPS)

A cellular quality control system that tags misfolded proteins for refolding or degradation.


A break in the skin or in the mucous membrane lining the alimentary tract that fails to heal and is often accompanied by inflammation.

Ulcerative colitis

A relatively common disease that causes inflammation of the large intestine (the colon). The cause is unknown.

Ultimate causation

The hypothetical evolutionary explanation for the existence of a certain pattern of animal behavior.

Ultrahigh molecular weight polyethylene - UHMWPE

A type of highly durable polyethylene used for replacement of joint surfaces or as entire components in prosthetic joints.

Ultrasonic / Ultrasound

Term used to describe a vibrating wave of a frequency above that of the upper frequency limit of the human ear. Generally includes all frequencies above 16 or 20,000 Hertz (kilocycles/second).

Ultrasonography (ultrasound)

A procedure using high frequencxy sound waves to visualize internal organs.  Primarily used to visualize abdominal and pelvic organs, such as the pregnant uterus.


The detailed structure of a specimen, such as a cell, tissue, or organ, that can be observed only by electron microscopy.

Ultraviolet (UV) radiation

Electromagnetic radiation in the region of 200 to 400 nm.


In addition to the four main taste components (sweet, sour, salty and bitter), there is the additional taste characteristic called "umami" or savory. One of the food components responsible for the umami flavour in foods is glutamate, an amino acid.

Umbilical cord blood stem cells

Stem cells collected from the umbilical cord at birth that can produce all of the blood cells in the body (hematopoietic).Cord blood is currently used to treat patients who have undergone chemotherapy to destroy their bone marrow due to cancer or other blood-related disorders.

Unassigned reading frame (URF)

An open reading frame (ORF) whose function has not yet been determined.

Uncertainty Principle (Heisenberg’s Uncertainity Principle)

A necessary consequence of the quantum mechanical description of matter. Quantum mechanics informs us that it is impossible to know exactly where something is and where it is going. Unlike in classical mechanics where the position of a large object in time and space (including its speed) can exactly be described and predicted based on measurements (Newton's laws of gravity, actio and reactio), the position and speed of individual subatomic particles like electrons, and thus the behavior of individual molecular interaction, can neither be measured nor predicted with certainty, yet perfectly described by statistical methods when studied in large numbers. This is a fundamental law of nature.


A condition in which the phenotypic expression of the heterozygote is less than that of either homozygote.

Undifferentiated cell

A cell that has not yet developed into a specialized cell type.

Undifferentiated schizophrenia

A condition manifested by definite signs of schizophrenic thought, affect and behavior that are of a sufficiently mixed or indefinite type that they defy classification into one of the other types of schizophrenia.


Another term for a eukaryotic flagellum.

Unequal crossing over

Non-reciprocal crossing over caused by mismatching of homologous chromosomes.
Usually occurs in regions of repetitive DNA.


Consisting of but a single cell.


Having, or relating to, one side. Unilateral is as opposed, for example, to bilateral (which means having, or relating to, two sides).

Unilateral Neglect

Paying little or no attention to things on one side of the body.
This usually occurs on the side opposite from the location of the injury to the brain because nerve fibers from the brain typically cross before innervating body structures. In extreme cases, the patient may not bathe, dress or acknowledge one side of the body.

Uninemic chromosome

A chromosome consisting of one double helix of DNA.


The union of two sets, A and B, is the set of all elements that are either in A, or in B, or in both A and B.

Unipolar prosthesis

A prosthesis used for hemiarthroplasties with no across the joint articulating component. Hip and shoulder prostheses are often unipolar. A unipolar device can be of monoblock or modular design. See bipolar prosthesis.

Unique DNA

A length of DNA with no repetitive DNA sequences.

Unique Radiolytic Products (URPs)

Chemicals produced in food when the food is irradiated that are different from chemicals produced during cooking.

Unit vector

A vector of length 1. It is customary to designate i as the unit vector in the x direction, j as the unit vector in the y direction, and k is the unit vector in the z direction.

Universal (or Ideal) Gas Law

PV=nRT where P=pressure; V=volume; n=number of moles of gas; R=gas constant, 0.0821 L atm/mol K; and T=temperature in Kelvins

Unmixed codon family

Group of four codons sharing their first two bases and coding for the same amino acid.

Unsaturated fatty acid

A fatty acid possessing one or more double bonds between the carbons in the hydrocarbon tail. Such bonding reduces the number of hydrogen atoms attached to the carbon skeleton.

Unsaturated hydrocarbon

A hydrocarbon molecule containing one or more double or triple bonds, and can thus absorb more hydrogen atoms.

Unsaturated zone

The zone immediately below the land surface where the pores contain both water and air, but are not totally saturated with water. These zones differ from an aquifer, where the pores are saturated with water.

Unspecified mental retardation

Mental retardation whose condition has no known cause.

Unstable mutation

A mutation that has a high frequency of reversion.  A mutation caused by the insertion of a controlling element, whose subsequent exit produces a reversion.

Unstructured scale

A line with start and end points which is used for measuring the intensity of a stimulus or sensation or product.

Untranslated region (UTR)

5' UTR is the portion of an mRNA from the 5' end to the position of the first codon used in translation. The 3' UTR is the portion of an mRNA from the 3' end of the mRNA to the position of the last codon used in translation.

Unusual bases

Other bases in addition to the normal adenine; cytosine; guanine; and uracil.
Found primarily in tRNAs and produced by post transcription modification of one of the normal bases.

Upper motoneurons

Those motorneurons that both directly and indirectly innervate the lower motorneurons of the brainstem and spinal cord.  (see Lower motoneuronsUpper motoneuron syndrome)

Upper motoneuron syndrome

A group of symptoms resulting from removal of supraspinal influences on spinal mechanisms.Damage to the UMN and its caudal projection results in muscle weakness, increased muscle tone, increased tendon reflexes but no muscle wasting; more specifically in spastic paralysis or paresis, increase muscle tone, increase reflex response (sometimes clonus, oscillating contraction following muscle stretch), Babinski sign.
(see Lower motoneurons, Upper motoneuron syndrome)

Upper respiratory infection (URI)

An infection of the ears, throat, and tonsils. Usually caused by a cold.

Upstream (in DNA)

A convention on DNA related to the position and direction of transcription by RNA polymerase (5'>3'). In the vicinity of a gene (or in a cDNA), the DNA has two strands, but one strand is virtually a duplicate of the RNA, so it's 5' and 3' ends determine upstream and downstream, respectively. Downstream (or 3' to) is in the direction of transcription whereas upstream (5' to) is in the direction from which the polymerase has come.
In genomic DNA, two adjacent genes may be on different strands and thus oriented in opposite directions. Upstream or downstream is only used on conjunction with a given gene.

Upstream (in RNA)

Anything towards the 5' end of a reference point is "upstream" of that point.
This orientation reflects the direction of both the synthesis of mRNA, and its translation - from the 5' end to the 3' end.

Upstream activator sequence (UAS)

A binding site for transcription factors, generally part of a promoter region.
A UAS may be found upstream of the TATA sequence (if there is one), and its function is (like an enhancer) to increase transcription. Unlike an enhancer, it can not be positioned just anywhere or in any orientation.


A nitrogenous base normally found in RNA but not DNA.
A pyrimidine base and one of four nitrogenous bases found in ribonucleic acid (RNA). It is part of UTP, the triphosphorylated nucleotide. The other three bases found in RNA are adenine, guanine, and cytosine.Uracil is capable of forming a base pair with adenine


A soluble form of nitrogenous waste excreted by mammals and most adult amphibians.


A duct leading from the kidney to the urinary bladder.


A tube that releases urine from the body near the vagina in females or through the penis in males; also serves in males as the exit tube for the reproductive system.


Inflammation of the urethra.

Uric acid

An insoluble precipitate of nitrogenous waste excreted by land snails, insects, birds, and some reptiles.

Urinary Tract Infection

When bacteria have reproduced to a large number in the bladder.
This can cause fever, chills, burning on urination, urgency, frequency, incontinence or foul smelling urine.


The liquid waste filtered from the blood by the kidney and stored in the bladder pending elimination through the urethra.


Universal Resource Identifier. URLs are the most familiar type of URI. A URI defines or specifies an entity, not necessarily by naming its location on the Web.


Uniform Resource Locator. The familiar codes (such as http://www.sciam.com/) that are used in hyperlinks.


The sagging of a standard gel under defined conditions.
Internationally recognized measure for the gel strength of high methoxyl pectins.


A female reproductive organ where eggs are fertilized and/or development of the young occurs.

UV radiation

Ultraviolet radiation, an invisible, high energy component of sunlight can cause skin damage including cancer.