base composition

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base com·po·si·tion

the proportions of the four bases (adenine, cytosine, guanine, and thymine or uracil) present in DNA or RNA; usually expressed as the percentage (mol %) of G plus C.

base composition

The ratio of guanine + cytosine (GC) to adenine + thymine (AT) nucleotides in a nucleic acid; BC is usually expressed as a guanine + cytosine (G+C) value—e.g., 60% G+C. The GC content is of interest given that guanine and cytosine share three hydrogen bonds in double-stranded nucleic acids, compared to the two shared by adenine and thymine; the higher the GC content, the more stable the nucleic acid.


1. the lowest part or foundation of anything. See also basis.
2. the main ingredient of a compound.
3. a molecule or ion with a tendency to take up a proton according to Bronsted and Lowry theory; a substance that combines with acids to form salts. In the chemical processes of the body, bases are essential to the maintenance of a normal acid-base balance. Excessive concentration of bases in the body fluids leads to alkalosis. See also basal.
4. the primary entity against which all other entities are compared.
5. the non-sugar components of nucleotides in DNA and RNA.

acid-base pairs
the two molecules forming the matching acid and conjugate base.
base composition
refers to the relative components of a nucleic acid.
conjugate base
the anion or uncharged molecule of an acid once it has given up its proton, e.g. Cl is the conjugate base of the acid, HCl.
base deficit
see base excess (below).
base excess
the amount of acid or base required to titrate a sample of whole arterial blood to the normal pH of 7.4. The base excess is determined mathematically by calculations that include measurement of the blood Pco2 and pH and take into account the hemoglobin level. It is negative (base deficit) in acidosis and positive in alkalosis.
heart base
the wide dorsal part of the heart carrying the atria and the large blood vessels and the attachment to the pericardial sac.
horn base
the widest part of the horn, at its attachment to the skin. In the adult horned animal the horn is hollow at this point, encloses the horn process of the frontal bone and merges with the skin. This is covered with a thin layer of horn similar to the periople of the hoof, called the epiceras.
narrow base
a mandible which is narrow relative to the maxilla; often causes the lower canine teeth to strike the hard palate. See also anisognathism.
nitrogenous base
an aromatic, nitrogen-containing molecule that serves as a proton acceptor, e.g. purine or pyrimidine.
omasal base
faces cranially and to the left where it is attached to the reticulum and the abomasum at the reticulo-omasal and omasoabomasal orifices.
base pair
two hydrogen bonded nucleotides in a DNA or RNA molecule.
purine b's
a group of compounds of which purine is the base, including uric acid, adenine, guanine, xanthine and theobromine.
pyrimidine b's
a group of chemical compounds of which pyrimidine is the base, including uracil, thymine and cytosine, which are common constituents of nucleic acids.
stapedal base
the footplate of the stapes in the middle ear from which the two legs originate. The stapes lies horizontally with the base facing medially and attached to the vestibular window by the annular ligament.
References in periodicals archive ?
The A:T:C:G base composition of 22 tRNA genes was exhibited in Supplementary Table S2, among which A+T content was the highest in O.
Because this broad-survey assay determines the base composition of a PCR product and not the sequence, it does not conclusively determine the exact position of specific mtDNA variants.
Since the total base composition does not reflect the base composition of the positions that are free to vary, the base composition used in the maximum likelihood calculations was based only on the base frequencies of the variable positions from Table 2.
We use broad-range PCR to generate amplification products from the broadest possible grouping of organisms, followed by electrospray ionization mass spectrometry and base composition analysis of the products (16,17).