- Two objects considered together because of similarity, for a common purpose, or because of some attracting force between them.- base p. (b.p.) the complex of two heterocyclic nucleic acid bases, one a pyrimidine and the other a purine, brought about by hydrogen bonding between the purine and the pyrimidine; base pairing is the essential element in the structure of DNA proposed by J. Watson and F. Crick in 1953; usually guanine is paired with cytosine (G C), and adenine with thymine (A T) or uracil (A U). SYN: nucleoside p., nucleotide p..- chromosome p. two chromosomes of the full diploid karyotype that are similar in form and function but that usually differ in content, one normally being inherited from each parent and one being transmitted to each progeny; in the heteromorphic sex (in humans, the male), one p., the sex chromosomes, differ markedly in appearance, content, and function.- conjugate acid-base p. in prototonic solvents ( e.g., H2O, NH3, acetic acid), two molecular species differing only in the presence or absence of a hydrogen ion ( e.g., carbonic acid /bicarbonate ion or ammonium ion/ammonia); the basis of buffer action.- line pairs a unit of resolution of radiographic screens and films or photographic films; greatest number of line pairs per cm that can be resolved.- p. production creation of a positron and electron, each of mass 0.511 MeV, when an incident photon of energy greater than 1.02 MeV is absorbed by matter; occurs in high-energy radiotherapy.
* * *(pār) 1. a combination of two related, similar, or identical entities or objects. 2. in cardiology, two successive premature beats, particularly two ventricular premature complexes. Called also couplet.
Medical dictionary. 2011.