Encyclopedia of Biology

the fractional values within this range denoting PARTIAL
AGONISTs. Ariëns’s original definition equates the
molecular nature of alpha to maximal response only
when response is a linear function of receptor occupan-
cy. This function has been verified. Thus, intrinsic
activity, which is a DRUGand tissue parameter, cannot
be used as a characteristic drug parameter for classifi-
cation of drugs or drug receptors. For this purpose, a
proportionality factor derived by null methods, namely,
relative EFFICACY, should be used. Finally, “intrinsic
activity” should not be used instead of “intrinsic effica-
cy.” A “partial agonist” should be termed “agonist
with intermediate intrinsic efficacy” in a given tissue.

intrinsic rate of increase (rmax) A mathematical
parameter that measures the maximum rate at which a
population will grow if resources are unlimited, using

birth and death rates in a population as the determi-

nants. It is the difference between the number of births

and the number of deaths; the maximum population

growth rate.

introgression The movement of genes from one pop-

ulation into another through hybridization followed by

backcrossing.

intron An intervening section of DNA that occurs

almost exclusively within a eukaryotic GENE, but which

is not translated to amino acid SEQUENCEs in the gene

product. The introns are removed from the premature

mRNA through a process called splicing, which leaves

the EXONs untouched, to form an active mRNA.

See alsoEUKARYOTE;MESSENGER RNA;TRANSLATION.

180 intrinsic rate of increase

Exon Intron Exon

Gene

DNA mRNA tRNA amino

acid

polypeptide

chain

transcription translation

Gene

Exon

Exon

ribosome

Intron

A C G C G A T A G G T G A A G T G T A C T G A A

U G C G C U A U C A C A U G A C U U

A C G C G A U A G U G U A C U G A A

Anintron is a noncoding sequence of DNA that is initially copied into RNA but is cut out of the final RNA transcript.(Courtesy of Darryl
Leja, NHGRI, National Institutes of Health)