On-Line Biology Book: GLOSSARY

R

race  A subdivision of a species that is capable of interbreeding with other members of the species.

radially symmetrical  In animals, refers to organisms with their body parts arranged around a central axis. Such animals tend to be circular or cylindrical in shape.

radiation  Energy emitted from the unstable nuclei of radioactive isotopes.

radioactive decay The spontaneous decay of an atom to an atom of a different element by emission of a particle from its nucleus (alpha and beta decay) or by electron capture.

radioisotope Term applied to a radioactive isotope, such as carbon-14 or uranium 238. Radioisotope nuclei are unstable and spontaneously breakdown and emit one of a number of types of radiation.

radiometric time Type of absolute time determined by the relative porportions of radioisotopes to stable daughter isotopes.

ray-finned  Taxonomic group of fish, such as trout, tuna, salmon, and bass, that have thin, bony supports holding the Þns away from the body and an internal swim bladder that changes the buoyancy of the body; one of the two main types of bony Þshes.

reabsorption  The return to the blood of most of the water, sodium, amino acids, and sugar that were removed during Þltration; occurs mainly in the proximal tubule of the nephron.

receptacle  The base that attaches a þower to the stem. PICTURE

receptor Protein on or protruding from the cell surface to which select chemicals can bind. The opiate receptor in brain cells allows both the natural chemical as well as foreign (opiate) chemicals to bind.

recessive  Refers to an allele of a gene that is expressed when the dominant allele is not present. An allele expressed only in homozygous form, when the dominant allele is absent.

recombinant DNA molecules  New combinations of DNA fragments formed by cutting DNA segments from two sources with restriction enzyme and then joining the fragments together with DNA ligase. Interspecies transfer of genes usually through a vector such as a virus or plasmid.

recombinant DNA technology  A series of techniques in which DNA fragments are linked to self-replicating forms of DNA to create recombinant DNA molecules. These molecules in turn are replicated in a host cell to create clones of the inserted segments.

recombination  A way in which meiosis produces new combinations of genetic inform