Receptor A molecule or a polymeric structure in or on a cell that specifically recognizes and binds a compound acting as a molecular messenger (neurotransmit-ter, hormone, lymphokine, lectin, drug, etc.).
Receptor mapping The technique used to describe the geometric and/or electronic features of a binding site when insufficient structural data for this receptor or enzyme are available. Generally the active-site cavity is defined by comparing the superposition of active molecules with that of inactive molecules.
Receptor-mediated endocytosis Cells use receptor-mediated endocytosis—a method where specific molecules are ingested into the cell—for ingestion of nutrients, hormones, and growth factors. The specificity results from a receptor-ligand (a molecule or ion that can bind another molecule) interaction. other ligands that can be ingested include toxins and lectins, viruses, and serum transport proteins and antibodies. A receptor, a specific binding protein such as clathrin, on the plasma membrane of the target tissue will specifically bind to ligands on the outside of the cell. An endocytotic process results, i.e., the cell folds inward with a portion of the plasma membrane, and the resulting clathrin-coated pit is pinched off to form a membrane-enclosed bubble or vesicle, called an endosome. After entering the cytoplasm, the endocytotic vesicle loses its clathrin coat, and the ligand (multiple ligands can enter the cell in the same coated pit) is ingested. The receptor can be recycled to the surface by vesicles that bud from the endosome targeting the plasma membrane. After these recycling vesicles fuse with the plasma membrane, the receptor is returned to the cell surface for binding and activity once more.
Receptor potential (end-plate potential) A change in a neuron’s membrane potential (a change in voltage across the receptor membrane) caused by redistribution of ions responding to the strength of the stimulus. If the potential is high enough, an action potential will be fired in an afferent neuron. The more action potentials fired, the more neurotransmitters released, and stronger the signals reaching the brain.
Recessive allele An allele that is not expressed phenotypically in a heterozygote due to the presence of a dominant or masking allele. However, it will be expressed in a specific phenotype when a counterpart recessive gene is present. However, often when individuals inherit two mutant copies of a gene on one of the autosomes, they suffer from autosomal recessive disorders. Examples like hemophilia occur where the recessive gene associated with it lies on part of the X chromosome (X-linked recessive disorder). Most genetic mutations produce recessive alleles.
Recessive disorder A disorder associated with a recessive allele.
Reciprocal altruism The belief that if one acts kindly toward another unrelated individual, that individual will be inclined to perform the same act back or to another individual. The individual giving does not necessarily expect anything back, only that the kindly act will be repeated and benefit society as a whole.
A recessive allele is expressed phenotypically only in offspring who have received two copies of the recessive gene, one from each parent.
Recognition species concept A concept of species as a set of individuals that recognize each other as potential mates; having a common or shared system of mate recognition, i.e., courtship displays, that ensures mating with members of the same population.
Recombinant An individual where the genotype is produced by the nonparental arrangement of alleles that results from independent assortment or crossing over (recombination); the phenotype individual is different from the parents; also, a term describing a new combination of genes that form DNA.
Recombinant DNA Altered DNA that has been joined, mostly by in vitro means, by genetic material from two different sources.
Recombinant DNA technology Refers to the modern techniques of gene cloning.
Redox potential Any oxidation-reduction (redox) reaction can be divided into two half reactions: one in which a chemical species undergoes oxidation, and one in which another chemical species undergoes reduction. If a half-reaction is written as a reduction, the driving force is the reduction potential. If the half-reaction is written as oxidation, the driving force is the oxidation potential related to the reduction potential by a sign change. So the redox potential is the reduction/oxidation potential of a compound measured under standard conditions against a standard reference half-cell. In biological systems, the standard redox potential is defined at pH = 7.0 versus the hydrogen electrode and partial pressure of hydrogen = 1 bar.
Redox reaction An abbreviated term for an oxidation-reduction reaction; a reaction that has both reduction (electrons are gained) and oxidation (electrons are lost) occurring; one does not occur without the other.
Reducing agent The reactant that donates its electrons and in turn becomes oxidized when another substance is reduced.
Reduction The part of a redox reaction where the reactant has a net gain of electrons and in which a different reactant must oxidize (lose electrons). See also oxidation.
Reductive elimination The reverse of oxidative addition.
Reductive speciation Complete fusion of two previously independent evolutionary species—a hybridization/intergradation phenomenon.
Reflex An active response to a stimulus that is usually involuntary and reproducible; many types exist from simple to complex. A common example is the kick response when a doctor hits a knee with a small hammer.
Refractory period At the end of the action potential, the absolute refractory period (ARP) is the interval immediately following the discharge of a nerve impulse (action potential) during which the cell cannot be induced to fire again. A relative refractory period is the time following the ARP where a neuron can be induced to discharge again but only if there is a more intense stimulus than normal.
Regulation Refers to control of activity of an enzyme (system) or gene expression.
Regulative development (indeterminate development) A development pattern in which cell fates are not determined until late in development; depends on interactions with neighboring cells. For example, in mice gastrulation, if the majority of embryonic cells are destroyed, a normal mouse can still develop from the remainder.
Reichstein, Tadeus (1897-1996) Swiss Chemist Tadeus Reichstein was born on July 20, 1897, at Wlo-clawek, Poland, to Isidor Reichstein and Gastava Brockmann. He was educated at a boarding school at Jena after his family moved to Zurich in 1906 (where he was naturalized). He had a private tutor and then attended the Oberrealschule (technical school of junior college grade) and the Eidgenossische Technische Hochschule (E.T.H.) (state technical college).
In 1916 he began to study chemistry at the E.T.H. at Zurich and graduated in 1920. In 1922 he began research on the composition of the flavoring substances in roasted coffee, a project that lasted for nine years.
In 1931 he turned to other scientific research, and by 1938 he was professor in pharmaceutical chemistry and director of the pharmaceutical institute in the University of Basel. From 1946 to 1967 he was professor of organic chemistry at the University of Basel.
In 1933 he synthesized vitamin C (ascorbic acid) and worked on plant glycosides. From 1953 to 1954 he worked with several other scientists and was the first to isolate and explain the constitution of aldo-sterone, a hormone of the adrenal cortex. He also collaborated with E. C. kendall and P. S. hench in their work on the hormones of the adrenal cortex. For this work, Reichstein, Kendall, and Hench were jointly awarded the Nobel Prize in physiology or medicine in 1950.
After 1967 he worked on the study of ferns and published many papers on the subject. He died on August 1, 1996, in Basel.
Relapsing fever A tropical disease associated with some 20 species of the bacteria genus Borrelia that is transmitted to humans by two vectors, soft ticks (Ornithodoros or Argas) or, in the case of Borrelia recurrentis, by lice. Louse-borne relapsing fever is more severe than the tick-borne variety.
Relative configuration The configuration of any stereogenic (asymmetric) center with respect to any other stereogenic center contained within the same molecular entity. A stereogenic unit is a grouping within a molecular entity that can be considered a focus of stereoisomerism.
Relative fitness One genotype contributes to the next generation. If the population is stable, i.e., neither increasing nor decreasing, then the genotype with a relative fitness of more than one will increase in frequency, whereas if the genotype has a relative fitness of less than one, it will decrease.
Relaxation If a system is disturbed from its state of equilibrium, it returns to that state, and the process is referred to as relaxation.
Releaser A signal stimulus that functions as a communication signal between individuals of the same species and initiates a fixed action pattern (FAP), a stereotyped species-to-species behavior.
Reniform Kidney shaped, such as a kidney bean.
Repetitive DNA Repeated DNA sequences that may occur in the thousands of copies in the chromosomes of eukaryotes; represents much of the human genome. These sequences of variable length can be repeated up to 100,000 (middle repetitive) or over 100,000 (highly repetitive) copies per genome. Much of the DNA in eukaryotes is repetitive.
Replication fork The Y-shaped portion of the replicating DNA where new strands are growing.
Repressible enzyme An enzyme whose synthesis is inhibited or regulated by a regulatory molecule, a specific metabolite.
Repressor A protein that prevents gene transcription; prevents RNA polymerase from commencing mRNA synthesis.
Reptilia The class of vertebrate animals that includes snakes, turtles, lizards, tuatara, crocodilians, and extinct fossil species. Reptiles have scales or modified scales, breathe air, are cold blooded, and usually lay eggs. The reptiles evolved from amphibians during the late Carboniferous or early Permian periods. Today there are about 2,500 species of snakes, 3,000 of lizards, 250 of turtles and tortoises, and 21 species of crocodilians and distributed worldwide throughout temperate and tropical regions.
A closeup of the head of a ribbonsnake, a member of the Reptilia class.
Reservoir host The host, a vertebrate, that harbors a particular parasite and acts as a long-term source of infection of other vertebrates or vectors.
Resistance The ability of an organism to resist microorganisms or toxins produced in disease.
Resolving power A property of instruments, like microscopes and telescopes, that distinguish objects that are close to each other; the smaller the minimum distance at which two objects can be distinguished, the greater the resolving power.
The process by which the DNA double helix unwinds and makes an exact copy of itself.
Resonance Raman spectroscopy A spectroscopic technique increasingly used in bioinorganic chemistry for characterization and assignment of vibrations directly connected with a chromophore, as well for the assignment of the chromophore. The excitation frequency is applied close to the absorption maximum of the chro-mophore. Particularly useful for deeply colored species.
Resource partitioning The process whereby coexisting species living in the same ecosystem each find a separate niche so that resources can be divided up and used without having to compete. often a few dominant species are able to exploit most of the resources, and the rest divide up the remainder.
Respiration The process where mitochondria in the cells of plants and animals release chemical energy from sugar and other organic molecules through chemical oxidation.
Respiratory system The respiratory system is the system by which oxygen, essential for life, is taken into the body and the waste product, carbon dioxide, is expelled from the body. The respiratory system consists of the mouth and nose, airways, and lungs. Air enters through the mouth and nose and passes down the pharynx (throat) and through the larynx (voice box). Air then continues down through the trachea (windpipe), which branches into two bronchi (singular: bronchus) to each of the two lungs. The inflammation of the bronchus is called bronchitis. The bronchi branch many times until becoming much smaller airways called "bronchioles." At the end of each bronchiole are tiny air-filled cavities called alveoli. Each alveolus is surrounded by many blood capillaries, which allow oxygen to move into the bloodstream and carbon dioxide out. This exchange of substances is the primary function of the respiratory system.
Resting potential The state of a neuron’s charge, the gradient of electric potential across the membrane, when it is in a resting state and ready to receive a nerve impulse (the action potential); usually consists of a negative charge on the inside of the cell relative to the outside. At rest, the cell membrane electrical gradient maintains a negative interior charge of -70 mv.
