R factor (resistance factor) • A plasmid that contains one or more genes that encode resistance to antibiotics.
Radial symmetry • The condition in which two halves of a body are mirror images of each other regardless of the angle of the cut, providing the cut is made along the center line. Thus, a cylinder cut lengthwise down its center displays this form of symmetry. (Contrast with biradial symmetry.)
Radioisotope • A radioactive isotope of an element. Examples are carbon-14 (14C) and hydrogen-3, or tritium (3H).
Radiometry • The use of the regular, known rates of decay of radioisotopes of elements to determine dates of events in the distant past.
Rain shadow • A region of low precipitation on the leeward side of a mountain range.
Ramet • The repeated morphological units of sessile, modular organisms. (Contrast with genet.)
Random genetic drift • Evolution (change in gene proportions) by chance processes alone.
Rate constant • Of a particular chemical reaction, a constant which, when multiplied by the concentration(s) of reactant(s), gives the rate of the reaction.
Reactant • A chemical substance that enters into a chemical reaction with another substance.
Reaction, chemical • A process in which atoms combine or change bonding partners.
Realized niche • The actual niche occupied by an organism; it differs from the fundamental niche because of the presence of other species.
Receptive field • Of a neuron, the area on the retina from which the activity of that neuron can be influenced.
Receptor potential • The change in the resting potential of a sensory cell when it is stimulated.
Recessive • See dominance.
Reciprocal altruism • The exchange of altruistic acts between two or more individuals. The acts may be separated considerably in time.
Reciprocal crosses • A pair of crosses, in one of which a female of genotype A mates with a male of genotype B and in the other of which a female of genotype B mates with a male of genotype A.
Recognition site (also called a restriction site) • A sequence of nucleotides in DNA to which a restriction enzyme binds and then cuts the DNA.
Recombinant • An individual, meiotic product, or single chromosome in which genetic materials originally present in two individuals end up in the same haploid complement of genes. The reshuffling of genes can be either by independent segragation, or by crossing over between homologous chromosomes. For example, a human may pass on genes from both parents in a single haploid gamete.
Recombinant DNA technology • The application of genetic tools (restriction endonucleases, plasmids, and transformation) to the production of specific proteins by biological "factories" such as bacteria.
Rectum • The terminal portion of the gut, ending at the anus.
Redox reaction • A chemical reaction in which one reactant becomes oxidized and the other becomes reduced.
Reducing agent • A substance that can donate electrons to another substance. The reducing agent becomes oxidized, and its partner becomes reduced.
Reduction (re duk´ shun) • Gain of electrons; the reverse of oxidation. Most reductions lead to the storage of chemical energy, which can be released later by an oxidation reaction. Energy storage compounds such as sugars and fats are highly reduced compounds. (Contrast with oxidation.)
Reflex • An automatic action, involving only a few neurons (in vertebrates, often in the spinal cord), in which a motor response swiftly follows a sensory stimulus.
Refractory period • Of a neuron, the time interval after an action potential, during which another action potential cannot be elicited.
Regulative development • A pattern of animal embryonic development in which the fates of the first blastomeres are not absolutely fixed. (Contrast with mosaic development.)
Regulatory gene • A gene that contains the information for making a regulatory macromolecule, often a repressor protein.
Releaser • A sensory stimulus that triggers a fixed action pattern.
Releasing hormone • One of several hypothalamic hormones that stimulates the secretion of anterior pituitary hormone.
REM sleep • A sleep state characterized by dreaming, skeletal muscle relaxation, and rapid eye movements.
Renal [L. renes: kidneys] • Relating to the kidneys.
Replication fork • A point at which a DNA molecule is replicating. The fork forms by the unwinding of the parent molecule.
Repressible enzyme • An enzyme whose synthesis can be decreased or prevented by the presence of a particular compound. A repressible opren often controls the syhthesis of such an enzyme.
Repressor • A protein coded by the regulatory gene. The repressor can bind to a specific operator and prevent transcription of the operon.
Reproductive isolating mechanism • Any trait that prevents individuals from two different populations from producing fertile hybrids.
Reproductive isolation • The condition in which a population is not exchanging genes with other populations of the same species.
Resolving power • Of an optical device such as a microscope, the smallest distance between two lines that allows the lines to be seen as separate from one another.
Resource • Something in the environment required by an organism for its maintenance and growth that is consumed in the process of being used.
Resource defense polygamy • A breeding system in which individuals of one sex (usually males) defend resources that are attractive to individuals of the other sex (usually females); individuals holding better resources attract more mates.
Respiration (res pi ra´ shun) [L. spirare: to breathe] • (1) Cellular respiration; the oxidation of the end products of glycolysis with the storage of much energy in ATP. The oxidant in the respiration of eukaryotes is oxygen gas. Some bacteria can use nitrate or sulfate instead of O2. (2) Breathing.
Respiratory chain • The terminal reactions of cellular respiration, in which electrons are passed from NAD or FAD, through a series of intermediate carriers, to molecular oxygen, with the concomitant production of ATP.
Resting potential • The membrane potential of a living cell at rest. In cells at rest, the interior is negative to the exterior. (Contrast with action potential, electrotonic potential.)
Restoration ecology • The science and practice of restoring damaged or degraded ecosystems.
Restriction endonuclease • Any one of several enzymes, produced by bacteria, that break foreign DNA molecules at very specific sites. Some produce "sticky ends." Extensively used in recombinant DNA technology.
Restriction map • A partial genetic map of a DNA molecule, showing the points at which particular restriction endonuclease recognition sites reside.
Reticular system • A central region of the vertebrate brain stem that includes complex fiber tracts conveying neural signals between the forebrain and the spinal cord, with collateral fibers to a variety of nuclei that are involved in autonomic functions, including arousal from sleep.
Retina (rett´ in uh) [L. rete: net] • The light-sensitive layer of cells in the vertebrate or cephalopod eye.
Retinal • The light-absorbing portion of visual pigment molecules. Derived from
b-carotene.
Retrovirus • An RNA virus that contains reverse transcriptase. Its RNA serves as a template for cDNA production, and the cDNA is integrated into a chromosome of the mammalian host cell.
Reverse transcriptase • An enzyme that catalyzes the production of DNA (cDNA), using RNA as a template; essential to the reproduction of retroviruses.
RFLP (Restriction fragment length polymorphism) • Coexistence of two or more patterns of restriction fragments (patterns produced by restriction enzymes), as revealed by a probe. The polymorphism reflects a difference in DNA sequence on homologous chromosomes.
Rhizoids (rye´ zoids) [Gr. rhiza: root] • Hairlike extensions of cells in mosses, liverworts, and a few vascular plants that serve the same function as roots and root hairs in vascular plants. The term is also applied to branched, rootlike extensions of some fungi and algae.
Rhizome (rye´ zome) [Gr. rhizoma: mass of roots] • A special underground stem (as opposed to root) that runs horizontally beneath the ground.
Rhodopsin • A photopigment used in the visual process of transducing photons of light into changes in the membrane potential of photoreceptor cells.
Ribonucleic acid • See RNA.
Ribosomal RNA (rRNA) • Several species of RNA that are incorporated into the ribosome. Involved in peptide bond formation.
Ribosome • A small organelle that is the site of protein synthesis.
Ribozyme • An RNA molecule with catalytic activity.
Ribulose 1,5-bisphosphate (RuBP) • The compound in chloroplasts which reacts with carbon dioxide in the first reaction of the Calvin-Benson cycle.
Risk cost • The increased chance of being injured or killed as a result of performing a behavior, compared to resting.
RNA (ribonucleic acid) • A nucleic acid using ribose. Various classes of RNA are involved in the transcription and translation of genetic information. RNA serves as the genetic storage material in some viruses.
RNA polymerase • An enzyme that catalyzes the formation of RNA from a DNA template.
RNA splicing • The last stage of RNA processing in eukaryotes, in which the transcripts of introns are excised through the action of small nuclear ribonucleoprotein particles (snRNP).
Rods • Light-sensitive cells (photoreceptors) in the retina. (Contrast with cones.)
Root cap • A thimble-shaped mass of cells, produced by the root apical meristem, that protects the meristem and that is the organ that perceives the gravitational stimulus in root gravitropism.
Root hair • A specialized epidermal cell with a long, thin process that absorbs water and minerals from the soil solution.
rRNA • See ribosomal RNA.
Rubisco (RuBP carboxylase) • Enzyme that combines carbon dioxide with ribulose bisphosphate to produce 3-phosphoglycerate, the first product of C3 photosynthesis. The most abundant protein on Earth.
Rumen (rew´ mun) • The first division of the ruminant stomach. It stores and initiates bacterial fermentation of food. Food is regurgitated from the rumen for further chewing.
Ruminant • An herbivorous, cud-chewing mammal such as a cow, sheep, or deer, having a stomach consisting of four compartments.