R factor (resistance factor) • A plasmid that contains one or more genes that encode resistance to antibiotics.

R gene • Resistance gene that functions in plant defenses against bacteria, fungi, and nematodes.

R group • The distinguishing group of atoms of a particular amino acid.

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.

Reactant • A chemical substance that enters into a chemical reaction with another substance.

Reaction • A chemical change in which changes take place in the kind, number, or position of atoms making up a substance.

Reaction center • A group of electron transfer proteins that receive energy from light-absorbing pigments and convert it to chemical energy by redox reactions.

Receptacle • The end of a plant stem to which the parts of the flower are attached.

Receptive field • Of a neuron, the area on the retina from which the activity of that neuron can be influenced.

Receptor • A site or protein on the outer surface of the plasma membrane or in the cytoplasm to which a specific ligand from another cell binds.

Receptor-mediated endocytosis • Endocytosis initiated by macromolecular binding to a specific membrane receptor.

Receptor potential • The change in the resting potential of a sensory cell when it is stimulated.

Recessive • In genetics, an allele that does not determine phenotype in the presence of a dominant allele. Contrast with dominance.

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 • See restriction site.

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 segregation, or by crossing over between homologous chromosomes.

Recombinant DNA • DNA generated in vitro, from more than one source.

Recombinant DNA technology • The application of restriction endonucleases, plasmids, and transformation to alter and assemble recombinant DNA, with the goal of producing specific proteins.

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 • Gain of electrons by a chemical reactant; any reduction is accompanied by an oxidation. (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 codes for a protein that controls the transcription of another gene(s).

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 • Pertaining to the duplication of genetic material.

Replication complex • The close association of several proteins operating in the replication of DNA.

Replication fork • A point at which a DNA molecule is replicating. The fork forms by the unwinding of the parent molecule.

Reporter gene • Marker genes included in recombinant DNA to indicate the presence of the recombinant DNA in a host cell.

Repressible enzyme • An enzyme whose synthesis can be decreased or prevented by the presence of a particular compound. A repressible operon often controls the synthesis 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.

Rescue effect • The process by which a few individuals moving among declining subpopulations of a species and reproducing may prevent their extinction.

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.

Respiration (res pi ra´ shun) [L. spirare: to breathe] • (1) Cellular respiration; the catabolic pathways by which electrons are removed from various molecules and passed through intermediate electron carriers to O₂, generating H₂O and releasing energy. (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.)

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 fragment length polymorphism • See RFLP.

Restriction site • A specific DNA base sequence recognized and acted on by a restriction endonuclease cutting the DNA.

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) • 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.

Risk cost • The increased chance of being injured or killed as a result of performing a behavior, compared to resting.

RNA (ribonucleic acid) • An often single stranded nucleic acid whose nucleotides use ribose rather than deoxyribose and in which the base uracil replaces thymine found in DNA. Serves as genome from some viruses. (See rRNA, tRNA, mRNA, and ribozyme.)

RNA editing • The alteration of bases on mRNA prior to its translation.

RNA polymerase • An enzyme that catalyzes the formation of RNA from a DNA template.

RNA primase • A replication complex enzyme that makes the primer strand of DNA needed to initiate DNA replication.

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 • The organ responsible for anchoring the plant in the soil, absorbing water and minerals, and producing certain hormones. Some roots are storage organs.

Root cap • A thimble-shaped mass of cells, produced by the root apical meristem, that protects the meristem; the organ that perceives the gravitational stimulus in root gravitropism.

Root hair • A long, thin process from a root epidermal cell that absorbs water and minerals from the soil solution.

Rough ER • That portion of the endoplasmic reticulum whose outer surface has attached ribosomes. Compare with smooth ER.

rRNA • See ribosomal RNA.

Rubisco (Ribulose bisphosphate carboxylase/oxygenase) • Acronym for the enzyme that combines carbon dioxide or oxygen with ribulose bisphosphate to catalyze the first step of the Calvin-Benson cycle.

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.