r abbr. Correlation coefficient.
R Symbol for Arginine.
R abbr. Gas constant = 8.314 472 J K−1 mol−1.
R5P abbr. Ribose-5-phosphate.
Ra Chemical symbol for radium.
rabid (also rabic) /RAB-id, -əd/ Pertaining to or affected by rabies.
rabies /RĀ-beez/ An infectious disease, commonly known as hydrophobia), primarily affecting mammalian carnivores (particularly canids and felids). Rabies virus (Lyssavirus), which is present in the saliva of infected individuals, is communicated to new hosts through bites. The virus travels from the wound to the central nervous system where it causes paralysis and eventual death, usually by respiratory failure. Until recently, once symptoms set in, this disease had no cure. However, a new treatment, known as the Milwaukee protocol, allows some patients to survive.
raceme /rə-SEEM/ An inflorescence with the flowers borne on short stalks (pedicles) of about equal length arising from an axial rachis.
Online Biology Dictionary (RAD-)
radical /RAD-ə-kəl/ A group of atoms viewed as a single unit, that can pass unchanged from one molecule to another, but that are not stable as independent entities.
radicle /RAD-ə-kəl/ The root of a plant embryo. The radicle is usually the first structure to emerge during germination.
radiation /RĀD-ee-Ā-shən/ (1) energy emitted in the form of waves or ionizing particles (2) adaptive radiation.
Online Biology Dictionary (RADIO-)
radioactive /RĀD-ee-ō-ACT-iv/ Emitting radiation such as alpha particles, beta particles, and gamma photons.
radiobiology /RĀD-ee-ō-bī-AWL-ə-jee/ The study of the effects of radiation on living living organisms.
radiocarpal /RĀD-ee-ō-bī-CARP-əl/ Pertaining to the radius and the carpus.
radiogenic element /RAYD-ee-ō-GEHN-ik/ An element produced by the radioactive decay of some other element.
radiograph /RĀD-ee-ō-graf/ An image made on photographic film by ionizing radiation, for example an x-ray image of some portion of a patient's body.
radiohumeral /RĀD-ee-ō-HYOOM-er-əl/ Pertaining to the radius and the humerus.
radioimmunoassay (RIA) /RĀD-ee-ō-im-MYOON-ō-ASS-ā/ A very sensitive, extremely specific technique for measuring antigen concentrations. For example, it allows exact measurement of tiny amounts of a hormone, such as insulin. To carry out a radioimmunoassay, an antigen is made radioactive, often with radioactive iodine attached to tyrosine. A measured quantity of this radiolabeled antigen and a known amount of antibody to that antigen are then mixed, and the two bind together. Subsequently, in a separate assay, a serum sample containing an unknown concentration of the same antigen (not radiolabeled) is added. The unlabeled antigen then competes with the radiolabeled antigen for antibody-binding sites. The proportion of antibody bound to unlabeled antigen can then be determined.
radioisotope /RĀD-ee-ō-Ī-sə-tōp/ A radioactive isotope.
radioulnar /RĀD-ee-ō-ƏL-ner/ Pertaining to the radius and the ulna.
radius /RĀD-ee-əs/ The smaller of the two bones of the forearm; it lies on the same side as the thumb. PICTURE
radula (pl radulae) /RAHD-jə-lə/ A rough appendage present in many mollusks. Used for scraping, it has minute chitinous teeth. PICTURE
ramet /RĀ-mət/ An individual member of a clonal line of individuals. Compare: ortet.
Ranidae /RAN-ee-dī, RAN-ee-dee/ The anuran family comprised of the true frogs (smooth, moist-skinned frogs, with extensively webbed feet, and strong hindlegs). PICTURE
Online Biology Dictionary (RANDOM)
random genetic drift Term used to refer to changes, from one generation to the next, in the population frequency of an allele or trait due to random deviation from the population frequencies expected on the basis of pre-existing frequencies. Such deviations can be quite large in small populations.
random mating (also panmixia and panmixis) In a population, mating in which all potential reproductive pairings between individuals occur with equal likelihood. Compare: assortative mating.
rare-cutter enzyme See: restriction site.
rats Rodents belonging to the genus Rattus. Rats transmit a wide variety of diseases to human beings. DISEASES CARRIED BY RATS
Online Biology Dictionary (RBC)
RBC Red blood corpuscle.
receptor /rə-SEPT-er/ A protein, either embedded in the plasma membrane (cell-surface receptor) or present the cytoplasm of a cell, to which one or more specific types of molecules can attach.
recessive allele /ə-LEEL/ In a diploid organism, an allele manifested only when two copies of it are present at a locus.
reciprocal translocation A condition (see figure at right) in which two non-homologous chromosomes trade blocks of chromatin (the process produces two new composite chromosomes each of which contain blocks from both of the pre-existing chromosomes).
Online Biology Dictionary (RECOMB-)
recombinant DNA technology In vitro techniques for combining DNA segments from distinct sources into a single DNA molecule.
recombination The occurrence of progeny having combinations of traits different from the combinations seen in the parents (due to crossing-over and independent segregation of chromosomes).
rectum /REK-təm/ A short (~12 cm), upright portion of the large intestine. It connects the sigmoid colon with the anal canal. PICTURE
regenerate /rə-JEHN-er-āt/ To form again (esp a body part by the formation of new tissue) — regeneration /rə-jehn-er-RĀ-shən/ — regenerative /rə-JEHN-er-ə-tiv/
regulatory sequence A DNA sequence controling gene expression.
regurgitate /rə-GER-jə-tāt/ To return stomach contents to the mouth.
regurgitation /rə-ger-jə-TĀ-shən/ (1) The return of stomach contents to the mouth; (2) passage of blood back through a heart valve.
Online Biology Dictionary (RENAL)
renal /REEN-əl/ Pertaining to the kidneys.
renal pelvis See: kidneys.
Online Biology Dictionary (REP-)
repetitive DNA Nucleotide sequences that occur repeatedly in a single genome.
replicated chromosome A chromosome that has undergone replication and that is therefore composed of two sister chromatids.
|Diagram of DNA Replication|
replication /REHP-lə-KĀ-shən/ The creation of a duplicate molecule from a template. During the replication of DNA the two strands of a duplex DNA molecule are first unzipped by helicase and topoisomerase enzymes to form a replication fork (see figure right), then each of the separated ("parental") strands is used as a template for the synthesis of a new strand. Single-strand binding proteins bind the separated strands to prevent them from re-annealing. On the leading parental strand (see figure) DNA polymerase proceeds continuously producing the new leading strand. Because the lagging replication complex (see figure) proceeds away from the fork, it produces the new lagging strand in a piecewise fashion; it must wait for the replication fork to open further before each of the new pieces (known as "Okazawi fragments" after their discoverer R. Okazawi) are synthesized. SEE AN ANIMATION OF DNA REPLICATION
replication fork See replication.
reproductive cycle (also life cycle) The series of forms that a particular type of organism takes on during the course of reproduction. For example, the human reproductive cycle involves two forms, a haploid gamete alternating with a diploid organism. Each form produces the other.
reptile /REP-tile/ An animal, such as a snake, lizard, or alligator belonging to the vertebrate class Reptilia. Reptiles differ from amphibians in having an amnion, a feature held in common with mammals and birds. Due to this trait, oviparous reptiles can lay their eggs on dry land — reptilian /rep-TILL-ee-ən/.
RER Rough endoplasmic reticulum.
resistance /rə-ZIST-əns/ An enhanced ability to survive exposure to, or avoid the adverse effects of, a toxic substance or pathogenic organism — resistant /rə-ZIST-ənt/.
respiration /RESP-er-RĀ-shən/ (1) the act of breathing; (2) the oxidation of energy-rich storage molecules, primarily glucose, to produce ATP. MORE INFORMATION
Online Biology Dictionary (RESTR-)
restriction endonucleases (also restriction enzymes) /END-ō-N(Y)OOK-lee-īz/ Enzymes that make cuts between the ends of a DNA or RNA polymer chain (exonucleases remove nucleotides from the ends of such chains). They cut the internal phosphodiester bonds of the molecule. Each type of restriction enzyme recognizes a specific sequence of nucleotides, 4 to 8 base pairs in length (the restriction site), and will cut only where that sequence occurs. Restriction endonucleases are one of the basic tools of recombinant DNA technology. ABOUT THE DISCOVERY OF RESTRICTION ENDONUCLEASES
restriction enzyme cutting site See: restriction site.
restriction fragment length polymorphism (RFLP) Variation between individuals in the sizes of the DNA fragments cut by specific restriction enzymes; polymorphic nucleotide sequences that result in RFLPs are used as markers on both physical maps and genetic linkage maps. RFLPs usually are caused by point mutation at a cutting site.
restriction site (short for restriction enzyme cutting site) A specific nucleotide sequence where a particular restriction enzyme cuts DNA or RNA. The restriction site of a "rare cutter enzyme" is itself rare in comparison with other nucleotide sequences of the same length.
reticulum /rə-TICK-yə-ləm/ The second stomach chamber of a ruminant. MORE INFORMATION
retina /RET-in-ə/ The interior, light-detecting coat of the eye — retinal /RET-in-əl/ MORE INFORMATION
Online Biology Dictionary (RETRO-)
retrotransposon /REH-trō-trans-PŌS-awn/ A retrovirus that is passed hereditarily because it is permanently inserted in the host genome.
retrovirus /REH-trō-VīR-əs/ An RNA virus that uses reverse transcriptase to create a DNA copy of itself. After reverse transcription, the DNA copy is inserted into the host genome and used as a template for the production of additional RNA copies of the virus
Online Biology Dictionary (REVERSE)
reverse mutation See: back mutation.
reverse transcriptase (RT) also RNA-dependent DNA polymerase /tran-SKRIP-tayz/ The enzyme used by retroviruses to create DNA copies of their RNA genomes, an essential step in their cycle of replication. MORE INFORMATION
RF (1) replicative form; (2) release factor.
RFLP See: restriction fragment length polymorphism.
Online Biology Dictionary (RHABDO-)
rhabdomyoblasts /RAB-doe-MY-ō-blasts/ Neoplastic ceils resembling the precursor cells of striated muscle.
rhabdomyoma /RAB-doe-my-Ō-mə/ A benign tumor of striated muscle.
rhabdomyosarcoma (RMS) /RAB-doe-MY-ō-sark-Ō-mə/ A sarcoma arising from rhabdomyoblasts.
rhamnose (Rha) /RAM-nōs, -nōz/ A deoxy sugar. MOLECULAR STRUCTURE
Rhincodon /RINK-ə-dawn/ The genus of the whale shark (Rhincodon typus), the world's largest fish. PICTURE
rhizome /RĪ-zōme/ An underground stem, produced by many plants.
rhodoplast /RŌD-ə-plast/ The plastid giving red algae its characteristic color.
rhodopsin /rŌ-DAWP-sin/ (also retinal purple) A pigment, sensitive to red light, present in the optic rod cells of the retina.
Online Biology Dictionary (RIBO-)
riboflavin /RĪ-bə-FLĀV-ən/ A water-soluble vitamin of the B complex group; essential for tissue repair; unstable in light.
ribonucleic acid (RNA) /RĪB-ō-n(y)oo-KLĀ-ək/ Any member of a class of single-stranded polynucleotides containing ribose sugar and the pyrimidine uracil. Molecules of this type play a variety of important roles within the cell. See: messenger RNA, ribosomal RNA, transfer RNA.
ribonucleotide See: nucleotide.
ribose /RĪ-bōs, -bōz/ The five-carbon monosaccharide component of RNA; occurs primarily as D-ribose. MOLECULAR STRUCTURE
ribose-5-phosphate /RĪ-bōs, -bōz/ An intermediate of the pentose phosphate pathway; molecular formula C₅H₁₁O₈P.
riboside /RĪB-ō-sīd/ See: nucleoside.
Online Biology Dictionary (RIBOSOM-)
ribosomal RNA (rRNA) /RĪB-ə-SŌM-əl/ The most abundant type of RNA. With ribosomal proteins, rRNAs form the structure of ribosomes.
ribosomes /RĪB-ə-sōm/ Cellular organelles, present in both prokaryotes and eukaryotes, that use mRNA templates to synthesize proteins, a process known as translation. See also: rRNA.
Rickettsia /rə-KEHT-see-yə/ A class of microorganisms often described as intermediate between viruses and bacteria; differ from bacteria in being obligate parasites (cannot grow in the absence of living cells); differ from viruses because they are retained in a Berkefeld filter.
right splicing junction (also acceptor splicing site) Boundary between an intron's right end and the adjacent exon's left end.
Ringer's solution (also Ringer solution) A solution containing calcium, potassium, and sodium chlorides; used in hospitals to alleviate dehydration and improve circulation, and in laboratories, to maintain cells and organs alive in vitro.
ringworm See: dermatophytosis.
RK HMG-CoA reductase kinase.
RT reverse transcriptase.
RKK Reductase kinase kinase.
Online Biology Dictionary (RNA)
RNA See: ribonucleic acid.
RNA polymerase /pə-LIM-ə-rayze/ An enzyme that catalyzes the synthesis of RNA polymers. To do this, it reads one strand (the template strand) of a double-stranded DNA molecule. This process is called transcription.
RNA virus A virus composed of RNA, not DNA.
Online Biology Dictionary (ROOT)
root pressure Upward pressure of water within vascular plants caused by root cells pumping water into the plant.
rRNA Ribosomal RNA.
RSV Rous sarcoma virus.
rumen /ROOM-ən/ The first stomach chamber of a ruminant. MORE INFORMATION
ruminant /ROOM-ə-nənt/ A mammal that chews cud. MORE INFORMATION