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.
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.
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.
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.
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.
radius /RĀD-ee-əs/ The smaller of the two bones of the forearm; it lies on the same side as the thumb. PICTURE
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 preexisting 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.
RBC Red blood corpuscle.
reciprocal cross n. A hybrid cross occurring between the same two types of organisms, but with the sexes of the parents reversed.
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 preexisting chromosomes).
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/
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.
renal /REEN-əl/ Pertaining to the kidneys.
renal pelvis See: kidneys.
repetitive DNA Nucleotide sequences that occur repeatedly in a single genome.
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/.
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.
retina /RET-in-ə/ The interior, light-detecting coat of the eye — retinal /RET-in-əl/ MORE INFORMATION
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
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.
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.
rhamnose (Rha) /RAM-nōs, -nōz/ A deoxy sugar. MOLECULAR STRUCTURE
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.
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-5-phosphate /RĪ-bōs, -bōz/ An intermediate of the pentose phosphate pathway; molecular formula C₅H₁₁O₈P.
riboside /RĪB-ō-sīd/ See: nucleoside.
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.
RKK Reductase kinase kinase.
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.
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.
ruminant /ROOM-ə-nənt/ A mammal that chews cud. MORE INFORMATION
ruminate /ROOM-ə-nate/ To digest with a multi-chambered stomach and by chewing cud. MORE INFORMATION
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