Online Biology Dictionary - R

Raceme

r Correlation coefficient.

R Arginine.

R Gas constant = 8.314 472 J K⁻¹ mol⁻¹.

R5P Ribose-5-phosphate.

raceme /ruh-SEEM/ An inflorescence with the flowers borne on short stalks (pedicles) of about equal length arising from an axial rachis (see figure at right).

radical /RAD-ik-uhl/ A group of atoms viewed as a single unit, that can pass unchanged from one molecule to another, but that cannot exist independently.

radicle /RAD-ik-uhl/ The root of a plant embryo.

radiation /RAYD-ee-AY-shun/ (1) energy emitted in the form of waves or ionizing particles (2) adaptive radiation.

radioactive /RAYD-ee-oh-ACT-iv/ Emitting radiation such as alpha particles, beta particles, and gamma photons.

radiobiology /RAYD-ee-oh-bye-AWL-uh-gee/ The study of the effects of radiation on living living organisms.

radiocarpal /RAYD-ee-oh-bye-CARP-uhl/ Pertaining to the radius and the carpus.

radiogenic element /RAYD-ee-oh-GEHN-ik/ An element produced by the radioactive decay of some other element.

radiograph /RAYD-ee-oh-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 /RAYD-ee-oh-HYOOM-er-uhl/ Pertaining to the radius and the humerus.

radioimmunoassay (RIA) /RAYD-ee-oh-im-MYOON-oh-ASS-ay/ 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.

Human Radius

radioisotope /RAYD-ee-oh-EYE-soh-tope/ A radioactive isotope.

radioulnar /RAYD-ee-oh-UHL-ner/ Pertaining to the radius and the ulna.

radius /RAYD-ee-uhs/ The smaller of the two bones of the forearm; it lies on the same side as the thumb. PICTURE

radula (pl radulae) /RAHD-juh-luh/ A rough appendage present in many mollusks. Used for scraping, it has minute chitinous teeth. PICTURE

ramet /RAY-muht/ An individual member of a clonal line of individuals. Compare: ortet.

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

RBC Red blood corpuscle.

recessive allele /uh-LEEL/ In a diploid organism, an alleles/a> that is manifested only when two copies of it are present at a locus.

Reciprocal Translocation

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).

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 A short (~12 cm), upright portion of the large intestine. It connects the sigmoid colon with the anal canal. PICTURE

regulatory sequence A DNA sequence controling gene expression.

renal /REEN-uhl/ Pertaining to the kidneys.

renal pelvis See: kidneys.

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-luh-KAY-shuhn/ 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.

RER Rough endoplasmic reticulum.

respiration /RESP-er-RAY-shuhn/ The oxidation of energy-rich storage molecules, primarily glucose, to produce ATP. In aerobic respiration, which occurs in plants, animals, and many prokaryotes, glucose and oxygen are converted to carbon dioxide and water (producing on average 38 ATP molecules per oxidized glucose molecule). In anaerobic respiration (which occurs during fermentation), less energy is extracted (only 2 ATP molecules per glucose molecule) because the products of the process, such as ethanol, contain more energy than does carbon dioxide, the product of aerobic respiration. In muscle, the product of anaerobic respiration is lactic acid. In yeast, it is ethanol. Both aerobic and anaerobic respiration begin with glycolysis to produce pyruvic acid, but thereafter the pathways differ. In aerobic respiration, the pyruvate is transported into a mitochondrion and metabolized there, whereas in anaerobic respiration it is processed in the cytoplasm. See: carbon cycle.

restriction endonucleases (also restriction enzymes) /END-oh-N(Y)OOK-lee-ayz/ 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. 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.

retrotransposon /REH-troh-trans-POSE-on/ A retrovirus that is passed hereditarily because it is permanently inserted in the host genome.

retrovirus /REH-troh-VIRE-uhs/ 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.

Two Enzymes in One: Reverse transcriptase performs several functions. As its name indicates, it can build DNA strands on an RNA template. This reaction is performed in the polymerase active site, formed by two sets of arms surrounding the RNA template and DNA (top of illustration). After building the DNA strand, the enzyme then removes the original RNA strand by cleaving it into pieces. This is performed by a nuclease active site at the opposite end of the enzyme. Finally, it builds a second DNA strand matched to the one that was just created to form the final DNA double helix. This reaction is also performed by the polymerase site.

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. It produces a single strand of DNA using an RNA template. In a natural setting, the template is the genome of a retrovirus. However, reverse transcriptase is also used in the laboratory to make cDNAs (complementary DNA sequences) from RNA sequences. Reverse transcription begins with the binding of a primer tRNA to the primer binding site. Reverse transcriptase performs a remarkable feat, reversing the normal flow of genetic information, but does a sloppy job. The polymerases that normally make DNA and RNA in cells are very accurate and make very few mistakes. Reverse transcriptase, on the other hand, makes many. The RT of the AIDS virus HIV makes about one mistake in every 2,000 bases it copies. One might suppose this would cause serious problems. But, in fact, this high error rate allows HIV to rapidly mutate and produce drug-resistant strains. Fortunately, the treatments combining several drugs are often effective in combating this problem since the virus cannot mutate to evade all of them at the same time. DIAGRAM OF REVERSE TRANSCRIPTION

RF (1) replicative form; (2) release factor.

RFLP See: restriction fragment length polymorphism.

rhabdomyoblasts /RAB-doe-MY-oh-blasts/ Neoplastic ceils resembling the precursor cells of striated muscle.

rhabdomyoma /RAB-doe-my-OH-muh/ A benign tumor of striated muscle.

rhabdomyosarcoma (RMS) /RAB-doe-MY-oh-sark-OH-muh/ A sarcoma arising from rhabdomyoblasts.

rhamnose (Rha) /RAM-nohs, -nohz/ A deoxy sugar. MOLECULAR STRUCTURE

Rhincodon /RINK-uh-dawn/ The genus of the whale shark (Rhincodon typus), the world's largest fish. SEE PICTURE

rhizome /RIGH-zohm/ An underground stem, produced by many plants.

rhodoplast /RODE-uh-plast/ The plastid giving red algae its characteristic color.

RIA Radioimmunoassay.

riboflavin /RIGH-uh-FLAYV-uhn/ A water-soluble vitamin of the B complex group; essential for tissue repair; unstable in light.

ribonucleic acid (RNA) /RIBE-oh-n(y)oo-KLAY-uhk/ 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 /RIGH-bohs, -bohz/ The five-carbon monosaccharide component of RNA; occurs primarily as D-ribose. MOLECULAR STRUCTURE

riboside /RIGH-boh-side/ See: nucleoside.

ribose-5-phosphate /RIGH-bohs, -bohz/ An intermediate of the pentose phosphate pathway; molecular formula C₅H₁₁O₈P.

ribosomal RNA (rRNA) /RIBE-uh-SOAM-uhl/ The most abundant type of RNA. With ribosomal proteins, rRNAs form the structure of ribosomes.

ribosomes /RIBE-uh-sohm/ Cellular organelles, present in both prokaryotes and eukaryotes, that use mRNA templates to synthesize proteins, a process known as translation. See also: rRNA.

Rickettsia /ruh-KEHT-see-yuh/ 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.

RK HMG-CoA reductase kinase.

RKK Reductase kinase kinase.

RMS Rhabdomyosarcoma.

RNA See: ribonucleic acid.

RNA polymerase /puh-LIM-uh-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.

rRNA Ribosomal RNA.

root pressure Upward pressure of water within vascular plants caused by root cells pumping water into the plant.

RSV Rous sarcoma virus.

Ru5P Ribulose-5-phosphate.

RuBP Ribulose-1,5-bisphosphate.

ruminant /ROOM-uh-nuhnt/ A mammal that digests plant matter by first softening it within a first stomach (the "rumen"), then regurgitating the partially digested mass ("cud"), and chewing it again. The rechewing of the cud to further break down the semi-digested food is called "ruminating." Ruminants include various many members of the order Artiodactyla, such as buffalo, cattle, goats, sheep, bison, yaks, water antelope, deer, camels, giraffes, alpacas, and llamas. Kangaroos and various other related Australian mammals also ruminate.