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Radioactive marker A radioactive atom used in the detection of a larger molecule in which it is  incorporated.

Radioimmunoassay (RIA)  A number of different, sensitive techniques for measuring antigen or antibody titres, using radiolabelled reagents.

RAG-1 and RAG-2  = Recombination activating genes.

Random amplified polymorphic DNA (RAPD) analysis A PCR technique that uses short random primers to amplify fragments that are representative of the genome being studied and which can be used to  make comparisons between the structures of the genomes of different organisms.

Random priming A method for DNA labeling that utilizes random DNA hexamers which anneal to single-stranded and act as primers for complementary strand synthesis by a suitable enzyme.

Receptor  Any protein that binds a specific extracellular signaling molecule (ligand) that induces a cellular response. Receptors for steroid hormones, which diffuse across the plasma membrane, are located within the cell; receptors for water-soluble hormones  (e.g., epinephrine) and peptide growth factors, as well as lipid soluble prostaglandins, are located in the plasma membrane with their lignad-binding domain exposed to the external medium. Receptors for neurotransmitters, located in postsynaptic membranes, can regulate ion flow through the cell membrane or activate other signal transduction pathways.

Receptor tyrosine kinases (RTKs)  An important class of cell-surface receptors whose cytosolic domain has tyrosine-specific protein kinase activity. This activity is activated by ligand binding and leads both to receptor autophosphorylation and to phosphorylation of target proteins. Both types of phosphorylations activate particular intracellular signal transduction pathways.

Recombinant A transformed cell that contains a recombinant DNA molecules.

Recombinant DNA molecule A DNA molecule created in the test-tube by ligating together pieces of DNA that are not normally contiguous.

Recombinant DNA technology All the techniques involved in the construction, study and use of recombinant DNA molecules.

Recombinant protein A polypeptide that is synthesized in a recombinant cell as the result of expression of a cloned gene.

Recombination The exchange of DNA sequences between different molecules, occurring either naturally   or as a result of DNA manipulation. This process also takes place during the somatic rearrangements of DNA which occur during the formation of genes encoding antibody molecules and T-cell antigen receptors.

Recombination activating genes (RAG-1 and RAG-2)  The genes, required for recombination of V, D and J gene segments during generation of functional antigen receptor genes.

Reflex arc  Neural or hormonal components that mediate a reflex usually includes receptor, afferent pathway, integrating center, efferent pathway, and effector.

Relaxed Refers to the non-supercoiled conformation of open-circular DNA.

Replacement vector A λ vector designed so that insertion of new DNA is by replacement of part of the non-essential region of the cDNA molecule.

Replica  plating A technique whereby the colonies on an agar plate are transferred en masse  to a new plate, on which the colonies grow in the same relative positions as before.

Replicative form of M13 The double-stranded form of the M3 DNA molecule found within infected E.coil cells.

Reporter gene A gene whose phenotype can be assayed in a transformed organism, and which is used in, for example, deletion analyses of regulatory regions.

Repression The switching off of expression of a gene or a group of genes in response to a chemical or other stimulus.

Renin  Hormone secreted by kineys that acts as an enzyme that catalyzes splitting off of angiotension I from angiotensinogen in plasma.

Respiration  General term for any cellular process involving the uptake of O2 coupled to production of CO2.

Restriction analysis Determination of the number and sizes of the DNA fragments produced when a particular DNA molecule is cut with a particular restriction endonuclease.

Restriction endonuclease An endonuclease that cuts DNA molecules only at a limited number of specific nucleotide sequences.

Restriction fragment length polymorphism (RFLP) A mutation that results in a detectable change in the pattern of fragments obtained when a DNA molecule is cut with a restriction endonuclease.

Restriction map A map showing the positions of different restriction sites in a DNA molecule.

Reticuloendothelial system  A diffuse system of phagocytic cells derived from the bone-marrow stem cells which are associated with the connective tissue framework of the liver, spleen, lymph nodes and other serous cavities. An old-fashioned term, rarely used-mononuclear phagocyte system is preferred.

Retrotransposon  Type of eukaryotic mobile DNA element whose movement in the genom is mediated by an RNA intermediate and involves a step of reverse transcription.

Retrovirus A virus with an RNA genome, able to insert into a host chromosome, derivatives of which have been used to clone genes in mammalian cells. Retrovirus replicates in cells by first making a DNA copy of the RNA, a process termed reverse transcription. This proviral DNA is inserted into cellular chromosomal DNA, and gives rise to further genomic RNA as well as the mRNAs for viral proteins.

Reverse transcriptase An RNA-dependent DNA polymerase, able to synthesize a complementary DNA molecule on a template of single-stranded RNA.

RFLP linkage analysis A technique that uses a closely linked RFLP as a marker for the presence of a particular allele in a DNA sample, often as a means of screening individuals for defective genes responsible for genetic diseased.

Ribonuclease An enzyme that degrades RNA.

Ribosome binding site The short nucleotide sequence upstream of a gene which after transcription forms the site on the mRNA molecule to which the ribosome binds.

RT-PCR A PCR technique in which the starting material is RNA. The first step in the procedure is conversion of the RNA to cDNA with reverse transcriptase.

Ri plasmid An Agrobacterium rhizogenes plasmid, similar to the Ti plasmid, used to clone genes in higher plants.

RNA splicing  A process that results in removal of introns and joining of exons in RNAs. Splicing in the pre-mRNA of higher eukaryotes occurs in large ribonucleoprotein complexes called spliceosomes.

Rosetting  A technique for identifying or isolating cells by mixing them with particles or cells to which they bind (e.g. sheep erythrocytes to human T cells). The rosettes consist of a central cell surrounded by bound cells.

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