- ribonucleic acid
- A macromolecule consisting of ribonucleoside residues connected by phosphate from the 3′-hydroxyl of one to the 5′-hydroxyl of the next nucleoside. RNA is found in all cells, in both nuclei and cytoplasm and in particulate and nonparticulate form, and also in many viruses; polynucleotides made in vitro are generally called such. Various RNA fractions are identified by location, form, or function.- antisense RNA the transcription product of the DNA antisense strand; it can play a role in the inhibition of translation. SEE ALSO: antisense DNA.- chromosomal RNA RNA associated with the chromosome (not mRNA, tRNA, or rRNA) that may have a role in transcription.- heterogeneous nuclear RNA (hnRNA) an ill-defined form of RNA, of high molecular weight, that never leaves the nucleus and is thought to be the precursor of messenger RNA.- informational RNA SYN: messenger RNA.- initiation tRNA tRNA in prokaryotes containing a formyl-methionyl residue that initiates translation. SYN: formyl-methionyl-tRNA, starter tRNA.- messenger RNA (mRNA) the RNA reflecting the exact nucleoside sequence of the genetically active DNA and carrying the “message” of the latter, coded in its sequence, to the cytoplasmic areas where protein is made in amino acid sequences specified by the mRNA, and hence primarily by the DNA; viral RNAs are considered to be natural messenger RNA s. SYN: informational RNA, template RNA.- nuclear RNA (nRNA) rNA found in nuclei, or associated with DNA, or with nuclear structures (nucleoli).- ribosomal RNA the RNA of ribosomes and polyribosomes.- small nuclear RNA (snRNA) small RNA ( i.e., about 90–300 nucleotides long) in the nucleus believed to have a role in RNA processing and cellular architecture.- starter tRNA SYN: initiation tRNA.- transfer RNA (tRNA) short-chain RNA molecules present in cells in at least 20 varieties, each variety capable of combining with a specific amino acid (see aminoacyl-tRNA). By joining (through their anticodons) with particular spots (codons) along the messenger RNA molecule and carrying their amino acyl residues along, they lead to the formation of protein molecules with a specific amino acid arrangement—the one ultimately dictated by a segment of DNA in the chromosomes. Each tRNA has about 80 nucleotides (MW about 25,000); most of the 20 varieties occur in multiple “isoacceptor” forms, separable by chromatography. Further subvarieties exist in, e.g., different strains of an organism, in subcellular organelles, and in different metabolic states. Cognate tRNAs are the tRNAs recognized by the specific amino acyl-tRNA synthetases. SYN: acceptor RNA, soluble RNA.
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(RNA) ri·bo·nu·cle·ic ac·id (RNA) (ri″bo-noo-kleґik) the nucleic acid in which the sugar is ribose, constituting the genetic material in the RNA viruses and playing a role in the flow of genetic information in all cells. Ribosyl moieties are linked via phosphate groups attached to their 5′ and 3′ hydroxyl groups to form the backbone of a linear polymer, with purine and pyrimidine bases attached to the sugars as side chains. The characteristic bases adenine (A), uracil (U), cytosine (C), and guanine (G) are specified by the presence of thymine (T), A, G, and C, respectively, in the gene being transcribed. Many RNA molecules contain bases modified by posttranscriptional processing (methylation, deamination, isomerization), and although mainly single-stranded, some contain regions of secondary structure such as base pairing between self-complementary sequences, which stabilizes specific conformations. In most viruses with an RNA genome, the RNA is single-stranded, but several families have a double-stranded RNA genome. For specific types of RNA, see under RNA.
Medical dictionary. 2011.
