ribonucleic acid: any of a class of single-stranded molecules transcribed from DNA in the cell nucleus or in the mitochondrion or chloroplast, containing along the strand a linear sequence of nucleotide bases that is complementary to the DNA strand from which it is transcribed: the composition of the RNA molecule is identical with that of DNA except for the substitution of the sugar ribose for deoxyribose and the substitution of the nucleotide base uracil for thymine. Cf. messenger RNA, ribosomal RNA, transfer RNA.[1945-50]
* * *One of the two main types of nucleic acid (the other being DNA), which functions in cellular protein synthesis in all living cells and replaces DNA as the carrier of genetic information in some viruses.Like DNA, it consists of strands of repeating nucleotides joined in chainlike fashion, but the strands are single (except in certain viruses), and it has the nucleotide uracil (U) where DNA has thymine (T). Messenger RNA (mRNA), a single strand copied from a DNA strand that acts as its template, carries the message of the genetic code from DNA (in chromosomes) to the site of protein synthesis (on ribosomes). Ribosomal RNA (rRNA), part of the building blocks of ribosomes, participates in protein synthesis. Transfer RNA (tRNA), the smallest type, has fewer than 100 nucleotide units (mRNA and rRNA contain thousands). Each nucleotide triplet on mRNA specifies which amino acid comes next on the protein being synthesized, and a tRNA molecule with that triplet's complement on its protruding end brings the specified amino acid to the site of synthesis to be linked into the protein. Various minor types of RNA also exist; at least some act as catalysts (ribozymes), a function long ascribed only to proteins.
* * *abbreviation of Ribonucleic Acid,complex compound of high molecular weight that functions in cellular protein synthesis and replaces DNA (deoxyribonucleic acid) as a carrier of genetic codes in some viruses. RNA consists of ribose nucleotides in strands of varying lengths. The structure varies from helical to uncoiled strands. One type, transfer RNA (tRNA), sometimes called soluble, or activator, RNA, contains fewer than 100 nucleotide units; other types contain thousands of units. The nitrogenous bases in RNA are adenine, guanine, cytosine, and uracil.There are three main types of RNA: messenger RNA (mRNA), transfer RNA, and ribosomal RNA (rRNA). In protein formation, mRNA carries codes from the DNA in the nucleus to the sites of protein synthesis in the cytoplasm (the ribosomes (ribosome)). Ribosomes are composed of rRNA and protein; they can “read” the code carried by the mRNA. A sequence of three nitrogenous bases in mRNA specifies incorporation of an amino acid; tRNA brings the amino acids to the ribosomes, where they are linked into proteins.In addition to the main types of RNA, a number of other small RNA molecules and ribonucleoproteins (molecules composed of RNA and protein) occur in the cell. The RNA portion of at least one cellular ribonucleoprotein has been shown to act as a biological catalyst, a function previously ascribed only to proteins. R.W. Holley (Holley, Robert William) described the structure of an RNA molecule in 1965.
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