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Ribonucleic acid (RNA)
One of the two major classes of nucleic acid, mainly involved in translating into proteins the genetic information that is carried in deoxyribonucleic acid (DNA). Ribonucleic acids serve two functions in protein synthesis: transfer RNAs (tRNAs) and ribosomal RNAs (rRNAs) function in the synthesis of all proteins, while messenger RNAs (mRNAs) are a diverse set, each member of which acts specifically in the synthesis of one protein. Messenger RNA is the intermediate in the usual biological pathway DNA → RNA → protein. Ribonucleic acid is a very versatile molecule, however. In addition to the roles in protein synthesis, other types of RNA serve other important functions for cells and viruses, such as the involvement of small nuclear RNAs (snRNAs) in mRNA splicing. In some cases, RNA performs functions typically considered DNA-like, such as serving as the genetic material for certain viruses, or roles typically carried out by proteins, such as RNA enzymes or ribozymes. See Deoxyribonucleic acid (DNA)
RNA is a linear polymer of four different nucleotides. Each nucleotide is composed of three parts: a five-carbon sugar known as ribose, a phosphate group, and one of four bases attached to each ribose, either adenine (A), cytosine (C), guanine (G), or uracil (U). The structure of RNA is basically a repeating chain of ribose and phosphate moieties, with one of the four bases attached to each ribose. The structure and function of the RNA vary depending on its sequence and length. See Nucleotide, Ribose
In its basic structure, RNA is quite similar to DNA. It differs by a single change in the sugar group (ribose instead of deoxyribose) and by the substitution of uracil for the base thymine (T). Typically, RNA does not exist as long double-stranded chains as does DNA, but rather as short single chains with higher-order structure due to base pairing and tertiary interactions within the RNA molecule. Within the cell, RNA usually exists in association with specific proteins in a ribonucleoprotein complex.
The nucleotide sequence of RNA is encoded in genes in the DNA, and it is transcribed from the DNA by a complementary templating mechanism that is catalyzed by one of the RNA polymerase enzymes. In this templating scheme, the DNA base T specifies A in the RNA, A specifies U, C specifies G, and G specifies C.
These small RNAs (70–90 nucleotides) that act as adapters to translate the nucleotide sequence of mRNA into protein sequence. They do this by carrying the appropriate amino acid to the ribosome during the process of protein synthesis. Each cell contains at least one type of tRNA specific for each of the 20 amino acids, and usually several types. The base sequence in the mRNA directs the appropriate amino acid-carrying tRNAs to the ribosome to ensure that the correct protein sequence is made. See Protein
Ribosomes are complex ribonucleoprotein particles that are the site of protein synthesis, that is, the process of linking amino acids to form proteins. The RNA components of the ribosome account for more than half of its weight. Like tRNAs, rRNAs are stable molecules and exist in complex folded structures. Each of these rRNAs is essential in determining the exact structure of the ribosome. In addition, the rRNAs, rather than the ribosomal proteins, are likely the basic functional elements of the ribosome. See Ribosomes
Whereas most types of RNA are the final products of their genes, mRNA is an intermediate in information transfer. It carries information from DNA to the ribosome in a genetic code that the protein-synthesizing machinery translates into protein. Specifically, mRNA sequence is recognized in a sequential fashion as a series of nucleotide triplets by tRNAs via base pairing to the three-nucleotide anticodons in the tRNAs. There are specific triplet codons that specify the beginning and end of the protein-coding sequence. Thus, the function of mRNA involves the reading of its primary nucleotide sequence, rather than the activity of its overall structure. Messenger RNAs are typically shorter-lived than the more stable structural RNAs, such as tRNA and rRNA. See Genetic code
Small nuclear RNA
Small RNAs, generally less than 300 nucleotides long and rich in uridine (U), are localized in the nucleoplasm (snRNAs) and nucleolus (snoRNAs) of eukaryotic cells. There they take part in RNA processing, such as intron removal during eukaryotic mRNA splicing and posttranscriptional modification that occurs during production of mature rRNA. See Intron
While most organisms carry their genetic information in the form of DNA, certain viruses, such as polio and influenza viruses, have RNA as their genetic material. The viral RNAs occur in different forms in different viruses. For example, some are single-stranded and some are double-stranded; some occur as a single RNA chromosome while others are multiple. In any case, the RNA is replicated as the genetic material and either its sequence, or a complementary copy of itself, serves as mRNA to encode viral proteins. The RNA viruses known as retroviruses contain an enzyme that promotes synthesis of complementary DNA in the host cell, thus reversing the typical flow of information in biological systems. See Animal virus, Retrovirus, Virus
Other types of RNA
There are RNAs that serve other important and diverse cellular functions. For example, a ribonucleoprotein enzyme is responsible for replication of chromosome ends. Also, there is an essential RNA component in a ribonucleoprotein complex that ensures that membrane and secreted proteins are synthesized in the appropriate cellular location.
RNA molecules can function both as carriers of genetic information and as enzymes. The discoveries of RNA catalysis and of the central role of rRNA in protein synthesis have led to an enhanced appreciation of RNA as the probable original informational macromolecule, preceding both the more specialized DNA and protein molecules in evolution. See Molecular biology, Nucleic acid
(RNA), a nucleic acid universally distributed in nature that contains ribose as its carbohydrate component and adenine and guanine (purine bases) and uracil and cytosine (pyrimidine bases) as its nitrogen bases. Several other derivatives of purine and pyrimidine are also found in small quantities in RNA.
RNA’s are linear polynucleotides that have chains consisting of several tens to tens of thousands of nucleotides and molecular weights that range from 10-20 × 103 to 5-6 × 106. Each individual RNA has a definite sequence of nucleotides. In the body, RNA is found mainly in ribonucleoproteins, which are complexes of RNA with proteins.
RNA is very important biologically because it participates in the realization of genetic information and in the biosynthesis of proteins in all living organisms. The macromolecular structure of RNA mainly consists of single-stranded polynucleotide chains that form double-helical segments according to the principle of the complementarity of bases. Many viruses contain RNA as their only nucleic component; the RNA may serve as the template for the biosynthesis of not only RNA but also of deoxyribonucleic acid (DNA), a process known as reverse transcription.
RNA is biosynthesized mainly in the cell nucleus from ribonucleoside triphosphates by the action of polymerases either on a DNA template (DNA-dependent RNA polymerases) or, in some viruses, on a RNA template (RNA-dependent RNA polymerases).
In bacteria, animal, and plant cells the various types of RNA carry out different biological functions. They also differ in structure and metabolism. A discussion of the most important types of RNA follows.
Ribosomal RNA (rRNA) is found in ribosomes and comprises the major portion of cellular RNA; it is represented by RNA with the sedimentation constants (S) 23S, 16S, and 5S. In colon bacillus the primary structure of rRNA, that is, the sequence of nucleotides, has been completely established for 5S RNA, almost completely established for 16S RNA, and partially established for 23S RNA. In different types of organisms the size and structure of rRNA are different. The biological role of rRNA has not been fully clarified. The integrity of rRNA molecules is necessary for the biosynthesis of the proteins in ribosomes.
Transfer RNA (tRNA) has a sedimentation constant of about 4S and a molecular weight of about 25,000. It exhibits relatively low polymerism, with about 80 nucleotide residues, and contains many methylated and other minor bases. The biological role of tRNA consists in binding activated amino-acid residues and transferring them to ribosomes, that is, to the site of the synthesis of polypeptide chains. Each amino acid has its own specific tRNA, usually more than one. Transfer RNA has a complex, partially double-helical, macromolecular structure that is represented in the shape of a clover leaf. This type of RNA contains segments that are joined to a ribosome—a triplet of nucleotides, or an anticodon—that is connected to the codon of messenger RNA (mRNA), and a terminal segment that binds the amino-acid residue. The primary structures of more than 60 tRNA’s have been fully established.
Messenger RNA is the most varied group of RNA. It acts as a template in the biosynthesis of proteins during the process of translation, which is the formation of a definite sequence of amino acids in a protein polypeptide chain based on a nucleotide code. RNA’s are synthesized in the cell on a template of DNA and form a sequence of ribonucleotides that are complementary to the sequence of deoxyribonucleotides in the DNA (the transcription process). Giant molecules that are the precursors of mRNA have been found in the cell nucleus. Most of these giant molecules decompose within the nucleus and only a relatively small portion of each molecule is transferred to the cytoplasm and forms actual mRNA.
The RNA that decomposes rapidly in the cell nucleus is probably involved in the regulation processes. Some other types of RNA have also been found, for example, a relatively stable nucleic RNA that exhibits low polymerism—its function is still unclear.
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I. B. ZBARSKII