Dictionary: ribonucleic acid   (rī'bō-nū-klē'ĭk, -klā'-, -nyū-)

See RNA.

[RIBO(SE) + NUCLEIC ACID.]

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 also Deoxyribonucleic acid (DNA).

Structure

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 also 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.

Transfer RNA

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 also Protein.

Ribosomal RNA

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 also Ribosomes.

Messenger RNA

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 also 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 also Intron.

Catalytic RNA

RNA enzymes, or ribozymes, are able to catalyze specific cleavage or joining reactions either in themselves or in other molecules of nucleic acid. See also Catalysis; Ribozyme.

Viral RNA

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 also 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 also Molecular biology; Nucleic acid.

A nucleic acid, found in both the nucleus and cytoplasm of cells, that transmits genetic instructions from the nucleus to the cytoplasm. RNA functions in the assembly of proteins.

See RNA.

nucleic acids present in all living cells which are particularly involved in cellular protein synthesis and replace DNA as a carrier of genetic information in some viruses; abbreviated RNA. RNA is similar in composition to DNA with two exceptions: the sugar in RNA is ribose (in DNA it is deoxyribose) and in RNA the pyrimidine uracil replaces the thymidine of DNA. The structure of RNA varies from helical to uncoiled linear strands of varying lengths, depending on the number of nucleotides forming the strand. This variance in structure is evident in the different types of RNA. For example, transfer RNA (tRNA) contains only about 75 nucleotides, while some mRNAs may contain thousands of nucleotides. Messenger RNA (mRNA) receives its name from its function of carrying the genetic code from DNA in the nucleus of the cell to the cytoplasm, where proteins are synthesized. The transfer of the genetic code from DNA to mRNA is called transcription. Molecules of mRNA migrate to the cytoplasm and bind to ribosomes, to form polyribosomes. RNA moves along the ribosomes and its information content as codons (three nucleotides) is translated into a particular sequence of amino acids, i.e. a protein or polypeptide. Stop codons, UAA, UAG and UGA, terminate translation of mRNA. Transfer RNA (tRNA) called also soluble RNA, brings about the transfer of specific amino acid molecules to the growing protein molecules during the synthesis of proteins. Each of the 20 common amino acids found in protein molecules has a corresponding type of tRNA. Thus, a specific tRNA carries the appropriate amino acid to its appropriate place in the nascent chain of the protein molecule being synthesized. tRNA has a triplet of nucleotides called the anticodon which is able, by base pairing to the codon triplet in mRNA as it moves along the ribosomes, to place the amino acids in the specific order characteristic of the particular protein. See also trna. Ribosomal RNA (rRNA) is so called because it is found as a major structural component of ribosomes are the cytoplasmic organelle required for linking of amino acids into protein molecules.
Large amounts of RNA are also found in the nucleus of cells. One class of nuclear RNA, called heterogeneous RNA (hnRNA), represents primary RNA transcripts, i.e. before processing including splicing of introns, to form mRNA. A second class of nuclear RNA is called small nuclear RNA (snRNA) one species of which, called small nuclear ribonucleoprotein (snRNP), is believed to play an important role in the processing of hnRNA to mRNA. In the splicing reaction RNA itself acts as an ‘enzyme’ in that it is able to cut and religate other RNA molecules.