RNA

in full ribonucleic acid

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.

ribonucleic acid [¦rī·bō¦nü¦klē·ik ′as·əd]

(biochemistry)

A long-chain, usually single-stranded nucleic acid consisting of repeating nucleotide units containing four kinds of heterocyclic, organic bases: adenine, cytosine, quanine, and uracil; they are conjugated to the pentose sugar ribose and held in sequence by phosphodiester bonds; involved intracellularly in protein synthesis. Abbreviated RNA.

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)

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 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 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 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 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

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 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 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