RNA, or ribonucleic acid, is a nucleic acid composed of chains of ribonucleotides and is involved in information transfer, protein synthesis, gene regulation, catalysis and numerous other cellular processes. Each RNA nucleotide contains a ribose sugar, a phosphate group and one of four principal bases, adenine, uracil, cytosine and guanine. RNA differs chemically from DNA because it contains ribose rather than deoxyribose and normally uses uracil instead of thymine.
Most cellular RNA is single stranded, although RNA molecules can fold back on themselves and form extensive internal base pairing, producing complex three dimensional structures. RNA therefore combines information carrying properties with structural and catalytic capabilities. It is found in all cellular organisms and plays particularly important roles in the expression and regulation of genetic information.
Messenger RNA, or mRNA, carries genetic information from DNA to ribosomes for protein synthesis in organisms where DNA is the primary genomic material. During transcription, RNA polymerase uses a DNA template to produce an RNA molecule with a sequence complementary to the template. In eukaryotic cells, many mRNA molecules are processed before translation. A 5 prime cap is added, introns are commonly removed through splicing, and a poly A tail is added at the 3 prime end.
Alternative splicing can allow a single gene to produce multiple related RNA transcripts and therefore multiple protein products. Mature mRNA is exported from the nucleus into the cytoplasm, where ribosomes read its sequence in groups of three nucleotides called codons. Each codon specifies an amino acid or a translation termination signal according to the genetic code.
Transfer RNA, or tRNA, is another essential RNA class. tRNA molecules act as adaptors during translation by carrying particular amino acids and pairing their anticodons with corresponding codons in mRNA. Their three dimensional structures allow them to interact with ribosomes, amino acids and enzymes called aminoacyl tRNA synthetases. Ribosomal RNA, or rRNA, forms a major structural and catalytic component of ribosomes. Ribosomes are therefore ribonucleoprotein complexes rather than purely protein machines. The catalytic centre responsible for peptide bond formation is primarily RNA based, making the ribosome a ribozyme containing molecular machine. These RNA classes demonstrate that RNA is not merely an intermediate copied from DNA but an active participant in the molecular machinery of life.
Many other RNA classes regulate gene expression and cellular organisation. MicroRNAs and small interfering RNAs can influence gene expression by guiding protein complexes to complementary RNA molecules and promoting translational repression or RNA degradation. Long noncoding RNAs can participate in transcriptional regulation, chromatin organisation and other processes, although their functions vary widely and many remain incompletely characterised. Small nuclear RNAs participate in RNA splicing, while small nucleolar RNAs help process and modify ribosomal RNA.
Circular RNAs form covalently closed structures and can have regulatory functions in particular biological contexts. There are also specialised RNAs involved in telomere maintenance, RNA modification, mitochondrial gene expression and other cellular processes. The term RNA therefore encompasses a very diverse family of molecules rather than a single functional category.
RNA can fold into elaborate structures because its single strand can form internal base pairs and interact with itself and with proteins or other nucleic acids. Secondary structures can include stems, loops, bulges and other motifs, which can combine into complex tertiary structures. These structures can determine RNA stability, localisation, binding specificity and catalytic activity. Ribozymes are RNAs that catalyse chemical reactions, demonstrating that biological catalysis is not exclusively a protein function.
Riboswitches are RNA elements that can bind small molecules and influence gene expression. Some viral genomes are themselves made of RNA, and RNA viruses can use RNA dependent RNA polymerases or reverse transcriptases depending on their replication strategy. RNA structure and chemistry therefore have major significance in molecular biology, evolution and medicine.
RNA is generally less chemically stable than DNA because the 2 prime hydroxyl group of ribose can participate in reactions that cleave the phosphodiester backbone under appropriate conditions. This reduced stability can be advantageous for transient information transfer because RNA molecules can be produced and degraded relatively rapidly. Cells use numerous proteins and regulatory mechanisms to control RNA processing, transport and degradation. RNA quality control pathways identify defective or improperly processed transcripts and prevent them from causing inappropriate protein production. RNA molecules can also undergo chemical modifications, including modifications of bases and sugars, which can affect stability, structure and translation. The dynamic nature of RNA makes it particularly suitable for roles that require rapid regulation rather than long term information storage alone.
RNA has become important in biotechnology and medicine because its properties can be manipulated directly. Messenger RNA can be delivered into cells to provide temporary instructions for producing a chosen protein. This principle underlies mRNA vaccine technologies, in which cells transiently produce an antigen encoded by the delivered RNA. RNA interference can be used experimentally and therapeutically to reduce expression of selected genes. Antisense oligonucleotides can bind particular RNA molecules and alter their processing or stability. RNA sequencing provides detailed information about gene expression and transcript structure in particular cells or tissues. These applications exploit RNA's ability to carry sequence specific information while remaining a transient and regulatable molecule.
The relationship between DNA and RNA is important but should not be oversimplified. DNA is generally more chemically stable and is used for long term genomic information storage in cellular organisms, whereas RNA performs numerous roles in information transfer, catalysis and regulation. The central information flow from DNA to RNA to protein is fundamental to molecular biology, but RNA can also serve as genetic material in many viruses and can participate in reverse information flow through reverse transcription.
The diversity of RNA supports the hypothesis that RNA could have played especially important roles in early molecular evolution, a concept known as the RNA world hypothesis, although the exact history of early life remains uncertain. RNA is therefore simultaneously a genetic molecule in some organisms, an information intermediate, a structural molecule, a catalyst and a major regulator of cellular processes.
