RNA

Ribonucleic acid (RNA) is a polymer of nucleotides that plays multiple roles in coding, decoding, regulation, and expression of genes. In cells, RNA largely functions as a messenger, adaptor, or component of the protein synthesis machinery. RNA differs from DNA in its sugar (ribose vs deoxyribose), in the use of uracil instead of thymine, and in its tendency to be single-stranded, though it can form complex secondary structures.

RNA molecules are built from a backbone of ribose phosphate with four bases. The presence of a 2'-hydroxyl group makes RNA chemically reactive and less stable than DNA. Bases pair via hydrogen bonding (adenine with uracil, guanine with cytosine) to form stems and loops that enable folding and interactions with proteins, other RNAs, and catalytic activities. These structures support diverse functions, including vision of enzymatic activity in some ribozymes.

The most abundant RNAs are messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). Additional classes include small nuclear RNAs (snRNA) and small nucleolar RNAs (snoRNA) involved in RNA processing, as well as regulatory RNAs such as microRNAs (miRNA), small interfering RNAs (siRNA), and Piwi-interacting RNAs (piRNA) that regulate gene expression and genomic stability.

RNA is synthesized from DNA templates by RNA polymerases during transcription. In eukaryotes, mRNA typically receives a 5' cap and a 3' poly(A) tail and often undergoes splicing to remove introns. During translation, ribosomes read mRNA codons to assemble amino acids with the help of tRNA adaptors. RNA also participates in catalysis as ribozymes and in gene silencing and defense against transposable elements. Some RNA genomes exist in RNA viruses; in cells, RNA is dynamic, regulated, and continually degraded as part of cellular physiology.