Ribonucleic acid (RNA) is a crucial biological macromolecule present in all living cells, playing a central role in converting the genetic information stored in DNA into functional proteins. It is a single-stranded nucleic acid, in contrast to the double-helix structure of DNA, which allows RNA to fold into complex secondary and tertiary structures that are essential for its diverse functions. The key difference between RNA and DNA lies in their sugar components: RNA contains ribose, which has an extra oxygen atom compared to the deoxyribose in DNA, making RNA less stable but more versatile.
RNA's primary function is as a messenger. When a cell needs to produce a specific protein, it transcribes the relevant gene from DNA into a messenger RNA (mRNA) molecule. This mRNA acts as a temporary, disposable copy of the genetic instructions, which is then transported out of the nucleus to the cytoplasm where ribosomes can read it. The ribosome translates the mRNA sequence, which is read in groups of three nucleotides called codons, into a specific sequence of amino acids to build a protein. Transfer RNA (tRNA) molecules act as adaptors, bringing the correct amino acids to the ribosome based on the mRNA codon, while ribosomal RNA (rRNA) forms the core of the ribosome and catalyzes the formation of peptide bonds between amino acids.
Beyond its role as a messenger, RNA has many other vital functions. It can act as an enzyme, known as a ribozyme, catalyzing chemical reactions such as peptide bond formation during protein synthesis and RNA splicing. This catalytic ability is a key piece of evidence for the "RNA world" hypothesis, which proposes that early life on Earth relied on RNA to both store genetic information and perform enzymatic functions before the evolution of DNA and proteins. Furthermore, RNA is the genetic material for many viruses, including influenza and HIV, which use its instability to their advantage by mutating rapidly to evade the host's immune system. Recent research has also revealed that a significant portion of the genome, beyond the 2% that codes for proteins, is transcribed into various non-coding RNAs (like lncRNAs and siRNAs) that play critical roles in regulating gene expression, cell division, and other cellular processes, with defects in these RNAs linked to diseases such as cancer and heart disease.