RNA

RNA, or ribonucleic acid, is a nucleic acid involved in the storage, expression, and regulation of biological information. It is fundamental to cellular life and serves as the genetic material of some viruses. Like DNA, RNA consists of chains of nucleotides, but its distinctive chemistry allows it to act not only as an information carrier but also as a structural component, molecular regulator, and catalyst. Most cellular RNA is single-stranded, although it often folds into extensively base-paired structures. (genome.gov)

Chemical composition and structure

RNA is a polymer built from nucleotides. Each nucleotide contains the sugar ribose, a phosphate group, and a nitrogen-containing base. The four principal bases are adenine (A), guanine (G), cytosine (C), and uracil (U). Adjacent nucleotides are joined by phosphodiester bonds, producing a sugar–phosphate backbone with a defined direction: from the 5′ end to the 3′ end. RNA sequences are conventionally written in this direction. (genome.gov)

RNA differs chemically from DNA in two major respects. Its sugar has a hydroxyl group at the 2′ carbon, where DNA has a hydrogen atom, and it ordinarily uses uracil rather than thymine. The 2′-hydroxyl group contributes to RNA’s folding and chemical reactivity, including reactions involved in RNA catalysis. (genome.gov)

“Single-stranded” does not mean that RNA lacks structure. Complementary regions within one molecule can pair, commonly through A–U and G–C interactions. G–U pairs and other noncanonical interactions also occur. These interactions produce stems, hairpin loops, bulges, and junctions, which can assemble into complex three-dimensional structures. Base stacking, hydrogen bonds, and interactions with ions and proteins help stabilize these folds. Even messenger RNAs contain structured regions that influence their function. (ibiology.org)

Major classes and functions

RNA classes are distinguished chiefly by their biological roles. The three central participants in protein synthesis are messenger RNA, ribosomal RNA, and transfer RNA; numerous other RNAs perform processing and regulatory functions. (genome.gov)

The functions of these classes are established through studies of protein synthesis, RNA processing, and small-RNA-mediated regulation. (ncbi.nlm.nih.gov)

Protein synthesis

During translation, the ribosome reads an mRNA sequence in three-nucleotide units called codons. Transfer RNAs act as adaptors: their anticodon regions recognize mRNA codons, while their attached amino acids become building blocks of the growing protein. Ribosomal RNA organizes this process and supplies the catalytic center responsible for peptide-bond formation. RNA therefore participates in protein synthesis as the template, adaptor, and core catalyst. (genome.gov)

Catalysis and regulation

An RNA capable of catalyzing a chemical reaction is called a ribozyme. Examples include self-splicing RNAs and the catalytic RNA component of RNase P, which processes precursor tRNAs. These discoveries established that biological catalysis is not restricted to protein enzymes. (nobelprize.org)

Regulatory RNAs can recognize other nucleic acids through complementary base pairing. In RNA interference, small guide RNAs work with proteins to silence matching targets. Related small-RNA mechanisms contribute to developmental regulation, defense against viruses in several groups of organisms, and suppression of mobile genetic elements. Different small-RNA pathways have distinct processing steps and biological functions; they are not interchangeable mechanisms. (nobelprize.org)

Synthesis and processing

Most cellular RNA is produced by transcription, in which RNA polymerase uses one strand of DNA as a template. The growing RNA is complementary to that template and is extended in the 5′-to-3′ direction. Transcription is a central stage of gene expression, whether the resulting RNA encodes a protein or functions directly as RNA. (ncbi.nlm.nih.gov)

In eukaryotes, a primary transcript often undergoes processing before becoming functional. Most nuclear protein-coding transcripts receive a 5′ cap and undergo polyadenylation at their 3′ end. During RNA splicing, intervening sequences called introns are removed and retained segments called exons are joined. Alternative splicing can generate different mature transcripts from the same gene, potentially producing different protein forms. These features are not universal to every RNA molecule or every organism. (ncbi.nlm.nih.gov)

Many functional RNAs also require cleavage, trimming, folding, or chemical modification. Processing is consequently not merely preparation for translation: it is part of the formation of functional tRNAs, rRNAs, and other noncoding RNAs. (ncbi.nlm.nih.gov)

RNA as genetic material

Some viruses have RNA rather than DNA genomes. Viral RNA genomes can be single-stranded or double-stranded. Their replication and expression depend on mechanisms that differ from the ordinary DNA-to-RNA pathway of cellular transcription. Retroviruses, for example, use reverse transcriptase to make DNA from an RNA template. RNA’s role as viral genetic material is distinct from its usual cellular role, where DNA stores hereditary information and RNA helps express it. (genome.gov)

Historical development

Understanding RNA expanded from its role in protein synthesis to recognition of its catalytic and regulatory capacities. Several discoveries marked this development:

• 1977: Studies of split genes revealed that mature mRNA need not correspond to an uninterrupted stretch of genomic DNA, establishing the importance of RNA splicing.
• 1980s: Discoveries of catalytic RNA demonstrated that RNA could perform reactions previously associated with protein enzymes. Sidney Altman and Thomas Cech received the 1989 Nobel Prize in Chemistry for this work.
• 1998: Andrew Fire, Craig Mello, and colleagues reported that double-stranded RNA could efficiently trigger sequence-specific gene silencing; Fire and Mello received the 2006 Nobel Prize in Physiology or Medicine.
• 2023: Katalin Karikó and Drew Weissman received the Nobel Prize in Physiology or Medicine for discoveries concerning nucleoside base modifications that enabled effective mRNA vaccines against COVID-19.
• 2024: Victor Ambros and Gary Ruvkun received the same prize for discovering microRNA and its role in post-transcriptional gene regulation. (nobelprize.org)

Research and biotechnology

RNA sequencing measures RNA sequences and their abundance, providing information about which genes are expressed and which transcript forms are present. The collection of RNA molecules in a cell or sample is called its transcriptome. Unlike the genome, the transcriptome can differ substantially among cell types and physiological states. Single-cell RNA sequencing examines this variation at the level of individual cells. (genome.gov)

RNA also provides a platform for biotechnology. Engineered mRNA can supply temporary instructions for producing a selected protein; in mRNA vaccines, that protein serves as an antigen. Chemical modifications and delivery systems are important because introduced RNA may otherwise provoke innate immune responses or produce insufficient protein. Small-RNA silencing is also widely used experimentally to investigate gene function. (nobelprize.org)

Evolutionary significance and unresolved questions

The RNA world hypothesis proposes that an early stage of life depended on RNA both to carry hereditary information and to catalyze reactions, before the modern division of labor among DNA, RNA, and proteins developed. RNA’s combined informational and catalytic capacities, together with its central position in the ribosome, provide reasons for investigating this hypothesis. (ncbi.nlm.nih.gov)

The hypothesis is not an established account of how life began. Important unresolved issues include how RNA or its precursors formed under prebiotic conditions, how sufficiently accurate replication emerged, and how an RNA-based system developed into cells using DNA genomes and protein enzymes. Present-day RNA biology supplies evidence about molecular capabilities, but it does not by itself determine the historical sequence of life’s origins. (ncbi.nlm.nih.gov)

References

1. Ribonucleic Acid (RNA)genome.gov
2. Ribonucleic Acid (RNA) Fact Sheetgenome.gov
3. From DNA to RNA — Molecular Biology of the Cellncbi.nlm.nih.gov
4. RNA Structure, Function, and Recognitionibiology.org
5. Overview — RNA, the Epicenter of Genetic Informationncbi.nlm.nih.gov
6. Thomas R. Cech — Articlenobelprize.org
7. Award ceremony speech — The Nobel Prize in Chemistry 1989nobelprize.org
8. The Nobel Prize in Physiology or Medicine 2006 — Advanced informationnobelprize.org
9. The Nobel Prize in Physiology or Medicine 2024 — Advanced informationnobelprize.org
10. Genetics — Medical Microbiologyncbi.nlm.nih.gov
11. Press release: The Nobel Prize in Physiology or Medicine 2023nobelprize.org
12. The Nobel Prize in Physiology or Medicine 2024nobelprize.org