RNAs

Ribonucleic acids (RNA) are a class of nucleic acids present in all living cells. They are polymers of ribonucleotides linked by phosphodiester bonds. RNA differs from DNA in sugar (ribose vs deoxyribose), the presence of uracil instead of thymine, and typically a single-stranded conformation that can fold into complex structures through base pairing.

RNA types include messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA), which participate in protein synthesis; small nuclear RNAs (snRNA) involved in pre-mRNA splicing; small nucleolar RNAs (snoRNA) guiding rRNA modification; and regulatory small RNAs such as microRNAs (miRNA) and small interfering RNAs (siRNA) that control gene expression by base pairing with target transcripts. Piwi-interacting RNAs (piRNA) defend against transposable elements, and long non-coding RNAs (lncRNA) regulate transcription and chromatin structure. Some RNA molecules, called ribozymes, have catalytic activity.

RNA is transcribed from DNA by RNA polymerases. In eukaryotes, RNA polymerase II makes mRNA, RNA polymerase I makes most rRNA, and III makes tRNA and some other RNAs. Mature mRNA often undergoes capping, polyadenylation, and splicing; tRNAs and rRNAs are processed and modified. In prokaryotes, transcription and translation are coupled, and many RNAs lack introns.

RNAs participate in decoding genetic information, catalysis, regulation, and defense. They are central to gene expression and cellular regulation, and disruptions can contribute to disease. RNA-based technologies, including siRNA therapeutics and mRNA vaccines, illustrate practical applications.