RNA

1. Chemical structure and distinguishing properties

RNA, or ribonucleic acid, is a polymer of nucleotides. Each nucleotide consists of a ribose sugar, a phosphate group and a nitrogenous base. The four principal bases are adenine, guanine, cytosine and uracil, abbreviated A, G, C and U. Adenine and guanine are purines; cytosine and uracil are pyrimidines. Nucleotides are linked by phosphodiester bonds, giving an RNA strand a chemical direction described as 5′ to 3′.

RNA differs from DNA in both sugar and base composition. DNA contains deoxyribose and generally uses thymine instead of uracil. RNA has a hydroxyl group at the 2′ carbon of its sugar, making it more susceptible to alkaline hydrolysis than DNA. This helps explain why DNA is generally better suited for stable, long-term storage of hereditary information in cellular organisms.

The statement that RNA is single-stranded is a useful generalisation, not an absolute rule. A single RNA strand can fold back on itself through complementary base pairing, producing stems, loops and three-dimensional structures. Some viruses possess double-stranded RNA genomes. Thus, single-stranded does not mean that an RNA molecule lacks paired regions or a definite structure.

• Common complementary pairing in RNA is A–U and G–C; G–U pairing also occurs in folded RNA.
• RNA occurs in the nucleus, cytoplasm, ribosomes, mitochondria and, in plants and algae, chloroplasts.

2. Major RNA types and their functions

Messenger RNA, or mRNA, carries protein-coding information transcribed from DNA. During translation, a ribosome reads its sequence in groups of three bases called codons. Each codon specifies an amino acid or a termination signal. Not every part of an mRNA molecule is translated: untranslated regions help regulate its stability, localisation and translation.

Transfer RNA, or tRNA, acts as an adaptor between the nucleotide sequence and amino acids. A tRNA contains an anticodon that recognises a corresponding mRNA codon and an attachment site for an amino acid. Aminoacyl-tRNA synthetases attach the appropriate amino acids to their corresponding tRNAs. Ribosomal RNA, or rRNA, forms the structural and catalytic core of ribosomes. The ribosome’s peptide-bond-forming activity is fundamentally RNA-catalysed.

Many other RNAs do not encode proteins. Small nuclear RNAs participate in the spliceosome, which removes introns from most eukaryotic precursor mRNAs. Small nucleolar RNAs help process and chemically modify rRNA. MicroRNAs and small interfering RNAs regulate gene expression, while long non-coding RNAs perform diverse regulatory functions. Classification as non-coding therefore does not mean biologically unimportant.

• mRNA provides the coding template; tRNA brings amino acids; rRNA helps assemble and catalyse protein synthesis.
• Ribozymes are RNA molecules with catalytic activity; not all enzymes are proteins.

3. RNA in gene expression and inheritance

In transcription, RNA polymerase uses a DNA template to synthesise RNA. For a protein-coding gene in a eukaryotic cell, the initial transcript commonly undergoes 5′ capping, splicing and addition of a 3′ poly-A tail. Splicing removes introns and joins exons. Alternative splicing can generate different mature transcripts from the same gene, helping increase protein diversity without a proportional increase in gene number.

Mature mRNA is generally exported from the nucleus to the cytoplasm, where ribosomes translate it. Translation includes initiation, elongation and termination. AUG usually functions as the start codon and codes for methionine, while UAA, UAG and UGA are stop codons in the standard genetic code. In bacteria, which lack a membrane-bound nucleus, transcription and translation can occur simultaneously.

The familiar information pathway is DNA to RNA to protein, but additional pathways exist. Retroviruses such as HIV use reverse transcriptase to produce DNA from RNA; this DNA can integrate into the host genome. Many RNA viruses copy RNA from an RNA template using RNA-dependent RNA polymerase. These mechanisms do not imply that sequence information normally flows from proteins back into nucleic acids.

• Transcription makes RNA; translation makes a polypeptide.
• Reverse transcription means RNA-to-DNA synthesis, not protein-to-RNA synthesis.

4. RNA viruses, evolution and laboratory detection

RNA serves as genetic material in numerous viruses. SARS-CoV-2, dengue virus and poliovirus have positive-sense single-stranded RNA genomes. Influenza viruses and rabies virus have negative-sense single-stranded RNA genomes, while rotaviruses have segmented double-stranded RNA genomes. Positive-sense RNA can function as mRNA; negative-sense RNA must first be copied into a complementary positive-sense form before translation.

Many RNA viruses evolve rapidly because their replication enzymes have limited proofreading. However, saying that all RNA viruses lack proofreading is incorrect: coronaviruses possess a proofreading mechanism. Genome segmentation also matters. Influenza viruses can exchange genome segments when different strains infect the same cell, a process called reassortment that can generate substantially altered viruses.

RNA is central to the RNA-world hypothesis, which proposes an early stage of evolution in which RNA performed both information-storage and catalytic roles. Catalytic RNA supports the hypothesis, but it is not a fully established reconstruction of life’s origin. In diagnostics, reverse-transcription PCR converts RNA into complementary DNA before amplification. Reverse transcription and real-time detection describe different features of a test, although they are often combined.

• A positive PCR result detects a target nucleic acid sequence; it does not by itself prove that infectious virus is present.
• RNA sequencing helps study gene expression and identify RNA transcripts.

5. RNA technologies: applications and limitations

RNA interference, or RNAi, is sequence-specific suppression of gene expression mediated by small RNAs. In a common pathway, double-stranded RNA is processed into small interfering RNAs. A guide strand directs the RNA-induced silencing complex to a complementary target mRNA, causing cleavage or repression. RNAi generally acts on gene expression rather than permanently changing the underlying DNA sequence.

In mRNA vaccination, delivered mRNA instructs host cells to produce an antigen that stimulates immune responses. Lipid nanoparticles commonly protect the RNA and assist cellular delivery. Conventional mRNA vaccines act in the cytoplasm, do not require entry into the nucleus and do not need to integrate into DNA. Self-amplifying RNA platforms additionally encode machinery that amplifies RNA within the cell, potentially allowing lower doses.

RNA technologies support medicines, crop protection and research. Guide RNA directs Cas proteins towards selected sequences in CRISPR systems, although the biological outcome depends on the particular system. Important limitations include RNA instability, delivery to the appropriate cells, unwanted immune activation and off-target effects. Storage requirements vary by formulation; all RNA products should not be assumed to require identical ultra-cold conditions.

• RNAi, mRNA vaccination and CRISPR are distinct technologies: gene silencing, antigen production and guided molecular targeting respectively.
• Safety, efficacy, delivery and manufacturing quality require evaluation for each product.

Previous year questions

No source-linked PYQ is available here yet. Use the practice questions below.

Practice questions