Q5(e). DNA and RNA: nucleotides, location, functions

Introduction. Both are polynucleotides: each nucleotide is a nitrogenous base linked by an N-glycosidic bond to C-1' of a pentose, with a phosphate on C-5'; nucleotides are linked by 3',5'-phosphodiester bonds. They differ in the sugar, one base, the strandedness and their roles.

4pt4ptxltabular@l>X>X@ & DNA & RNA & DNA (continued) & RNA (continued) Sugar & 2'-deoxy-D-ribose (no 2'-OH) & D-ribose (2'-OH)* Bases & A, G, C, T & A, G, C, U Strands & two antiparallel strands, double helix (B form; A=T two, G≡C three H-bonds; Chargaff: A = T, G = C) & usually single strand, folded into local hairpins and loops; no Chargaff equality Stability & stable; resists alkali (no 2'-OH) & hydrolysed by dilute alkali (2'-OH attacks the phosphate) Size & very large (millions of base pairs) & smaller (about 75 nucleotides in tRNA to a few thousand in mRNA and rRNA) Location & mainly the nucleus (chromosomes); also mitochondria and chloroplasts & made in the nucleus (rRNA in the nucleolus); works mainly in the cytoplasm (ribosomes)* Function & stores and transmits genetic information; template for replication and transcription & protein synthesis: mRNA carries the code, tRNA brings amino acids, rRNA forms ribosomes and catalyses peptide bonds; regulatory and catalytic RNAs xltabular

Scheme: The two structural differences: 2'-OH in RNA; thymine (5-methyluracil) in DNA, uracil in RNA.

Conclusion. DNA: deoxyribose, thymine, double helix, in the nucleus, stores heredity. RNA: ribose, uracil, single strand, made in the nucleus and active in the cytoplasm, expresses the information as protein.

Application. The 2'-OH difference makes RNA labile, which is why RNA viruses mutate fast and mRNA vaccines need cold storage; DNA's stability allows its use in forensic fingerprinting and PCR.