Final answer:
Double-stranded RNA (dsRNA) adopts an A-helical structure due to intramolecular hydrogen bonding. This structure is tighter and more compact than DNA's B-helical form and is crucial for RNA's stability and functional roles in biological processes.
Step-by-step explanation:
Double-stranded RNA (dsRNA) always adopts an A-helical structure. This is due to intramolecular hydrogen bonding between complementary nucleotides which stabilizes the structure. Unlike DNA, which typically has a B-helical form, RNA has a narrower, more tightly wound helix, the A-form, that is suitable for its functional roles within cells. The A-helical structure of dsRNA is essential for its involvement in various biological processes, and it plays a significant role in the functioning of many organisms. For example, in some viruses, dsRNA forms the basis of their genomes and is surrounded by a helical capsid, indicative of its stability and functional importance. Furthermore, in the CRISPR/Cas system, RNA guides (gRNA) base pair with DNA to direct Cas endonucleases to specific sites, illustrating the importance of RNA structure in genetic regulation. The ability of single stranded RNA to fold back on itself due to intramolecular hydrogen bonding, forming secondary structures such as the hairpin structure, underpins the versatility and functionality of RNA.