DNA and RNA Structure

DNA's two strands run antiparallel, one 5' to 3' and the other 3' to 5', and they wind into a double helix because that twisting packs the bulky nitrogenous bases into the protected core while the charged sugar-phosphate backbones face outward toward the watery cell interior. The backbone is a repeating chain of deoxyribose sugars linked by phosphate groups; each sugar also holds one of four bases pointing inward, where A pairs with T and G pairs with C through hydrogen bonds. That pairing and the helix's twist let DNA store information densely and stay chemically stable for the long haul, which is exactly what a permanent genetic archive needs.

RNA breaks from that model in three linked ways: it is usually single-stranded, it uses ribose instead of deoxyribose, and it swaps thymine for uracil. The ribose sugar carries an extra reactive hydroxyl group on its 2' carbon that deoxyribose lacks, and that hydroxyl makes RNA's backbone more prone to breakdown. Being single-stranded removes the protective pairing that shields DNA's bases, so RNA degrades faster. None of that is a flaw: RNA's job is to carry instructions briefly from gene to ribosome, not to store them for a lifetime, so instability suits its transient role.

The antiparallel structure also explains a quirk of copying DNA. Polymerase only extends a chain by adding to a free 3' end, working 5' to 3'. Since the two template strands point opposite directions, one new strand extends continuously toward the opening replication fork, while the other must be built backward, away from the fork, in short fragments that get stitched together as the fork keeps opening. This sets up the leading and lagging strand mechanics covered next.