The twist was the answer. DNA's paired strands, wound around a common axis, made heredity legible because each base faced a specific partner across the helix: adenine with thymine, guanine with cytosine, their pairing faces aligned inside sugar-phosphate rails. Chemistry set the rules.
Once hydrogen bonds identified the permitted partners, the molecule did not need an outside clerk to remember the sequence; either exposed strand could direct construction of its mate. The strands part. Their antiparallel orientation lets DNA polymerase read a template in a defined direction, adding nucleotides to a growing strand through phosphodiester bonds along its sugar-phosphate backbone. Errors had consequences. Mismatch repair could inspect departures from pairing rules, making the structure a working account of fidelity rather than an attractive sketch.
No master blueprint existed. In semiconservative replication, each daughter molecule retains one parental strand and gains one newly synthesized complement, so sequence becomes both message and instruction. That was the trick. At a replication fork, helicase opens the duplex while polymerases copy exposed bases, converting molecular form into a repeatable process. The cost was asymmetry. The opposed directions of the strands require continuous synthesis on one and discontinuous synthesis on the other, a prediction that forced the copying story into experimental reach. A spiral became grammar.
