Rungs and Skein

Rungs went first, because her half is the ladder. She ran her legs along the model: two rails twisting, and between them, rung after rung, each a pair — A with T, G with C. "This is what the code actually is," she said. "A long, long ladder of pairs — every letter holding hands with its one partner. That's my half: the ladder that holds the whole code, orderly all the way up." She stretched it out — and out, and out. "But look how long it is. All this, from just one cell. It's thinner than anything, but longer than your arm. Orderly, yes — but there's no way this fits inside a tiny cell just lying loose like this. It'd be a hopeless sprawl. Being an orderly ladder isn't enough to fit. That's Skein's half."

Skein made it fit. She took the end of Rungs' long ladder and began to wind it around her spool, loop after patient loop. "Rungs makes the orderly ladder — I wind it up so it fits," she said. "The same arm's-length of ladder, coiled tighter and tighter into a neat little bundle — a chromosome — small enough to tuck inside a cell." She held up a seed-sized wound bundle. "That's my half: the winding that makes the impossibly long fit the impossibly small." Then she nodded at Rungs' ladder. "But my winding only works because it's her ladder — orderly, paired, all in a line. If I tried to wind a random knotted mess, I'd just make a worse knot. It's because the ladder is neat and orderly that I can wind it up neat, and unwind exactly the piece we need. Her order is what makes my winding possible."

Then they did the move that is theirs — the one neither can do alone. They followed one length of code all the way: Rungs showed it was a long orderly ladder of pairs, and Skein wound that very ladder, loop by loop, into a small dense chromosome that tucked easily into a cell — and then, to read a gene, Skein unwound just that one stretch, and Rungs read its rungs, and wound it back. Long code, made to fit, still readable. They showed each half alone: Rungs' ladder unwound was a sprawl that fit nowhere; Skein's winding of a random mess was just a tighter knot. Orderly ladder and careful winding, together — and the whole code fit, and stayed readable. "My ladder alone is too long to fit," Rungs said. "My winding alone, without her order, is just knotting," Skein said. "Together — an arm's-length of code fits in a cell, and unwinds just where you need it." One long strand, between a millipede and a silkworm, tucked its whole vast code into a space too small to see.