TWIST · WRITHE · REPLICATE
Where does the twist go?
Constrain a linear DNA molecule, move either replication fork, and rotate the junction. Watch linkage move between supercoils ahead and precatenanes behind without changing DNA contour length.
RELAXED STARTING STATE
A relaxed duplex spans two rotationally fixed tethers
Add a turn, or move a replication fork, to create a topological constraint.
Magnifiers resolve crossings; TOP1 and TOP2 callouts mark their duplex-supercoil substrate ahead.
APPLY TOPOLOGY
Rotate the right tether
Viewed from the left tether toward the right: rightward overwinds (+); leftward underwinds (−).
MOVE THE JUNCTIONS
Replicate from either side
Each step unwinds one modeled turn and traps it in the duplex ahead.
CHANGE DUPLEX TOPOLOGY
Use a topoisomerase
Both act only on supercoiled parental duplex ahead; fork ssDNA is excluded. One TOP2 click repeats ΔLk = 2 passages to the 0/±1 endpoint.
Fixed boundariesLinking number cannot escape through either end.
Fork migrationParental unwinding generates positive supercoils ahead.
Fork rotationThe junction rolls around the DNA x axis; linkage moves behind.
GUIDED CHECK
Can you redistribute three turns?
Advance the left fork three times, then rotate it twice. Finish with +1 ahead, +2 behind, and +3 introduced.
Topology conserved. Fork rotation redistributed linking number; it did not remove it.