The helix clock: a potential biomechanical cell cycle timer

A model based upon helical geometry that provides cylindrically shaped cells with a means to measure their length during growth and to time cell cycle events is presented. The helix clock arises from the change in pitch angle that accompanies the parallel packing of strands on a cylinder surface. A single strand inserted into the cylinder surface nearly perpendicular to the long axis of the cylinder starts the clock running. As additional strands are inserted parallel to those in place, the pitch angle of all strands must reorient. A limit is reached when all strands lie parallel to the long axis of the cylinder. By sensing either the pitch angle or a physical ramification thereof, cells can measure their length during growth and time events of the cell cycle. The helix clock model is discussed in relationship to the bacterial cell cycle. The idea that bacterial cells use one helix hand for cylinder elongation, the other for septation is presented. The negative twist so generated apparently drives folding in the helical bacterial macrofiber system of Bacillus subtilis.