molecule grows from 23 to 72 nts.

fast: let molecule 400 arbitrary time units to fold per elongation step; if
      tanscription is complete molecule has 1e11 arbitrary time units to
      reach the Td equilibrium distribution.
      (time of end of trascription: 19600)

slow: let molecule 4000 arbitrary time units to fold per elongation step; if
      tanscription is complete molecule has 1e11 arbitrary time units to
      reach the Td equilibrium distribution.
      (time of end of trascription: 196000)