Phases and states কী?

In a fresh flask (a batch culture) the cells first pause — the lag phase, while they induce the enzymes the new medium needs — then grow exponentially, then stop when a nutrient is exhausted or a waste accumulates: the stationary phase, in which the cells shrink, thicken their envelope, condense their nucleoid around protective proteins, switch on stress genes through the alternative sigma factor RpoS, and can survive for months; a death phase follows in which most cells lyse and a few live on the remains. Some species answer starvation with a developmental programme: Bacillus subtilis divides asymmetrically and the larger cell engulfs the smaller and builds the spore around it, a sequence of eight hours controlled by a cascade of sigma factors — the subunits that direct RNA polymerase to one set of promoters — alternating between the two compartments, the first bacterial example of differentiation and of cell-to-cell signalling through a wall.