Question

How does bacteriophage lambda "decide" whether to enter the lytic or lysogenic cycle when it infects a bacterium and how does it execute the decision? Which of the statements below are correct?

a. In a population of healthy and vigorously growing bacteria cells it propagates lytically.

b. In a population of poorly growing bacteria cells it propagates lysogenically.

c. In the lysogenic state, the prophage genome is inactive (repressed).

d. In the lytic phase, the phage genes for reproduction are active (induced).

Answer 1

The bactriophage lambda is a virus that infects bacteria as part of its life cycle. It ha two modes of propogatio, namely:

1) the lytic cycle

2) the lysogenic cycle

The way the lambda phage decides which life cycle to choose depends on the following

There is a very important late transcribed protein called the cII protein which is the lynchpin of the decision.

In cells not under stress and with high nutrient conbtent protease activity is high this protease degrades cII. This causes induction of lysis cycle. In cells which are under stress and not healthy there is a low amount of protease making cII stable. This leads to the lysogenic lifestyle. cIII tabilizes cII, directly and also as a competitive inhibitor to the required proteases. This means that a cell having problems, lacking in nutrients and in dormancy , is more likely to follow the lysogenic lifecycle. The phage can now be dormant until the bacteria gets better and so the phage can create more copies of itself when there are more number of bacteria around.

As per this logic points no a and b are correct.

and c and d are wrong because in option C the prophage genome is active during lysogeny and in option D the phage reproduction genes are repressed by CRO protein.