Search for the Genetic Material

How to write this in exams

1. 1

   Start with the exact idea

   The search for genetic material refers to experimental evidence showing that DNA carries hereditary information in most organisms, mainly through transformation and bacteriophage experiments.

2. 2

   Then show how to use it

   Answer in experiment sequence, not as a mixed story. For Griffith, name Streptococcus pneumoniae strains, state that live R strain became virulent only when mixed with heat-killed S strain, and conclude that a transforming principle moved from S to R cells. For Avery-MacLeod-McCarty, say that extracts were treated separately with protease, RNase, and DNase; transformation continued after protein or RNA destruction but stopped after DNA destruction, so DNA was identified as the transforming principle. For Hershey-Chase, mention separate labelling of protein with radioactive sulfur and DNA with radioactive phosphorus, infection of bacteria, blending to remove phage coats, centrifugation, and detection of DNA label in the bacterial pellet. End with the conclusion that DNA is the genetic material in most organisms, while RNA is genetic material in some viruses. Marks are often lost when students say heat-killed bacteria revived or when they omit the observation before the conclusion.

3. 3

   Add one concrete example

   In the Hershey-Chase experiment, phosphorus-labelled DNA was detected inside bacterial cells after phage infection, supporting that DNA, not the protein coat, entered and controlled new phage formation.

4. 4

   Avoid this incomplete answer

   Writing that heat-killed S bacteria came back to life in Griffith's experiment.