Search for the Genetic Material
The search for genetic material refers to experimental evidence showing that DNA carries hereditary information in most organisms, mainly through transformation and bacteriophage experiments.
In one line: Griffith saw the effect, Avery named the molecule, Hershey-Chase tracked its entry.
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- Griffith observed transformation in bacteria when harmless live R strain became virulent after exposure to heat-killed S strain material.
- Avery, MacLeod, and McCarty showed that the transforming principle was DNA because DNase destroyed transformation, while protease and RNase did not.
- Hershey and Chase used bacteriophages labelled with radioactive sulfur for protein and radioactive phosphorus for DNA.
- After infection and blending, labelled DNA entered bacterial cells and directed viral multiplication.
- These experiments established DNA as genetic material in most organisms, while RNA serves this role in some viruses.
Example
Rule to remember
Evidence rule: if destroying DNA prevents transformation but destroying protein or RNA does not, DNA is the hereditary material responsible for that effect.
See it worked out
Follow the example line by line, then use the same method on new questions.
Worked example
If a question says DNase treatment stopped bacterial transformation, infer that DNA was required for transformation and explain why protein was ruled out.
Method you can reuse
- 1Answer in experiment sequence, not as a mixed story.
- 2For 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.
- 3For 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.
- 4For 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.
- 5End with the conclusion that DNA is the genetic material in most organisms, while RNA is genetic material in some viruses.
- 6Marks are often lost when students say heat-killed bacteria revived or when they omit the observation before the conclusion.
Visual tip
A flowchart is useful: labelled phage infects bacteria, blender separates coats, centrifugation forms pellet and supernatant, radioactivity location is observed.
Check yourself
Answer the question in your head before opening the answer.
Try it yourself
Why did Hershey and Chase use radioactive sulfur and phosphorus?
Think first, then show answerHide answer
Common wrong answer
Writing that heat-killed S bacteria came back to life in Griffith's experiment.
Don't mix this up
Exam tip
Write it in the exam
Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.
How CBSE asks it
Answer plan
- 1
Enough for 1 mark
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
Add for 2 marks
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.
- 3
Add for 3 marks
Griffith observed transformation in bacteria when harmless live R strain became virulent after exposure to heat-killed S strain material.
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