Question

[5]
(a) State the 'Central dogma' as proposed by Francis Crick. Are there any exceptions to it? Support your answer with a reason and an example.

(b) Explain how the biochemical characterisation (nature) of 'Transforming Principle' was determined, which was not defined from Griffith's experiments.

Answer 1

[2.5]
(a)



Yes, there are exceptions to it. In some viruses, flow of information is in the reverse direction in RNA viruses due to the presence of reverse transcription. e.g., HIV Also, some virus species are so primitive that they use only RNA $\to$ proteins, having not developed DNA. With the discovery of prions, a new exception to the central dogam has been discovered.
Protein $\to$ Protein.


[2.5]
(b) Protein, DNA and RNA were purified from heat killed S-strain of Streptococcus.

(i) Protein + Protease $\to$ transformation occured (R cell to S type).

(ii) RNA + RNAase $\to$ transformation occured (R cell to S type).

(iii) DNA + DNAase $\to$ transformation inhibited.
Hence DNA alone is the transforming material.