(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.
Hint: Central dogma states that all the information required to make proteins are present in the DNA, whereas RNA acts as messenger to carry this information to the ribosomes, by which translation takes place. Griffith performed an experiment on the pneumonia bacteria and found the transforming principle.
Complete answer: a) Central dogma was proposed by Francis crick. It states that during the expression of the protein coding gene, the information passes from the DNA and from it is transferred to the RNA and finally to proteins. This flow of information during the formation of protein is called as central dogma of the molecular biology It is universally accepted, but there is one exception for this, it is Retrovirus, because in this RNA is transferred to DNA with the help of enzyme reverse transcriptase. b) Based on previous Mendel experiments, Griffith conducted the experiments on streptococcus pneumonia. Where they collect the streptococcus pneumonia and cultured the bacteria on cultured plates. While culturing they identified two strains, one is smooth shiny bacteria said to be S strain, and rough bacteria said to be R strain. The difference between the S and R is the presence of a mucous coat on S strain, which is absent in R strain. They injected into Mice and observed, Mice injected with S strain develops pneumonia, while Mice injected with R strain was alive. Later they heated the strains and injected and concluded that transferring things is the genetic material.
Note: Actually in the year of 1940 after the pandemic of Spanish flu, so many deaths are again noticed and it is identified that this is because of pneumonia, so Griffith started experiments to find out the vaccine for the pneumonia, during this research he identified DNA as the transforming material.