Discussion
During in vitro DNA synthesis which of the following radionucleotide is most preferred ?
Answer: A
The preferred label among these is \(^{3}H\) labelled dTTP.
When DNA polymerase adds a nucleotide, it attaches the innermost (alpha) phosphate to the growing chain and releases the beta and gamma phosphates as pyrophosphate. A tritium label on the base of dTTP is carried into the new DNA, so the amount of radioactivity in acid-insoluble material measures DNA synthesis.
Gamma- and beta-labelled dTTP fail for the reason above: their label leaves in the pyrophosphate and never enters the DNA. (The alpha-labelled nucleotide would work, but it is not offered.) dUTP is not a normal substrate for DNA synthesis, since uracil is excluded from DNA, so carbon-14 labelled dUTP is not the reagent of choice either.
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