DNA was isolated from wild type (Gal+) and mutant (Gal-) E. coli cells and separated by density gradient centrifugation technique. DNA from Gal- strain acquired a lower position. This indicates that the mutation is caused by:
(a) deletion (b) insertion (c) mis-sense mutation (d) point mutation
Me and my friends had an extended discussion on this question and we all agreed that since the Gal- strain acquired a lower position, it was obviously more dense than Gal+ strain. Now, one of them claimed that if the volume were to remain constant, then mass is directly proportional to density hence the insertion mutation should cause increase in mass corresponding to a similar increase in density. I am not convinced by this explanation. They also said that, "Keeping the volume constant is a necessity since the experiment would lose its meaning otherwise". I am curious to know how far these assertions are correct.
After doing some research on this topic on the internet, I found a paper related to this question :
The answer given by the organizing committee of the 2nd Indian National Biology Olympiad was (b) insertion.
What could be the possible explanation for such an occurrence? Me and my friends are still unable to grasp the logic behind it even after going through different papers and books.