Discussion
A linear DNA is 100% labelled at one end and has 3 restriction sites for EcoRI. If it is digested by EcoRI so that possible fragments are produced, how many of these fragments will be labelled and how many non-labelled ?
Answer: A
There will be 4 labelled and 6 unlabelled fragments.
Three EcoRI sites divide the molecule into 4 segments. 'Possible fragments' means every fragment a partial digest can give, any run of adjacent segments. With \(n\) segments the count is:
\[ \frac{n(n+1)}{2} = \frac{4 \times 5}{2} = 10 \]
A fragment is labelled only if it includes the labelled end. Those are the runs starting at that end: segment 1, segments 1-2, 1-3 and 1-4, so 4 are labelled and \(10 - 4 = 6\) are not.
A complete digest would give only 4 fragments, 1 labelled. Options with 6 labelled reverse the count, and 3 and 3 or 3 and 5 have no consistent total.
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