Question

A linear DNA fragment of 5 kb gives 2.5 kb, 1.5 kb, and 1.0 kb fragments on complete EcoRI digestion; the same 5 kb fragment gives 3.5 kb and 1.5 kb fragments on complete XbaI digestion. What set of fragments will appear when the 5 kb piece is fully digested with EcoRI and XbaI simultaneously?

• A. 3 kb and 2 kb
• B. 2 kb and 1 kb
• C. 2 kb, 1.5 kb, 1 kb, and 0.5 kb
• D. 2.5 kb, 1.5 kb, 0.75 kb, and 0.25 kb

Hint:

For double digests on linear DNA, place each enzyme’s cut(s) on a 0–L axis and then sort all cut positions; the consecutive differences give the fragment sizes.

Answer 1

The Correct Option is C

Let the 5 kb fragment be represented as a line from 0 to 5.
EcoRI makes two cuts and produces three fragments of sizes \(2.5,\ 1.5,\ 1.0\ \mathrm{kb}\).
A consistent placement is at \(E_1 = 2.5\ \mathrm{kb}\) and \(E_2 = 4.0\ \mathrm{kb}\), giving segments: \(0 \to 2.5 = 2.5\), \(2.5 \to 4.0 = 1.5\), \(4.0 \to 5.0 = 1.0\).
XbaI makes a single cut that yields \(3.5\) and \(1.5\ \mathrm{kb}\), so the cut position is \(X = 3.5\ \mathrm{kb}\) (or equivalently \(1.5\ \mathrm{kb}\) from the other end).
When both enzymes act simultaneously, the ordered cut positions are \(0 < E_1 = 2.5 < X = 3.5 < E_2 = 4.0 < 5\).
Thus, the 2.5-kb EcoRI block is split into two pieces of \(2.0\) and \(0.5\ \mathrm{kb}\). The remaining EcoRI fragments remain \(1.5\ \mathrm{kb}\) and \(1.0\ \mathrm{kb}\).
Therefore, the final fragments obtained are: \(2.0,\ 1.5,\ 1.0,\ 0.5\ \mathrm{kb}\).