Discussion
A linear DNA fragment of 5 kilobase (kb) when completely digested with EcoRI produces 2.5 kb, 1.5 kb, and 1 kb fragments. Complete digestion of the same 5 kb fragment with XbaI produces 3.5 kb and 1.5 kb fragments. Which one of the following sets of fragments will be obtained if the 5 kb fragment is fully digested with EcoRI and XbaI simultaneously?
Answer: C
The double digest gives 2 kb, 1.5 kb, 1 kb, and 0.5 kb.
EcoRI cuts the 5 kb fragment twice (three pieces), and XbaI cuts it once, 1.5 kb from one end. Place the EcoRI pieces in the order 1, 2.5, 1.5 kb, so EcoRI cuts at 1 kb and 3.5 kb. An XbaI cut at 1.5 kb from the left end then falls inside the 2.5 kb piece and splits it into 0.5 and 2 kb.
\[ 1 + 0.5 + 2 + 1.5 = 5\ \text{kb} \]
Whatever the order of the pieces, a double digest can never give fewer fragments than EcoRI alone, so the two-fragment answers (3 and 2 kb, 2 and 1 kb) are impossible. Every cut lies a whole number of 0.5 kb steps from the ends (the pieces are 1, 1.5, 2.5 and 3.5 kb), so pieces of 0.75 and 0.25 kb cannot arise either. The set 2, 1.5, 1 and 0.5 kb is the only option that fits both single digests.
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