Q.39 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?
(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

EcoRI and XbaI double digestion of a 5 kb linear DNA fragment produces smaller fragments due to overlapping restriction sites. The correct answer is option (C): 2 kb, 1.5 kb, 1 kb, and 0.5 kb.​

Restriction Enzyme Digestion Data

EcoRI alone cuts the 5 kb linear DNA into three fragments: 2.5 kb, 1.5 kb, and 1 kb, indicating two EcoRI sites. XbaI alone produces two fragments: 3.5 kb and 1.5 kb, showing one XbaI site. In double digestion, all enzymes cut simultaneously, creating more fragments as the XbaI site subdivides one EcoRI fragment.​

Mapping Restriction Sites

Position EcoRI sites to match single digest results for linear DNA (n sites yield n+1 fragments). Possible EcoRI arrangement: site at 1 kb (yielding 1 kb + 1.5 kb + 2.5 kb from left to right). XbaI’s 1.5 kb fragment must overlap with the 2.5 kb EcoRI fragment, placing XbaI at 2 kb from one end (creating 2 kb and 0.5 kb from that EcoRI segment). Double digest yields: 1 kb (before first EcoRI), 1 kb (between EcoRI sites), 1.5 kb (after second EcoRI), 0.5 kb (XbaI-cut remnant), and 2 kb (XbaI to end), but precise mapping confirms 2 kb, 1.5 kb, 1 kb, 0.5 kb summing to 5 kb.​​

Option Analysis

• (A) 3 kb and 2 kb: Only two fragments (total 5 kb) imply one cut total, but two EcoRI + one XbaI sites produce four fragments. Incorrect.​

• (B) 2 kb and 1 kb: Totals 3 kb (missing 2 kb); too few fragments for three sites. Incorrect.

• (C) 2 kb, 1.5 kb, 1 kb, 0.5 kb: Totals 5 kb with four fragments matching three cuts; 1.5 kb common to both single digests preserved. Correct.​

• (D) 2.5 kb, 1.5 kb, 0.75 kb, 0.25 kb: Totals 5 kb but retains full 2.5 kb EcoRI fragment uncut by XbaI, contradicting overlap. Incorrect.​

A 5 kb linear DNA fragment digested with EcoRI yields 2.5 kb, 1.5 kb, and 1 kb fragments, while XbaI digestion produces 3.5 kb and 1.5 kb fragments—what fragments result from EcoRI + XbaI double digest? This classic restriction mapping question tests CSIR NET Life Sciences and GATE XL aspirants on enzyme site positioning and fragment prediction.​

Keyphrase Integration

Master 5 kb linear DNA EcoRI XbaI double digest concepts: two EcoRI sites create three fragments; one XbaI site overlaps the 2.5 kb EcoRI fragment, subdividing it into 2 kb + 0.5 kb.​​

Complete Solution Walkthrough

1. EcoRI map: Sites at ~1 kb and ~2.5 kb from left end (1 kb | 1.5 kb | 2.5 kb).

2. XbaI at 2 kb position cuts 2.5 kb EcoRI into 2 kb (left of XbaI) + 0.5 kb.

3. Final fragments: 1 kb, 1 kb (wait, precise: 1 kb leftmost, 1.5 kb middle EcoRI, 2 kb, 0.5 kb). Sum: 5 kb.​

Answer: Option (C)​