Q.44 X different sized DNA fragments can be observed upon incomplete EcoRI digestion of identical DNA
molecules with two EcoRI sites as shown below. The maximum value of X is ____ .

EcoRI digestion of linear DNA with two specific sites produces exactly 3 different sized fragments. The diagram shows two EcoRI sites separating 500 bp segments, with the maximum value of X being 1500 bp for distinct fragment sizes. This MCQ tests understanding of restriction enzyme action on linear DNA molecules.​

EcoRI Enzyme Basics

EcoRI is a type II restriction endonuclease that recognizes the palindromic sequence 5′-GAATTC-3′ and cuts between G and A, producing sticky ends. It cleaves DNA at these sites, fragmenting the molecule into pieces whose sizes depend on site positions. For linear DNA, n cut sites yield n+1 fragments.​

Diagram Analysis

The diagram depicts linear DNA with EcoRI sites at positions creating three labeled 500 bp segments and an unlabeled X bp arc at one end. Cuts occur at the two EcoRI sites, yielding fragments of 500 bp, 500 bp, and (X + 500) bp. To observe 9 different sized fragments is impossible with only two sites, as maximum fragments are 3, potentially 2 distinct sizes if equal.​

Correct Answer: 3 Fragments

With two EcoRI sites on linear DNA, complete digestion always produces 3 fragments regardless of distances. Here, two are 500 bp each (identical size), and the third is X + 500 bp, totaling 3 pieces but possibly 2 bands on gel if X + 500 ≠ 500 bp. The question specifies different sized fragments observed, but standard interpretation confirms 3 total observable pieces post-digestion.​

Maximum X Value

For 9 different sized fragments, the setup cannot produce that; maximum distinct sizes with two sites is 3 (if all unequal). Maximum X ensuring distinct sizes (500, 500, X+500 all different? Wait, two 500s same) but to have 3 distinct? Impossible with two 500s. Likely, X=1500 bp makes third 2000 bp, distinct from 500 bp, maximizing total length ~2500 bp for 2 distinct sizes. Exact max X for distinct observation is when X+500 ≠ 500, so X ≠ 0, but context implies largest X for 3 observable (though 2 sizes).​

Options Explained

• 3 fragments: Correct, as two sites on linear DNA yield 3 pieces (500, 500, X+500 bp).​

• 2 fragments: Incorrect for linear; applies to circular DNA where two sites give 2.​

• 1 fragment: No cuts, but two sites ensure cutting.​

• 9 fragments: Impossible; requires 8 sites for linear DNA.​