Question

How many of the following statements are correct?
I. If a linear DNA molecule is digested with a restriction enzyme at four recognition sites, 4 fragments would be produced.
II. If a closed circular DNA molecule is digested with a restriction enzyme at six recognition sites, 6 fragments would be produced.
III. If a linear DNA molecule is digested with a restriction enzyme at four recognition sites on the DNA, 5 fragments would be produced.
IV. If a closed circular DNA molecule is digested with a restriction enzyme at six recognition sites, 7 fragments would be produced.

• A. 1
• B. 2
• C. 3
• D. 4

Answer 1

The correct option is B 2

Restriction endonucleases are the enzymes that cleave the phosphodiester bonds present between the consecutive nucleotides within or near to specific DNA sequences that they identify. These identifiable sequences are known as recognition sequences.

• If a linear structure is cut into fragments, the number of fragments is always one greater than the number of cuts being done in it. If ‘n’ is the number of cuts made in the linear structure, ‘n+1’ would be the number of fragments produced.
• If a circular structure is cut into fragments, the number of fragments is always equal to the number of cuts being done in it. If ‘n’ is the number of cuts made in the circular structure, ‘n’ would be the number of fragments produced.

Here, the number of recognition sites signify the number of cuts being done by the restriction enzymes.

• If a linear DNA molecule is digested with a restriction enzyme at four recognition sites, 5 fragments would be produced.
• If a closed circular DNA molecule is digested with a restriction enzyme at six recognition sites, 6 fragments would be produced.

Thus, statements II and III are correct and statements I and IV are incorrect.