Question

Which of the following statements regarding EcoR I is not correct?
I. It is isolated from Escherichia coli RY13.
II. Its recognition sequence is
5' GAATTC 3'
3' CTTAAG 5'
III. It produces complementary blunt ends.
IV. It was the first restriction endonuclease to be characterised.

• A. I and II
• B. II and IV
• C. III and IV
• D. I and IV

Answer 1

The correct option is C III and IV

Restriction endonucleases are the enzymes that cleave the phosphodiester bonds present between the consecutive nucleotides within or near to specific DNA sequences known as recognition sequences which are generally palindromic.

EcoR I is a restriction endonuclease. It has been isolated from the bacteria Escherichia coli RY13. It recognises the base sequence 5’ GAATTC 3’ in the double stranded DNA and makes the cuts between G and A. It results in the formation of DNA fragments with single stranded free ends. These single stranded free ends on either strands of a double stranded DNA are called sticky ends.

The figure below depicts the formation of two complementary sticky ends 3’-TTAA-5’ and 5’-AATT-3’.

The first restriction endonuclease discovered was Hind II.
When a double stranded DNA is digested by the restriction enzyme exactly in the middle of the recognition sequence such that there are no single stranded free ends (overhangs) produced in the ends of the fragments. These ends are called blunt ends.
Therefore, statements III and IV are incorrect.