Assertion : Restriction endonuclease EcoRI cuts the DNA strand exactly at the centre of its palindromic recognition sequence. Reason : The cutting action of EcoRI leaves single-stranded overhanging stretches called sticky ends on each strand.

Step-by-step solution

Assertion is false because restriction endonuclease EcoRI cuts the DNA strand a little away from the centre of the palindromic recognition sequence, specifically between the G and A bases on both strands of the 5' -GAATTC- 3' sequence. Reason is true because this staggered cut leaves single-stranded overhanging stretches at the ends, which are called sticky ends. Answer: If assertion is false but reason is true.