Question

Which of the following statements are correct?
(i) Restriction enzymes may cut the DNA strand a little away from the centre of the palindromic sequence, but between the same two bases on the opposite strands.
(ii) Hind ll always cuts DNA molecules at a particular site by recognising a specific sequence of six base pairs.
(iii) DNA is a positively charged molecule.

• A. (i) and (iii)
• B. (i) and (ii)
• C. (i), (ii) and (iii)
• D. (ii) and (iii)

Answer 1

The correct option is B (i) and (ii)

Restriction enzymes are the endonucleases that cleave the phosphodiester bonds present between the nucleotides at the specific position within or near to the specific DNA sequences identified by them, known as recognition sites. These recognition sites are usually palindromic in nature. Palindromic sequences are those DNA sequences that are read the same when the polarity is kept the same in both the strands.

• Some restriction enzymes cut the DNA strand a little away from the centre of the palindrome site, but between the same two bases on the opposite strands. This results in the formation of complementary sticky ends (single stranded overhangs).
• Some restriction enzymes make cuts in the middle of the recognition site and produce double stranded ends called blunt ends.

For example, EcoR I is a restriction endonuclease enzyme isolated from the bacteria Escherichia coli RY13. It recognises the base sequence 5’ GAATTC 3’ in the double stranded DNA and makes the cuts a little away from its centre at a specific point between G and A.
The figure given below shows the functioning of the restriction enzyme EcoR I:

The first restriction endonuclease isolated was Hind II. It was isolated from Haemophilus influenzae. It recognises a specific sequence of 6 base pairs.

DNA is a negatively charged molecule. This negative charge is due to the phosphate groups present in it.

Thus, statements (i) and (ii) are correct.