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Which one of the following palindromic base sequences in DNA can be easily cut at about the middle by some particular restriction enzyme?
A
5′ CACGTA 3′; 3′ CTCAGT 5′
B
5′ CGTTCG 3′; 3′ ATGGTA 5′
C
5′ GATATG 3′; 3′ CTACTA 5′
D
5′ GAATTC 3′; 3′ CTTAAG 5′
Detailed Solution
A palindromic sequence in DNA is a sequence of base pairs that reads the same on the two strands when the orientation of reading is kept the same (5′ → 3′ on both).
5′ GAATTC 3′ / 3′ CTTAAG 5′: the upper strand read 5′ → 3′ is GAATTC, and the lower strand read 5′ → 3′ is also GAATTC. The two strands are complementary and the sequence is a palindrome.
This is the recognition sequence of EcoRI, which cuts between G and A on each strand, a little away from the centre of the palindrome, producing sticky ends.
In the other three pairs the two strands are not even complementary to each other, so they are not palindromes.
Hence the sequence that can be cut is 5′ GAATTC 3′; 3′ CTTAAG 5′.
5′ GAATTC 3′ / 3′ CTTAAG 5′: the upper strand read 5′ → 3′ is GAATTC, and the lower strand read 5′ → 3′ is also GAATTC. The two strands are complementary and the sequence is a palindrome.
This is the recognition sequence of EcoRI, which cuts between G and A on each strand, a little away from the centre of the palindrome, producing sticky ends.
In the other three pairs the two strands are not even complementary to each other, so they are not palindromes.
Hence the sequence that can be cut is 5′ GAATTC 3′; 3′ CTTAAG 5′.