Identify the correct palindromic sequence for Eco RI.

The correct answer is Option (2) → 5′—GAATTC—3′ 3′—CTTAAG—5′

A palindromic sequence in DNA reads the same on the two strands when both are read in the 5′ → 3′ direction. The restriction enzyme Eco RI recognises one particular palindrome, GAATTC, and cuts within it.

(1) 5′—GTTTTC—3′ 3′—CAAAAG—5′ — Incorrect. Read 5′ → 3′, the lower strand is GAAAAC. It differs from the upper strand GTTTTC, so this is not a palindrome.

(2) 5′—GAATTC—3′ 3′—CTTAAG—5′ — Correct. The lower strand read 5′ → 3′ is also GAATTC. This is the site of Eco RI.

(3) 5′—AAATTT—3′ 3′—TTTAAA—5′ — Incorrect. This is a palindrome, because the lower strand read 5′ → 3′ is also AAATTT. But it is not the sequence that Eco RI recognises.

(4) 5′—GAGTTC—3′ 3′—CTCAAG—5′ — Incorrect. Read 5′ → 3′, the lower strand is GAACTC. It differs from GAGTTC, so this is not a palindrome.

Note: A similar question was also asked in CUET (UG) 2026 on 14-05-2026 (Slot 2).

NCERT Extract:
Restriction Enzymes – “… As against a word-palindrome where the same word is read in both directions, the palindrome in DNA is a sequence of base pairs that reads same on the two strands when orientation of reading is kept the same. For example, the following sequences reads the same on the two strands in 5’ 3’ direction. This is also true if read in the 3’ 5’ direction. 5’ —— GAATTC —— 3’ 3’ —— CTTAAG —— 5’ Restriction enzymes cut the strand of DNA a little away from the centre of the palindrome sites, but between the same two bases on the opposite strands.”