Which of the following is a complementary sequence of 5' ---- GAATTC…

The correct answer is Option (3) → 3'--- CTTAAG--- 5'

Bases pair A with T and G with C, and the complementary strand runs the opposite way. For 5′ GAATTC 3′ the complementary strand is 3′ CTTAAG 5′.

Option (1) 3'--- GAATTC --- 5' — Incorrect. It repeats the same bases instead of pairing them.

Option (2) 5' --- CTTAAG --- 3' — Incorrect. The bases are right, but the strand must run 3′ to 5′.

Option (3) 3'--- CTTAAG--- 5' — Correct. In DNA, A pairs with T and G with C, and the two strands run antiparallel. So the strand complementary to 5′-GAATTC-3′ is 3′-CTTAAG-5′.

Option (4) 5'----GAATTC ---- 3' — Incorrect. This is the original strand itself.

Note: A similar question was also asked in CUET (UG) 2026 on 29-05-2026 (Slot 2), and in CUET (UG) 2023 on 13-06-2023 (Slot 3).

NCERT Extract:
Restriction Enzymes – “… Each restriction endonuclease recognises a specific palindromic nucleotide sequences in the DNA.”
Restriction Enzymes – “… These are groups of letters that form the same words when read both forward and backward, e.g., “MALAYALAM”. 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’”