Restriction endonuclease
Read the following passage carefully and answer the given questions.
Hind II always cuts DNA molecules at a particular point by recognizing a specific sequence of six base pairs. Besides Hind II, today we know more than 900 restriction enzymes that have been isolated from over 230 strains of bacteria, each of which recognizes different recognition sequence. The convention for naming these enzymes is the first letter of the name comes from the genus and the second two letters come from the species of the prokaryotic cell from which they were isolated. Roman numbers following the names indicate the order in which the enzymes were isolated from that strain of bacteria. Restriction enzymes belong to a larger class of enzymes called nucleases. These are of two kinds: exonucleases and endonucleases. Exonucleases remove nucleotides from the ends of DNA, whereas endonucleases make cuts at specific positions within the DNA. Each restriction endonuclease functions by ‘inspecting’ the length of a DNA sequence. Once it finds its specific recognition sequence, it will bind to the DNA and cuts each of the two strands of the double helix at specific points in their sugar-phosphate backbones. Each restriction endonuclease recognizes a specific palindromic nucleotide sequence in the DNA. These are groups of letters that form the same word when read both forward and backward, e.g., “MALAYALAM”.
Which one of the following is a palindromic sequence?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 5′GAATTC3′ 3′CTTAAG5′
To test a palindrome, read the upper strand from 5′ to 3′. Then read the lower strand also from its 5′ end to its 3′ end, that is, from right to left. If both give the same sequence, it is a palindrome.
(1) 5′CCTTAA3′ 3′GGAATT5′ — Incorrect. Upper strand: CCTTAA. Lower strand read 5′ → 3′: TTAAGG. They are not the same.
(2) 5′GAATTC3′ 3′CTTAAG5′ — Correct. Upper strand: GAATTC. Lower strand read 5′ → 3′: GAATTC. Both read the same, so this is a palindrome. It is the recognition sequence of EcoRI.
(3) 5′CCCGGT3′ 3′GGGCCA5′ — Incorrect. Upper strand: CCCGGT. Lower strand read 5′ → 3′: ACCGGG. They are not the same.
(4) 5′TTAACC3′ 3′AATTGG5′ — Incorrect. Upper strand: TTAACC. Lower strand read 5′ → 3′: GGTTAA. They are not the same.
Note 1: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.
Note 2: A similar question was also asked in CUET (UG) 2026 on 24-05-2026 (Slot 1), and in CUET (UG) 2025 on 03-06-2025 (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’”