Read the passage and answer the following question :
In 1963, enzymes that restricted the growth of bacteriophage in Escherichia coli were discovered. Among them, restriction endonucleases played a vital role. Five years later, Hind II, the first restriction endonuclease, was isolated and characterized. Hind II was known for its ability to cut DNA molecules at a specific point by recognizing a unique six-base pair sequence, which was termed the "recognition sequence" for Hind II. Currently, over 900 restriction enzymes have been identified from more than 230 bacterial strains, each recognizing different recognition sequences. For example, EcoRI is derived from Escherichia coli RY 13, where the letter 'R' represents the name of the strain, and the Roman numeral indicates the order of enzyme isolation from that strain. Restriction enzymes belong to a larger class of enzymes that can be divided into two types: exonucleases and endonucleases. Exonucleases remove nucleotides from the ends of DNA, while endonucleases make cuts at specific positions within the DNA. Each restriction endonuclease "inspects" the length of a DNA sequence until it finds its specific recognition sequence. Once located, it binds to the DNA and cuts both strands of the double helix at specific points in their sugar-phosphate backbones.Notably, each restriction endonuclease recognizes a specific palindromic nucleotide sequence in the DNA.
Which of the following double-stranded DNA sequences is a palindrome?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 5′–GGATCC–3′
3′–CCTAGG–5′
A word palindrome, such as MALAYALAM, reads the same forward and backward. A palindrome in DNA is different: it is a stretch of base pairs that reads the same on the two strands when both are read in the same direction, 5′ → 3′. To test an option, read the upper strand from 5′ to 3′, then read the lower strand from its own 5′ end (right to left).
Option (1) 5′–GAATCC–3′
3′–CTTAGG–5′ — Incorrect. The upper strand reads GAATCC, but the lower strand read from its 5′ end gives GGATTC. The two do not match.
Option (2) 5′–GATTAG–3′
3′–CTAATC–5′ — Incorrect. GATTAG reads the same backward on the upper strand, so it is only a word-type palindrome. The lower strand read 5′ → 3′ gives CTAATC, which is not the same.
Option (3) 5′–GGATCC–3′
3′–CCTAGG–5′ — Correct. The upper strand reads GGATCC, and the lower strand read from its 5′ end also gives GGATCC. Both strands read alike, so this is a DNA palindrome.
Option (4) 5′–GTTAAG–3′
3′–CAATTC–5′ — Incorrect. The upper strand reads GTTAAG, while the lower strand read 5′ → 3′ gives CTTAAC. The two strands differ.
NCERT Extract:
Restriction Enzymes – “… Do you know what palindromes are? 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′”