Molecular scissors
Read the following passage carefully and answer the given questions.
Hind II always cut DNA molecules at a particular point by recognising a specific sequence of six base pairs. This specific base sequence is known as the recognition sequence for Hind II. Besides Hind II, today we know more than 900 restriction enzymes that have been isolated from over 230 strains of bacteria each of which recognises different recognition sequences. 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, e.g., EcoRI comes from Escherichia coli RY 13. Restriction enzymes belong to a larger class of enzymes called nucleases. These are of two kinds: exonucleases and endonucleases. 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 cut each of the two strands of the double helix at specific points in their sugar-phosphate backbones.
Identify the specific palindromic nucleotide sequences for EcoRI.
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → GAATTC
Each restriction enzyme recognises a specific palindromic sequence in DNA. A DNA palindrome is a sequence of base pairs that reads the same on the two strands when both are read in the same direction (5′ → 3′). EcoRI recognises 5′-GAATTC-3′. To test a sequence, write the complementary strand and read it 5′ → 3′:
5′-GAATTC-3′
3′-CTTAAG-5′, which read 5′ → 3′ is GAATTC. The two strands read the same.
(1) GTTAAC — Incorrect. Its other strand also reads GTTAAC, so it is a palindrome. But it is not the sequence that EcoRI recognises.
(2) GATAAC — Incorrect. Its other strand reads GTTATC. The two strands differ, so it is not a palindrome.
(3) GCTTAC — Incorrect. Its other strand reads GTAAGC. It is not a palindrome.
(4) GAATTC — Correct. This is the palindrome recognised by EcoRI.
Note: 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.
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.”