Comprehension:
Restriction Enzymes
Read the following passage carefully and answer the given questions.
Genetic engineering is the combination of techniques to alter the chemistry of genetic material (DNA and RNA), to introduce these into host organisms and thus change the phenotype of the host organism. The first restriction endonuclease–Hind II, whose functioning depended on a specific DNA nucleotide sequence was isolated and characterised five years later. It was found that 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 recognise different recognition sequences. Restriction enzymes belong to a larger class of enzymes called nucleases namely exonucleases and endonucleases. The DNA fragments can be separated according to their size through sieving effect by gel electrophoresis. Hence, the smaller the fragment size, the farther it moves.
Choose the correct palindrome sequence of 5'-GAATTC-3'
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 3′-CTTAAG-5′
A palindrome in DNA is a sequence of base pairs that reads the same on the two strands when both are read in the same direction, that is from 5′ to 3′. The restriction enzyme EcoRI recognises such a site, 5′-GAATTC-3′.
(1) 3′-GAATTC-5′ — Incorrect. These are the same letters as the first strand, written in the opposite direction. The strand opposite to GAATTC is made of the paired bases, so it cannot have the same letters.
(2) 5′-AAGTTC-3′ — Incorrect. Pairing G with C and A with T gives CTTAAG for the opposite strand, not AAGTTC.
(3) 3′-CTTAAG-5′ — Correct. A pairs with T and G pairs with C, so the strand opposite to 5′-GAATTC-3′ is 3′-CTTAAG-5′. Read from 5′ to 3′, this strand is GAATTC again. So both strands read the same, and the site is a palindrome.
(4) 5′-CTTAAG-3′ — Incorrect. This is the complementary letters written in the wrong direction. Read from 5′ to 3′, it is CTTAAG, which does not read the same as GAATTC.
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) 2023 on 22-05-2023 (Slot 1).
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’”