Restriction endonucleases
Read the following passage carefully and answer the given question.
The cutting of DNA at specific locations became possible with the discovery of the so-called ‘molecular scissors’– (restriction enzymes). They cut DNA molecule at particular points by recognition a specific sequence of six base pairs. The piece of DNA was then linked with the plasmid DNA. These plasmid DNA act as vectors to transfer the piece of DNA attached to it. 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. Each restriction endonuclease recognizes a specific palindromic nucleotide sequences in the DNA. Palindromes 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 following sequences indicates:
5′-GAATTC-3′
3′-CTTAAG-5′
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Palindromic sequence
A word palindrome, such as MALAYALAM, reads the same forward and backward. Here the upper strand read 5′ → 3′ is GAATTC. The lower strand read 5′ → 3′ is also GAATTC. So the sequence is palindromic. Each restriction endonuclease recognises such a sequence. GAATTC is the recognition site of EcoRI.
(1) Palindromic sequence — Correct. Both strands read GAATTC in the 5′ → 3′ direction.
(2) Repetitive sequence — Incorrect. Repetitive DNA is a short stretch repeated many times, one copy after another. It is the basis of DNA fingerprinting. The given sequence is not repeated.
(3) Initiation sequence — Incorrect. An initiation sequence marks where replication or transcription starts, such as the origin or the promoter.
(4) Terminating sequence — Incorrect. A terminator marks where transcription ends.
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.”