Read the following passage carefully and answer the given questions.
Restriction endonuclease on finding its specific recognition sequence, 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 recognises a specific palindromic nucleotide sequences in the DNA. Since the DNA is enclosed within the membranes, we have to break the cell open to release DNA along with other macromolecules such as RNA, proteins, polysaccharides and also lipids. This can be achieved by treating the bacterial cells/plant or animal tissue with enzymes such as lysozyme (bacteria), cellulase (plant cells), chitinase (fungus). Now a days the most commonly used matrix is agarose which is a natural polymer extracted from sea weeds. The DNA fragments separate (resolve) according to their size through sieving effect provided by the agarose gel. 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. In addition to ‘ori’, the vector requires a selectable marker, which helps in identifying and eliminating non-transformants and selectively permitting the growth of the transformants. Transformation is a procedure through which a piece of DNA is introduced in a host bacterium. Normally, the genes encoding resistance to antibiotics such as ampicillin, chloramphenicol, tetracycline or kanamycin, etc., are considered useful selectable markers for E. coli.
Each restriction endonuclease recognises a specific ______________________ in the DNA.
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Palindromic nucleotide sequences
The passage says that each restriction endonuclease recognises a specific palindromic nucleotide sequence in the DNA. A DNA palindrome reads the same on the two strands when both are read in the same direction (5′→3′), for example 5′-GAATTC-3′ for EcoRI.
Option (1) Homologous nucleotide sequences — Incorrect. Homologous sequences are similar sequences in different DNAs; they are not what the enzyme recognises.
Option (2) Non-palindromic nucleotide sequences — Incorrect. This is the opposite of the right answer.
Option (3) Inverted nucleotide sequences — Incorrect. An inverted sequence is only a reversed stretch; it need not read the same on both strands, which is what the enzyme looks for.
Option (4) Palindromic nucleotide sequences — Correct. Each restriction endonuclease recognises a specific palindromic nucleotide sequence in the DNA: a sequence that reads the same on the two strands when read in the same direction (5′→3′), as in 5′-GAATTC-3′.
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.