DNA Types and Nuclease Enzymes — NEET Biology

Correct answer: D — A–ii–p, B–i–q, C–iii–s, D–iv–r

Let's match each item from Column I to its correct function in Column II and resulting product/feature in Column III: A. Exonuclease: These enzymes remove nucleotides from the ends of the DNA (Column II - ii). This action typically results in blunt ends (Column III - p) if both strands are removed evenly, or can create specific single-stranded overhangs depending on the enzyme. The combination A-ii-p is correct. B. Endonuclease: These enzymes make cuts at specific positions within the DNA (Column II - i). Restriction endonucleases (a type of endonuclease) can generate either sticky or blunt ends (Column III - p), or specifically DNA fragments with specific overhangs (sticky ends) (Column III - q). Given the options, B-i-q is a more specific and accurate match as restriction endonucleases are key in rDNA technology for generating sticky ends. While p is also possible, q highlights the common outcome used in cloning. The combination B-i-q is correct. C. DNA ligase: This enzyme acts on cut DNA molecules and joins their ends by forming phosphodiester bonds (Column II - iii). This linking of DNA fragments is essential for creating a new combination of circular autonomously replicating DNA, known as recombinant DNA (Column III - s). The combination C-iii-s is correct. D. EcoRI: This is a restriction endonuclease that recognises a specific palindromic nucleotide sequence (Column II - iv). It cuts DNA a little away from the centre of the palindrome sites, leaving single-stranded portions called sticky ends that are complementary (Column III - r) to other fragments cut with the same enzyme. The combination D-iv-r is correct. Therefore, the correct match is A–ii–p, B–i–q, C–iii–s, D–iv–r. The core concept tested here is the role and specific actions of various enzymes and molecules crucial for recombinant DNA technology, as described in NCERT Chapter 9.