Q.What is genetic engineering? How does it differ from conventional hybridisation used by plant and animal breeders to create genetic variation?

Enzymes are the workhorses of molecular biology, acting as catalysts for a vast array of biochemical reactions within living cells. In the realm of genetic engineering and biotechnology, specific enzymes are indispensable tools, allowing scientists to manipulate DNA with precision. Understanding the distinct roles of these molecular instruments is fundamental to comprehending processes like recombinant DNA technology and Polymerase Chain Reaction (PCR).

Let's delve into the functions of the enzymes listed:

• DNA Ligase: This enzyme plays a crucial role in joining DNA fragments. When DNA is cut, either naturally during replication and repair or artificially during genetic engineering, there are breaks in the phosphodiester backbone. DNA ligase forms phosphodiester bonds between the 3'-hydroxyl end of one nucleotide and the 5'-phosphate end of another, effectively sealing these nicks. In recombinant DNA technology, it is used to join a desired DNA fragment (e.g., a gene of interest) into a vector plasmid.

  Important

  DNA ligase is often referred to as 'molecular glue' because it covalently links DNA fragments together.

• Restriction Exonuclease: The term 'exonuclease' itself provides a strong clue to its function. 'Exo' means outside or end. An exonuclease is an enzyme that cleaves nucleotides one at a time from the end (either 3' or 5') of a polynucleotide chain. It progressively removes nucleotides from the terminal positions of a DNA molecule. This is distinct from endonucleases, which cut within the DNA strand.

• Taq Polymerase: This is a DNA polymerase enzyme isolated from the thermophilic bacterium Thermus aquaticus. Its key characteristic is its heat stability, meaning it can withstand high temperatures without denaturing. This property makes it invaluable in the Polymerase Chain Reaction (PCR), where cycles of heating and cooling are required. Its primary function is to synthesize new DNA strands by extending primers on a DNA template, using deoxyribonucleotides as building blocks.

  Note

  The heat stability of Taq polymerase is critical for PCR, as it allows the enzyme to remain active even after the high-temperature denaturation step that separates DNA strands.

• Restriction Endonuclease: These enzymes are often called 'molecular scissors' because they cut DNA at specific internal positions. 'Endo' means inside, indicating they cleave phosphodiester bonds within a polynucleotide chain, not just at the ends. Each restriction endonuclease recognizes a specific palindromic nucleotide sequence (a recognition site) on the DNA and cuts both strands of the DNA helix at or near that site. These cuts can result in either 'sticky ends' (overhangs) or 'blunt ends'. They are fundamental to genetic engineering for cutting DNA into manageable fragments.

  Important

  Restriction endonucleases are highly specific, recognizing and cutting only at particular DNA sequences. This specificity is what makes them such precise tools for genetic manipulation.

Based on these explanations, we can match the enzymes with their functions:

• P. DNA Ligase matches with iii. Joins the DNA fragments.
• Q. Restriction exonuclease matches with i. Removes nucleotides from ends of DNA.
• R. Taq polymerase matches with ii. Extends primer on a DNA template.
• S. Restriction endonuclease matches with iv. Cuts DNA at a specific position.

Therefore, the correct option is (d).