Q.Which of the following is not required in the preparation of a recombinant DNA molecule?

Preparing a recombinant DNA molecule is an in vitro process that happens in a test tube, not inside a living cell. You need three molecular tools to physically construct the recombinant molecule:

• Restriction endonuclease – cuts both the vector DNA and the foreign DNA at specific recognition sites, generating compatible sticky or blunt ends

• DNA fragments – the foreign gene or DNA segment you want to insert into the vector

• DNA ligase – seals the sugar-phosphate backbone by forming phosphodiester bonds between the vector and inserted fragment, completing the recombinant molecule …

The question asks us to distinguish between what is needed to physically construct a recombinant DNA molecule and what comes into play only later, when we want to use that molecule. Think of it as the difference between building a car in a factory and then driving it on the road — the road isn't part of the manufacturing process.

Recombinant DNA technology hinges on cutting and pasting DNA from different sources. To create a recombinant DNA molecule in vitro (in the test tube), you need three essential ingredients working together.

DNA fragments are the raw material — the foreign gene or DNA sequence you want to insert, and the vector (plasmid or phage DNA) that will carry it. Without these pieces, there's nothing to recombine.

Restriction endonucleases are the molecular scissors. These enzymes recognize specific palindromic sequences in DNA and cut both strands, often leaving "sticky ends" — short single-stranded overhangs. When you cut your gene of interest and your vector with the same restriction enzyme, their sticky ends are complementary and can base-pair with each other. This is the recognition and cutting step that makes recombination possible.

DNA ligase is the molecular glue. Once the sticky ends of the foreign DNA and vector have annealed (paired up), ligase seals the sugar-phosphate backbone by forming phosphodiester bonds. Without ligase, you'd have DNA fragments merely hydrogen-bonded together, not covalently joined into a stable recombinant molecule. …

Alternative Approach: An Assembly-Line Walkthrough With an Excluded Stage

Trace recombinant DNA construction as a literal assembly line, and identify exactly

where each listed item belongs -- and where it does not.

Step 1 -- Raw materials station: DNA fragments (the vector plus the foreign gene of

interest) arrive here. Without raw material, there is nothing to assemble. Required.

Step 2 -- Cutting station: a restriction endonuclease cuts both the vector and the

foreign DNA fragment at matching recognition sites, generating complementary sticky (or

matching blunt) ends. Required.

Step 3 -- Temporary joining station: the complementary sticky ends of the cut vector

and insert base-pair with each other spontaneously by hydrogen bonding -- this step

needs no enzyme, just correct end-matching from Step 2.

Step 4 -- Permanent sealing station: DNA ligase seals the remaining nicks in the

sugar-phosphate backbone by forming covalent phosphodiester bonds, locking the

recombinant molecule together permanently. Required. …