Q.The basic scheme of the essential steps involved in the process of recombinant DNA technology is summarised below in the form of a flow diagram. Study the given flow diagram and answer the questions that follow. Step 1: Vector DNA (Plasmid) (cut using Restriction Enzyme) + Alien DNA (cut using Restriction Enzyme) Step 2: Recombinant DNA molecule Step 3: Transfer of recombinant DNA molecule in E. coli (Host) Step 4: Replication of the recombinant DNA molecule in E. coli to form multiple copies of the alien gene
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Start your 14-day free trial to unlock the full solution →Step 4 is called cloning or gene cloning; combination (ii) using the same restriction enzyme (EcoRI) for both plasmid and alien DNA is correct because it produces complementary sticky ends that can anneal.
The flow diagram you've been given traces the heart of recombinant DNA technology—the process by which we insert a foreign gene into a bacterial host so that the bacterium becomes a living factory, churning out copies of that gene and often the protein it encodes. Each step builds on the last, and understanding why certain choices matter at Step 1 determines whether the entire process succeeds or fails.
The Technical Term for Step 4
Once the recombinant DNA molecule—plasmid stitched together with the alien gene—enters the E. coli host cell, the bacterium treats it like any other piece of DNA it needs to maintain. The plasmid replicates autonomously inside the cell, and every time the bacterium divides, the daughter cells inherit copies of the recombinant plasmid. Over a few hours, a single transformed bacterium multiplies into millions, each carrying the foreign gene. This amplification of the gene of interest, producing multiple identical copies, is what we call cloning or more precisely gene cloning. The term captures the idea that we're making clones—exact genetic replicas—of the inserted DNA segment.
Some texts also use the phrase "amplification of recombinant DNA" or refer to the host cells as "clones," but the standard term for the overall process at this step is cloning.
Choosing the Right Restriction Enzyme Combination
Now to the more subtle question: which enzyme strategy works in Step 1? The answer hinges on how restriction enzymes cut DNA and how the cut ends must match for ligation to occur.
Restriction enzymes recognize specific palindromic sequences and make cuts that leave either blunt ends or staggered cuts called sticky ends. When a restriction enzyme like EcoRI cuts DNA, it leaves short single-stranded overhangs—sticky ends—with a defined sequence. The magic of recombinant DNA technology is that if you cut two different DNA molecules (the plasmid vector and the alien DNA) with the same restriction enzyme, both will have complementary sticky ends. These ends can base-pair with each other through hydrogen bonding, and DNA ligase can then seal the sugar-phosphate backbone, permanently joining the two molecules.
Let's examine the two combinations:
- (i) EcoRI to cut the plasmid and HindIII to cut the alien DNA: This creates a mismatch. EcoRI produces one type of sticky end on the plasmid, and HindIII produces a completely different sticky end on the alien DNA. The overhangs have different sequences and cannot anneal to each other. No base pairing means no ligation—the recombinant DNA molecule simply won't form. …
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