Biology · Biotechnology: Principles and Processes · NEET
NCERT gives a fixed order: isolation of DNA, fragmentation (cutting) of DNA by restriction endonucleases, isolation of the desired DNA fragment, ligation of the fragment into a vector, transfer of recombinant DNA into the host, culturing the host cells at large scale, and finally extraction of the desired product (downstream processing). NEET loves asking you to arrange these in sequence, so memorise the order, not just the names.
The three basic steps are a simplified summary: (i) identifying and isolating the desired DNA, (ii) introducing that DNA into the host, and (iii) maintaining the DNA in the host and passing it to progeny. The full process expands these into the seven detailed steps (isolate, cut, amplify, ligate, insert, culture, extract). Both are correct; the three-step version is the high-level view and the seven-step version is the working laboratory sequence.
PCR (Polymerase Chain Reaction) comes after cutting and isolating the gene of interest, to amplify (make many copies of) that gene before it is ligated into the vector. Having many copies makes ligation and cloning much easier. So the order is: cut DNA, isolate the desired fragment, amplify by PCR, then ligate. A common NEET trap is placing amplification before cutting - that is wrong.
The same restriction enzyme produces the same sticky ends (identical single-stranded overhangs) on both the gene and the vector. These complementary ends form hydrogen bonds with each other, and DNA ligase then seals them permanently. If different enzymes were used, the overhangs would not be complementary and ligation would fail, so no recombinant DNA would form.
The last step is extraction of the desired product. After the host cells are cultured and the foreign gene is expressed to make the protein, the product is separated and purified - this separation and purification together are called downstream processing. It also includes formulation with preservatives and quality-control testing before the product is sold. Downstream processing happens after the reaction, not before.
Main steps in the formation of Recombinant DNA are given below. Arrange these steps in a correct sequence. A. Insertion of recombinant DNA into the host cell. B. Cutting of DNA at specific location by restriction enzyme. C. Isolation of desired DNA fragment. D. Amplification of gene of interest using PCR.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
NCERT lists seven working steps: isolation of DNA, cutting (fragmentation) by restriction endonucleases, isolation of the desired fragment, ligation into a vector, transfer into the host, large-scale culturing, and extraction of the product. These can be summarised as three basic steps for quick revision.
The first step is isolation of the genetic material (DNA). The cell is broken open using enzymes like lysozyme (bacteria), cellulase (plant), or chitinase (fungi) so the DNA can be released and later purified, because DNA must be pure before restriction enzymes can cut it.
The last step is extraction of the desired product. After large-scale culturing in a bioreactor and expression of the foreign gene, the product is separated and purified through downstream processing, then formulated and quality-tested before sale.
DNA ligase joins the fragments. After a restriction enzyme cuts both the gene of interest and the vector to make matching sticky ends, DNA ligase seals the sugar-phosphate backbone to create a single recombinant DNA molecule.
Yes. NEET repeatedly asks students to arrange the steps in the correct order (for example NEET 2023). Learning only the names is not enough; you must know the sequence: isolate, cut, isolate fragment, amplify, ligate, insert, culture, extract.
