Question
Describe the steps involved in recombinant DNA (rDNA) technology, from isolation of the gene of interest to extraction of the final product.
(NEET, CBSE Class 12 — Biotechnology: Principles and Processes)
Solution — Step by Step
The gene to be cloned is isolated from the source organism’s DNA using restriction endonucleases (molecular scissors). These enzymes cut DNA at specific palindromic sequences (e.g., EcoRI cuts at GAATTC). The cut produces sticky ends or blunt ends.
The isolated gene is inserted into a cloning vector (plasmid, bacteriophage, or cosmid). The vector is also cut with the same restriction enzyme to produce compatible sticky ends. DNA ligase joins the gene and the vector, creating recombinant DNA (rDNA).
The rDNA is introduced into the host cell (usually E. coli). Methods include: heat shock (CaCl treatment + 42 degree C), electroporation, or microinjection. The host cell that takes up the rDNA is called a transformant.
Not all host cells take up the rDNA. We select transformants using selectable markers (antibiotic resistance genes on the vector). Insertional inactivation: if the gene inserts into the antibiotic resistance gene, that resistance is lost — blue-white screening with lacZ gene and X-gal identifies recombinants.
Confirmed recombinant cells are grown in bioreactors (large-scale fermenters) under optimal conditions. The desired protein is produced by the host. Downstream processing: extraction, purification (chromatography), and formulation of the final product (e.g., human insulin, interferon).
graph TD
A["Gene of interest"] -->|"Restriction enzymes cut"| B["Isolated gene fragment"]
C["Vector (plasmid)"] -->|"Same restriction enzyme"| D["Cut vector"]
B --> E["DNA ligase joins"]
D --> E
E --> F["Recombinant DNA"]
F -->|"Transformation<br/>Heat shock / Electroporation"| G["Host cell (E. coli)"]
G -->|"Selection: antibiotic markers<br/>Screening: blue-white"| H["Confirmed recombinants"]
H -->|"Bioreactor scale-up"| I["Product extraction<br/>Downstream processing"]Why This Works
rDNA technology works because the genetic code is universal — a human gene inserted into a bacterial cell will produce the same protein. Restriction enzymes provide precision cutting, ligase provides precise joining, and selectable markers allow us to find the needle in the haystack (the one cell that has the correct rDNA).
The entire process is essentially: cut, paste, insert, select, grow, harvest.
Alternative Method — PCR as an Alternative to Cloning
If you need many copies of the gene (not the protein), PCR (Polymerase Chain Reaction) amplifies DNA in vitro using Taq polymerase. PCR takes 2-3 hours to make billions of copies, while cloning takes days. PCR is used for diagnosis (COVID testing) and forensics.
For NEET, know the tools of rDNA: restriction enzymes (scissors), ligase (glue), vectors (vehicles), and host organisms (factories). Also remember: EcoRI is the most commonly cited restriction enzyme — it was isolated from E. coli strain RY13.
Common Mistake
Students confuse the vector with the host. The vector is the DNA molecule that carries the gene (plasmid, phage). The host is the living cell that takes up the vector (E. coli, yeast). The gene rides on the vector into the host — like a passenger on a bus.