Question
Outline the steps of recombinant DNA (rDNA) technology from gene isolation to the extraction of the desired product.
Solution — Step by Step
flowchart TD
A[Identify and Isolate Gene of Interest] --> B[Cut Gene using Restriction Enzymes]
B --> C[Cut Vector with Same Restriction Enzyme]
C --> D[Join Gene + Vector using DNA Ligase]
D --> E[Recombinant DNA - rDNA]
E --> F[Transfer rDNA into Host Cell]
F --> G[Select Transformed Cells]
G --> H[Clone and Culture at Scale]
H --> I[Extract and Purify Protein Product]The desired gene is identified and isolated from the source organism’s DNA. This can be done using restriction endonucleases (molecular scissors) that cut DNA at specific palindromic sequences. Alternatively, the gene can be synthesised chemically or obtained as cDNA from mRNA using reverse transcriptase.
Both the gene of interest and the vector (a carrier molecule, usually a plasmid like pBR322 or a bacteriophage) are cut with the same restriction enzyme to produce compatible sticky ends. The gene is then inserted into the vector using DNA ligase, which seals the sugar-phosphate backbone. The result is recombinant DNA (rDNA).
The rDNA is introduced into a competent host cell (usually E. coli). Methods include: heat shock (42 degrees C briefly), electroporation (electric pulses), microinjection, or using gene guns (biolistics) for plant cells. The host cell that takes up the rDNA is called a transformant.
Not all cells take up the rDNA. We select transformants using selectable markers — genes for antibiotic resistance on the vector. For example, pBR322 has ampicillin and tetracycline resistance genes. Insertional inactivation or blue-white screening (using lacZ gene) identifies cells with the recombinant plasmid.
Selected transformants are cultured in large bioreactors (fermenters) for mass production. The desired protein is extracted and purified through downstream processing — filtration, chromatography, and other purification steps. Example: human insulin (humulin) is produced this way in E. coli.
Why This Works
rDNA technology allows us to insert a gene from one organism into another, making the host cell produce a protein it normally would not. This is possible because the genetic code is universal — human genes produce human proteins even when expressed in bacterial cells. The vector ensures the gene is replicated and expressed in the host.
Common Mistake
Students forget that both the gene AND the vector must be cut with the same restriction enzyme to produce compatible sticky ends. Using different enzymes creates incompatible ends that DNA ligase cannot join. Also, restriction enzymes cut at specific palindromic sequences (like GAATTC for EcoRI) — they do not cut randomly.