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Biochemistry · Topic

Recombinant DNA and cloning !

Recombinant DNA technology cuts and joins DNA from different sources so that a host cell makes a protein or copies a gene.

Updated 2026-11-01

Key facts

  • Restriction enzymes cut specific palindromic sequences (e.g. EcoRI, GAATTC), leaving sticky or blunt ends.
  • DNA ligase joins the fragment to the vector (plasmid or virus).
  • The plasmid carries an origin of replication, an antibiotic resistance gene (selection) and a multiple cloning site.
  • cDNA is made from mRNA with reverse transcriptase: it has no introns, so bacteria can express it.
  • Classic products: human insulin, growth hormone, erythropoietin, factor VIII, hepatitis B vaccine.

In detail

  • 1. Get the gene: by PCR, from cDNA or by chemical synthesis.
  • 2. Cut the gene and the vector with the same restriction enzyme so their ends match.
  • 3. Join with DNA ligase and put the vector into the host cell (transformation in bacteria, transfection in animal cells).
  • 4. Select the cells that took it up with the antibiotic; blue/white screening (disrupted lacZ) spots those carrying an insert.
  • 5. Express the protein: bacteria (cheap, no glycosylation), yeast or mammalian cells (human-type glycosylation, as for erythropoietin).
  • Restriction fragment length polymorphisms (RFLP): a mutation that creates or destroys a cut site changes fragment sizes; the basis of classic molecular diagnosis of sickle cell disease.

Clinical notes

  • Recombinant human insulin replaced animal insulin and reduced immune reactions.
  • Transgenic and knockout animals let researchers study human diseases in the lab.

Test yourself

Think of the answer, then tap the question to see it.

Why is cDNA rather than genomic DNA cloned to express a human protein in bacteria?

Bacteria do not splice; cDNA, copied from mature mRNA, has no introns.

What kind of sequence do restriction enzymes recognise?

Short palindromic sequences (reading the same 5'→3' on both strands), such as GAATTC for EcoRI.

Why does a plasmid carry an antibiotic resistance gene?

For selection: only bacteria that took up the plasmid survive on the antibiotic medium.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: Recombinant DNA Technology (National Human Genome Research Institute (NHGRI), 2026); Restriction Enzyme (National Human Genome Research Institute (NHGRI), 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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Representative model, for educational purposes only.