Learning from Nature's Own Engineers

Moving a gene into a plant or an animal cell might sound like something we had to invent from scratch. In fact, we learnt the trick from bacteria and viruses, which have been transferring their genes into eukaryotic cells for ages. These pathogens know exactly how to deliver DNA into a host and force it to do what they want.

So the strategy is a borrowed one: study how a pathogen naturally delivers genes to transform its host, then take that same delivery machinery, strip out the harmful part, and use it as a vector to carry a gene of our choosing. Two natural gene-delivery agents illustrate this beautifully — Agrobacterium tumefaciens for plants and retroviruses for animals.

Agrobacterium and the Ti Plasmid

Agrobacterium tumefaciens is a pathogen of several dicot plants. In nature it delivers a piece of DNA called T-DNA into a normal plant cell and transforms it into a tumour. It then directs these tumour cells to make the chemicals that the pathogen itself needs — in effect, hijacking the plant to run a small factory for the bacterium.

The DNA responsible sits on the bacterium's tumour-inducing (Ti) plasmid. Genetic engineers have taken this plasmid and modified — or disarmed — it into a cloning vector. In its reworked form it is no longer pathogenic to plants, yet it keeps the very machinery that lets it slip genes into plant cells. So we now use the disarmed Ti plasmid to deliver genes of interest into a wide variety of plants.

Retroviruses for Animal Cells

Animals have their own natural gene-delivery specialists: retroviruses. Left to themselves, retroviruses can transform normal animal cells into cancerous ones — again by inserting their genetic material into the host.

That same ability, once understood, becomes a tool. Retroviruses have been disarmed so they can no longer cause disease, and they are now used to deliver desirable genes into animal cells. The logic is identical to the plant case: a pathogen that was good at forcing genes into a eukaryotic host is stripped of its harmful traits and repurposed as a safe, efficient vector.

After the Vector Does Its Job

Whether the carrier is a plasmid, a disarmed Ti plasmid or a disarmed retrovirus, the pattern that follows is the same. Once the gene or DNA fragment has been ligated into a suitable vector, that recombinant construct is transferred into a host — bacterial, plant or animal — and inside the host it multiplies.

This is the pay-off of the whole approach. The vector supplies the means of entry and, through its origin of replication, the means of copying. The host supplies the machinery. Between them, a single chosen gene can be carried across the species barrier and reproduced faithfully in cell after cell.

Quick Recap

  • We learnt gene delivery into eukaryotic cells from bacteria and viruses, which naturally transform such cells.
  • Agrobacterium tumefaciens, a pathogen of several dicot plants, delivers T-DNA to turn normal plant cells into a tumour and make them produce chemicals the pathogen needs.
  • Its tumour-inducing (Ti) plasmid has been modified/disarmed into a cloning vector that is no longer pathogenic but still delivers genes of interest into plants.
  • Retroviruses naturally transform normal animal cells into cancerous ones; they have been disarmed and now deliver desirable genes into animal cells.
  • Once ligated into a suitable vector, a gene or DNA fragment is transferred into a bacterial, plant or animal host, where it multiplies.

Solved Examples — Section 9

Q1. From where did scientists learn how to deliver genes into plant and animal cells?

Answer: From bacteria and viruses, which naturally transform eukaryotic cells by delivering their genes into them.

Q2. What does Agrobacterium tumefaciens naturally do to a plant cell?

Answer: It delivers a piece of DNA called T-DNA that transforms a normal plant cell into a tumour and directs the tumour cells to make chemicals the pathogen needs.

Q3. What is the Ti plasmid, and how has it been changed for use as a vector?

Answer: The tumour-inducing (Ti) plasmid of Agrobacterium tumefaciens has been modified (disarmed) into a cloning vector that is no longer pathogenic but can still deliver genes of interest into plants.

Q4. How are retroviruses used in animal cell engineering?

Answer: Retroviruses, which naturally transform normal cells into cancerous cells, are disarmed and then used to deliver desirable genes into animal cells.

Q5. Agrobacterium tumefaciens is a pathogen of which group of plants?

Answer: Several dicot plants.

Q6. After a gene is ligated into a suitable vector, what happens next?

Answer: The recombinant vector is transferred into a bacterial, plant or animal host, where it multiplies.