Process of genetic modification

Genetic modification — also called genetic engineering — means changing an organism's DNA by adding a gene from a different organism. The new organism then has a useful new feature.

The whole point is this: a gene is a length of DNA that codes for a protein. If we can put a useful gene into the cells of another organism, those cells will read the gene and make that protein.

A classic example is making human insulin. The human gene for insulin is put into bacteria. The bacteria then make human insulin, which can be collected and used to treat people with diabetes.

To do this we need a way to carry the gene into the new cell, and a way to cut out the gene and stick it in place. Those jobs are done by a vector and by enzymes — the rest of these notes explain how.

Exam tip. Keep the big idea clear: we transfer a gene so the new cell makes a protein. Many marks come simply from saying the host cell expresses the gene and produces the protein.

A vector is something that carries a piece of DNA (the gene) into a host cell. On its own, a gene cannot get inside a cell — it needs a vector to deliver it.

For the Double Award, you need to know two types of vector:

1. Plasmids. A plasmid is a small, circular loop of DNA found in bacteria (separate from the main bacterial DNA). The gene can be inserted into the plasmid, and the plasmid is then taken up by a bacterium.
2. Viruses. A virus naturally injects its own genetic material into cells. This makes it useful as a vector: the desired gene is put into the virus, and the virus then carries the gene into the host cell for us.

Both work the same way overall — they take up the piece of DNA and then insert it into another cell.

Exam tip. If asked to name two vectors, write plasmid and virus. A common one-mark question is simply 'state what is meant by a vector' — answer: something that carries the gene/DNA into the host cell.

To put a gene into a vector we use two enzymes:

• A restriction enzyme cuts out the desired gene from the original DNA. The same restriction enzyme is also used to cut open the vector (the plasmid or virus). Because the same enzyme is used, the cut ends match and can join together.
• The enzyme ligase then joins (sticks) the gene into the cut vector.

The result is a vector that now contains the inserted gene. DNA made by joining DNA from two different sources like this is called recombinant DNA — "re-combined" DNA.

Here is the idea as a picture:

Exam tip. Match the enzyme to its job: restriction enzyme = cuts, ligase = joins. Mixing these up is the most common error in this topic.

Putting the steps together, the overall process of genetic modification is:

1. Isolate the gene. A restriction enzyme cuts out the desired gene (e.g. the human insulin gene).
2. Insert the gene into a vector. The same restriction enzyme cuts open the vector (a plasmid or virus), and ligase joins the gene into it. This makes recombinant DNA.
3. Vector carries the gene into the host cell. The vector takes the recombinant DNA into a host cell — for example a bacterium takes up the plasmid.
4. Host expresses the gene. The host cell reads (expresses) the gene and makes the protein it codes for. As the host reproduces, all the new cells also carry the gene and make the protein.

So a useful gene ends up working inside a completely different organism, which then produces the protein we want (such as insulin) in large amounts.

Exam tip. A common 4-mark question asks you to describe the steps in order. Use the chain: cut out gene (restriction enzyme) → join into vector (ligase) → vector into host cell → host makes the protein. Keeping them in order earns the marks.