6. Genetic Engineering

What you need to know...
  • Genetic information can be transferred from one cell to another naturally or by genetic engineering
  • Stages of genetic engineering to include: 
    • identify section of DNA that contains required gene from source chromosome, 
    • extract required gene, 
    • insert required gene into vector/bacterial plasmid
    • insert plasmid into host cell, 
    • grow transformed cells to produce a GM organism.

Source: SQA

You now know what DNA is and how it works. In this topic we're going to learn how DNA can be transferred from cell to cell. This occurs naturally in some species, but we've also made use of this for our own benefits - we'll find out how.

Natural Transfer of Genetic Information

In the first topic in this unit, we looked at the structures of bacteria cells. One of those structures which was unique to bacteria cells you'll remember was plasmids.

Plasmids are small circles of DNA which can be easily transferred from one bacterial cell to another. This is one way in which genes which code for antibiotic resistance are transferred so quickly through a population of bacteria cells (which happens to be a big news story on the day I'm writing this). Viruses can also transfer DNA into living cells naturally as they work by tricking cells into creating virus proteins by injecting their own viral genetic information into cells such as ours.

Genetic Engineering

One of the most amazing things about life on earth which we often take for granted is that all life uses the same DNA code to store its information and create proteins (Professor Brian Cox discusses the implications of this in evolutionary terms on Wonders of Life). Because all of life uses the same DNA code, we are now able to combine DNA from one species with another in order to produce proteins in new and different ways which is revolutionising science and medicine. The bacterial plasmids which you already know about are often very crucial for these genetic engineering techniques as we'll see below.

In order to describe the processes of genetic engineering, we'll use an example. Insulin is a protein hormone produced by the pancreas which helps regulate the concentration of sugar in the blood. One form of diabetes results from some people's inability to produce functioning insulin. People with this form of diabetes must inject insulin into their bloodstream in order to control their blood sugar levels. But where does this insulin come from? Until the 1980s the only source of insulin was from animals. Animal insulin was extracted from the pancreases of pigs and cows. This was not ideal. Today people with this form of diabetes can be given pure human insulin. How? Genetic engineering.

There are a number of steps involved in the genetic engineering process, which we'll look at here. First of all, the gene we wish to copy and express needs to be identified in the source chromosome and extracted. The diagram below shows the human insulin gene being identified and extracted from a human chromosome. Scissors aren't really used to remove the gene, an enzyme is. 

Next, the DNA which has been extracted from the source chromosome must be added to a vector. A vector is just a piece of DNA which carries our wanted gene. In our example the vector is a bacterial plasmid. The plasmid is cut open using the same enzyme which was used to cut out our wanted gene and then the two are joined together using another enzyme. 

The plasmid can now be inserted into a host cell. Remember bacterial cells naturally exchange plasmids so they will readily take up any plasmids they are exposed to. Once the plasmid is in the host bacterial cell, the cell will make many copies of it. When they do this, they are unwittingly also making many copies of of our human insulin gene. 
Of course, given the right conditions the host bacteria cells will then go on to divide just like any other bacteria cells. As they do so, once again they will be making many many copies of our human insulin gene also.

Now we'll not only have many copies of our wanted gene, but the bacteria cells will even begin expressing the gene too. This means they will actually make the human insulin protein using the process we discussed in a previous topic. This insulin can then be collected, purified and given to people with diabetes.

There's more on this process of Genetic Engineering on the BBC website. There are many other applications of Genetic Engineering and we'll discuss how this approach can be applied in agriculture in a later unit.