Genetic engineering- How?

For this science, all the work happens in test tubes.
First the gene from species A that we want to insert into species B is isolated. This is done by first isolating DNA, then, using appropriate methods, determining its sequence (base sequence).
Once we know this, and we know which genes to use, specific enzymes are used to cut out the gene that we are interested in. After purification we have a copy of the gene we want. It can then be studied, multiplied and subsequently used for insertion into another species.

There are two ways of getting a gene into a cell of species B:

Plasmids are small rings of DNA found in bacteria. They only have a limited number of genes and hence their sequence is very easy to determine. As they replicate quickly it is simple to study them.

To insert our gene A into the plasmid we need to do the following:

  1. Take a large number of a known plasmid and place it into a test tube.
  2. Add a cutting enzyme: an enzyme that recognises a certain base sequence and will cut the DNA strand at a particular point.
  3. Leave this so that the enzyme can cut all the DNA, then purify it.
  4. Add the gene we want to incorporate into the cell B, i.e. gene A. Gene A will add to the openend plasmid ring.
  5. Add an enzyme called ligase, which will join the ends together and reform a plasmid with our gene in.
  6. Put the plasmid back into bacteria and leave these to grow and multiply.

But now we need to work out which bacteria contain the modified plasmid. This is done by using plasmids containing a marker gene, usually a gene that makes bacteria resistant to an antibiotic. The culture is then treated with this antibiotic and all the cells without the plasmid die whilst the others mutliply.