Cloning a gene (polymerase chain reaction)

We can make exact genetic copies of whole organisms, cells or pieces of DNA. These copies are called clones. A clone is a copy of a plant, animal or micro-organism derived from a single common ancestor cell or organism. Clones are genetically identical.

A PCR gel

Janine Young

However, the word 'clone' can also be applied to genes. A gene is said to be cloned when its sequence is multiplied many times in a common laboratory procedure called polymerase chain reaction (PCR).

To study genes, researchers need large amounts of DNA to work with. PCR copies the cell’s natural ability to replicate its DNA and can generate billions of copies within a couple of hours.

There are four main stages:

  1. The DNA to be copied is heated, which causes the paired strands to separate. The resulting single strands are now accessible to short lengths of DNA called primers. Primers match a short section of the DNA to be copied.

  2. Large amounts of primers are added to the single strands of DNA. The primers bind to matching sequences along the DNA sequence, in front of the gene that is to be copied. The reaction mixture is then cooled. This allows double-stranded DNA to form again. Because of the large amounts of primers, the two strands will always bind to primers, instead of to each other.

  3. DNA polymerase is added to the mixture. This is an enzyme that makes DNA strands. It can synthesise strands from all the DNA primer combinations and dramatically increases the amount of DNA present. One enzyme used in PCR is called Taq polymerase which originally came from a bacterium that lives in hot springs. It can withstand the high temperature necessary for DNA strand separation and therefore, can be left in the reaction and still functions.

  4. The above steps are repeated until enough DNA is obtained.

This whole process is automated and happens in a couple of hours. The reaction occurs in a small tube placed inside a specialised machine that can make the big temperature adjustments quickly.

Researchers use the many gene copies to research the function of the gene.