Sometimes, more than one DNA sequence might be copied. Gel electrophoresis can be used to check whether or not this happened. If only the sequence of interest has been copied, you should get a single band in the gel (all the copied sequences will be the same size, and run the same distance through the gel). If more than one sequence has been copied, you will get more than one band (each band represents different sized DNA pieces). If you know the size of the DNA fragment that you were wanting to copy, you can match the bands up with the bands from a DNA ladder (a mixture of DNA pieces of different sizes which is also added to the gel). This will help you to identify the band containing the DNA of interest. The DNA can then be extracted from the gel and used.
Transcript
Anassuya Ramachandran (Faculty of Medical and Health Sciences, The University of Auckland)
We had done a PCR earlier on in the day and we needed to actually see if the PCR has worked.
Dr Martin Philpott (Faculty of Medical and Health Sciences, The University of Auckland)
In theory a PCR should produce one product - the gene which you are interested in. In practice, nothing ever works quite that simply, and what you will find is you will have got multiple products produced.
Anassuya Ramachandran (Faculty of Medical and Health Sciences, The University of Auckland)
So to confirm that the PCR products are indeed there, we have to run it on the gel, and then photograph the gel. Then we can carry on analysing those fragments that we have.