Gel Electrophoresis – VCE Biology Unit 4

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What is Gel Electrophoresis?

Gel Electrophoresis is an important biological process which allows a scientist to separate strands DNA strand of different sizes. Recall that in genetic engineering scientists use restriction enzymes can cut up DNA into many segments. Essentially gel electrophoresis allows the “mixture” of DNA fragments to be separated according to their size in kilobases (kb).Gel Electrophoresis can also be used to separate RNA and protein molecules – it is not solely limited to DNA.gel-electrophoresis-apparatus

How does Gel Electrophoresis work?

(1) DNA is negatively charged – it will move towards positive terminal

Gel Electrophoresis uses an agarose gel and a voltage (potential difference) so that the DNA fragments have motivation to move! Remember that DNA is negatively charged, due to the phosphate groups within the backbone of the structure. Hence the DNA will move towards a positive terminal via electrostatic (plus to minus) attraction.

(2) The gel will separate the DNA strands according to their size

The larger DNA fragments will encounter more resistanceand will move through the gel at a slower rate. On the other hand the smaller DNA fragments will encounter less resistance and will therefore move through the gel at a faster rate.

Remember:The separation process occurs for a limited time only. This means that the gel electrophoresis machine should only be switched on for 1-2 hours (can vary according to sample). This is because eventually even the large DNA fragments can get to the positive voltage and this would not allow separation to occur.

What is the standard?

The standard is a pre-made sample of DNA with known fragment sizes. The purpose of the standard is that it enables the scientist to determine the size of the “sample” DNA fragments.

How are the DNA fragments identified?

Remember that DNA fragments are not coloured, they must be stained with a particular dye. Usually DNA is stained with a dye that can be seen under UV light which allows all the DNA bands to be illuminated. Other times, particularly when interested in only finding a specific DNA sequence a probe is used. A probe is a short section of single stranded DNA or RNA which has a known base sequence and can identify a sequence of interest. Say for example that you were interested in locating the insulin gene – you may know the code but not know the location of the strand. By using a probe we can identify the position of this insulin gene.