Gel Electrophoresis- Genetic Technology

Gel Electrophoresis

Gel electrophoresis is a technique used to separate DNA fragments (or other macromolecules, such as RNA and proteins) based on their size and charge.

The suffix phoresis means "migration" or "movement."  The prefix electro tells us that we are using electricity to make molecules migrate.

Electrophoresis involves running a current through a gel containing the molecules of interest. Based on their size and charge, the molecules will travel through the gel in different directions or at different speeds, allowing them to separate from one another.

Gel Electrophoresis of DNA

•      DNA is extracted from a sample, such as sperm, hair, etc.

•      DNA is chopped using enzymes called Restriction Endonucleases.

NB: Restriction endonucleases are enzymes that cut specific regions in the DNA sequence. They cut the sugar phosphate bone to give either sticky ends or blunt ends.

•      Restriction Endonucleases cleave the DNA close to VNTR region.

NB: Variable tandem number repeat is a repeat sequence in the DNA that is different in each person.

•      The DNA is ready for electrophoresis

NB: If you’re comparing DNA, cut both DNA with the same restriction enzyme.

•      Place the DNA fragments in a well with an electric current applied using a teat pipette. The DNA should be on the negative side or the cathode since it is negatively charged. Phosphates in nucleotides of DNA have negative charge. This gives the DNA a negative charge. 

•      All DNA molecules have the same amount of charge per mass. Because of this, gel electrophoresis of DNA fragments separates them based on size only.Since DNA is negatively charged, it will travel to the positive side. Smaller fragments travel furthest as they have smaller mass. Larger fragments are slower and would travel the least.

•      When a gel is stained with a DNA-binding dye and placed under UV light, the DNA fragments will glow, allowing us to see the DNA present at different locations along the length of the gel.

•      Probes can also be added to make the film go dark when placed on an X-Ray film

•      A well-defined “line” of DNA on a gel is called a band. Each band contains a large number of DNA fragments of the same size that have all travelled as a group to the same position

•      One well is reserved for a DNA ladder, a standard reference that contains DNA fragments of known lengths. By comparing the bands in a sample to the DNA ladder, we can determine their approximate sizes. Helps give estimates of how long the other base pairs are when run alongside it.

Gel electrophoresis has many uses ranging from criminal investigations to paternity tests. It is used in conjunction with other techniques to determine if someone has a particular allele of a gene, such as screening for sickle cell anaemia.  

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