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Allozyme Electrophoresis and Population Structure in the Snowy Campion, Silene latifolia ssp.alba*

* also known as Silene alba

Theory of allozyme electrophoresis
Setting up and running an allozyme gel
PTC taste test

Please note the revised learning objectives for test 4!

Allozyme Electrophoresis

A mixture of soluble enzymes is loaded on a gel (made of starch or cellulose acetate).  Electric current causes each protein to move through the gel at a speed determined by its size and charge.

The proteins are not visible as they move through the gel.  To visualize them, we take advantage of the fact that they are enzymes.  The substrate is added to the gel and anywhere an enzyme is present that reacts with that particular substrate, product will form.  The product is generally not visible either.  However, a stain that binds specifically to that product is also added, and any place where the enzyme was present will appear as a colored band on the gel. 

Isozymes and Allozymes  

Enzymes that perform the same function, but which are coded for by genes located at different loci are called isozymes.  

Variant forms of an enzyme that are coded for by different alleles at the same locus are called allozymes.  

Allozymes and isozymes move at different speeds through a gel because they differ from each other in size and charge.

In an allozyme gel, the lanes represent genotypes of different individual organisms.  Each different zone of activity contains a different isozyme.  By convention, they are numbered according to their position on the gel.  Within the a zone of activity, the different bands represent allozymes coded for by different alleles.  By convention, the alleles that code for these allozymes are named according to the speed at which their enzymes move through the gel (i.e. fast or slow).

Monomeric enzymes

If an enzyme is monomeric (consists of a single polypeptide unit), each different band position in the zone of activity results directly from a different polypeptide coded for by a different allele.

Dimeric enzymes

If an enzyme is dimeric (has a quaternary structure consisting of two polypeptide units), the different band positions in the zone of activity represent the different combinations of polypeptide units.  If there are two different alleles in an individual, there will be three positions because there are three possible combinations of the polypeptide subunits coded for by the alleles.  

The PTC taste test

Two types of papers are available at the front bench.  Those in the blue packet have been soaked in a solution of phenylthiocarbamide (PTC).  The white packet contains plain paper for comparison. 

To perform the test, remove a piece of each of the two types of papers and touch them to your tongue.  Some individuals will taste no difference between the two papers, while others will taste bitter, sour, salty, or sweet with the PTC paper.  Record the results of your test on the sheet on the front bench.