Gel Electrophoresis
Gel electrophoresis is a method used in laboratories to separate DNA fragments of different sizes. These sizes can then be estimated using a "DNA ladder" reference.
DNA, such as a PCR product, is loaded in small wells in one end of a gel block. An electric current is then applied to the gel (the positive electrode opposite to the wells). Because DNA is negatively charged, it will move through the gel toward the positive electrode. Smaller DNA molecules move faster through the gel than larger DNA molecules, leading to size separation. After electrophoresis, DNA is visualized and appears as bands under each well. Only a large amount of DNA (millions of copies)can be visualized. A DNA ladder containing a mixture of DNA fragments of known sizes is also loaded on the gel. The sizes of the DNA samples can then be estimated by comparing with the DNA ladder (Figure 1).
Figure 1. Fragments of different length from a PCR reaction are run on a gel. The fragments will move at a speed and a distance relative to their size: smaller DNA molecules will move down the gel faster and further than longer DNA molecules. 
By mixing the PCR product with loading buffer, we can visualize how far the PCR product has traveled during gel electrophoresis. Loading buffer is also useful in making the sample heavier so that when you pipette it onto the gel, the sample will sink to the bottom of the gel.
There are many compositions for loading buffer; however, it will typically contain the following:
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Coloring reagent, for example, xylene cyanol, cresol red, bromophenol blue, or orange G. This will add color to the sample and help you visualize the sample position on the agarose gel.
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High-viscosity reagent, for example, Ficoll, sucrose, or glycerol. This will make the sample heavier by increasing the overall sample viscosity.
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Water to dilute the above mentioned reagents.