Final answer:
SDS is used in PAGE to denature proteins and mask their charges, leading to protein separation by size when an electric current is applied.
Step-by-step explanation:
The purpose of having sodium dodecyl sulfate (SDS) when running a polyacrylamide gel electrophoresis (PAGE) is to denature proteins and to mask their native charges. By adding SDS, proteins become uniformly negatively charged regardless of their original charge. This allows the separation of proteins in the gel based on molecular weight or size only, since all the proteins will migrate towards the positive electrode when an electric current is applied. The SDS-PAGE is particularly useful because it provides a reliable method for protein separation which can be visualized after staining with dyes such as Coomassie blue or a silver stain. SDS-PAGE is a critical technique in various forms of biochemical analysis, including the study of protein purity, the identification of protein complexes, and the determination of protein molecular weight.