Polyacrylamide gel electrophoresis, called "Native PAGE" or "Nondenaturing PAGE" when SDS is not added, conserves the charge of the proteins and separates them according to their molecular weight-charge ratio. When SDS is added, it linearizes the proteins (destroys their tertiary structure) and coats them with the negatively charged molecule to offset the charge of their individual R groups. This is called SDS-PAGE. This means that SDS-PAGE separates according to their molecular weight. The intention of denaturing is to remove their 3-dimensional shapes that could slow down their progression through the gel (imagine you are comparing a linear protein and a ring protein with a higher surface area) and in this way standardize the proteins' solely on their molecular weight.
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