Why proteins move through the gel in SDS/PAGE in western blotting?

Keeping me busy. same reason as the blotting. First they are denatured by boiling in SDS (a detergent) and beta-mercaptoethanol (a reducing agent that dirsupts disufide bridges). The linearised proteins are now coated with molecules of negatively charged SDS along their length. the gel is placed in an ionic buffer tank containing an electrolyte (laemelli buffer), and a platinum wire is placed at each end of the gel. At the bottom of the gel is the positive electrode (anode) and at the top the negative electrode (cathode), and a current is placed across the electrodes. The SDS-coated negatively charged proteins move towards the anode through the gel.