Final answer:
In two-dimensional gel electrophoresis, proteins are separated horizontally by their isoelectric point, followed by vertical separation based on molecular mass.
Step-by-step explanation:
In a two-dimensional gel electrophoresis experiment, proteins are separated horizontally by their isoelectric point (pI). This technique involves an initial separation of proteins based on their pI via isoelectric focusing (IEF), followed by a second dimension of separation where proteins are further separated by their molecular mass using SDS-PAGE. Two-dimensional gel electrophoresis is capable of resolving complex mixtures of proteins, displaying a unique gel pattern where the location of each protein is determined not only by its pI but also by its relative molecular mass (Mr). Visualization of the proteins is typically achieved using sensitive stains like silver stain or Coomassie Brilliant Blue, which indicate the abundance of the proteins in the gel as a function of the stained density.