Practice question

Supershift assay extends EMSA to identify protein composition within DNA-protein complexes. Labeled DNA probe is first incubated with nuclear extract to form specific retarded complex. Then antibody recognizing candidate transcription factor is added. If antigen is present within complex, antibody binds, dramatically increasing molecular weight, bulk and hydrodynamic drag, producing even slower migrating band above original shift. Appearance of supershifted species confirms presence of specific protein and indicates multiprotein assembly or protein-protein interaction on DNA. Free probe migrates normally, degraded probe shows loss rather than additional retardation.