Integrating Content Review with Timed Passage Practice at 510+

At the 515+ level, B/B passages frequently present experimental data that students must interpret in combination with passage text. Western blot (immunoblot): proteins are separated by size via SDS-PAGE gel electrophoresis (smaller proteins migrate faster, lower on the gel), then transferred to a membrane and probed with antibodies against a specific protein. A band on a Western blot indicates the presence of that protein at a specific molecular weight. Comparing band intensity (thickness/darkness) across lanes represents relative protein quantity. Absence of a band indicates protein absence or below-detection levels. ELISA (enzyme-linked immunosorbent assay): quantifies protein or antibody concentration using enzyme-linked antibodies that produce a colorimetric signal proportional to target concentration — the higher the absorbance reading, the more target protein present. PCR and gel electrophoresis: DNA fragments are separated by size — larger fragments run more slowly and appear higher on the gel. The ladder (molecular weight standard) provides size reference. Interpreting experimental controls is essential: the positive control (known to produce the expected result) validates that the experimental system is working; the negative control (known to produce no result) validates the absence of contamination or non-specific signal. A MCAT question will often ask what the experiment cannot tell us — the answer usually involves the absence of a control that would have been required to rule out an alternative explanation.