Liver RNA. Electrophoresis gel containing RNA (ribonucleic acid) isolated from liver tissue. The RNA molecules (white bands) are being observed under ultraviolet light, which causes a chemical present in the gel to fluoresce. Electrophoresis separates molecules according to their size, by applying an electrical current to samples loaded onto the gel. The bands present in each sample represent different types of RNA. RNA is a nucleic acid that is involved in directing protein synthesis. Different RNA sequences are present in different tissues, representing differing patterns of gene expression.