Which substance is used to increase the density of the sample, allowing it to sink into wells?
DNA samples must sink into wells despite buffer overlay. Loading buffers contain high-density solutes such as sucrose, glycerol, or Ficoll to increase sample specific gravity above that of the running buffer, preventing diffusion. Sucrose at 10 to 40 percent is commonly used because it is uncharged, chemically inert, and does not interfere with migration. Ethidium bromide is an intercalating fluorophore for visualization. Bromophenol blue is an anionic tracking dye marking front migration. TEMED catalyzes polyacrylamide polymerization. Thus, sucrose functions purely as a densifying agent for well loading.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.