Silver staining is more sensitive than ethidium bromide for double stranded DNA, and detects single stranded DNA or RNA with no loss in sensitivity. Silver staining relies on the reduction of silver cations to insoluble silver metal by nucleic acids. This chemical reaction is insensitive to the macrostructure of the DNA molecule. Reduced silver grains deposit in the gel around the DNA bands, creating a “latent image”. The latent image is developed to visibility by soaking the gel in a solution of silver cations and a reducing agent. The silver granules in the latent image catalyze the further reduction and deposition of silver from the solution. Bands manifest as dark brown or black regions which appear before significant background develops. Development is stopped by altering the pH of the gel to a point where silver reduction is no longer favored.

The mechanism whereby nucleic acids reduce silver is not well defined. Staining is enhanced by treatment with oxidants, and it may be that such treatment oxidizes vicinal sugar diols to highly reductive aldehydes, which are known to reduce silver (e.g. Tollens test). However, if this were the primary mechanism, silver staining would be expected to destroy the DNA. Recent reports indicate that enough DNA survives silver staining to provide a template for PCR amplification. This may reflect either the nondestructive nature of silver staining or the extreme sensitivity and power of the PCR reaction.

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