text: Biotin is commonly used as a "tag" in this instance to amplify the hybridization signal in the southern blot.  Older procedures for southern blot hybridization used radioactive 32P labeled DNA or RNA probes for the detection method.  Because of the expense and risk of dealing with radioactive waste, alternate methods that would give comparable signals were developed.  Chemiluminesence, fluorescence, and biotin-amplification systems will all give comparable sensitivities.\n\nIn the case you're describing, it's a sandwich detection system akin to an ELISA.  The biotin-tagged (biotinylated) lamda DNA is used to hybridize to the DNA on the Southern blot.  The researcher would then wash the blot and re-probe with a substance which binds to the biotin (ie. avidin, streptavidin or neutravidin) which itself was labeled either with an enzyme (for a colormetric reaction) or a fluorescent label.  If the reaction was colormetric it's likely the scientist used a streptavidin-HRP (HRP=horseradish peroxidase) for detection of the biotinylated DNA probe.  The HRP is then exposed to a substrate (TMB) to cause the color change.  \n\nThe benefit for using a biotin/steptavidin detection system is that multiple streptavidin molecules bind to a single biotin molecule, so you get an amplification of the signal.\n\nHope this clarifies things.