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Repetitive sequences blocking
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Repetitive sequences blocking: Cot-1 DNA vs. salmon sperm DNA
Repetitive sequences (especially Alu and L1 families in human) have to be blocked with competitor DNA prior to FISH, to avoid high background hybridizations. The unlabeled, competitor DNA prevents binding of the labeled repetitive sequences from binding to the nuclei and chromosomes on the slide, thus decreasing hybridization background.
The most common suppressor DNAs tested were Cot1 DNA (Life Technologies) and salmon sperm DNA (Sigma). Suppression of the repetitive sequences of all human probes tested worked with both types of competitor DNA (Fig. 8h-j), but Cot1 DNA was about 10 times more efficient (1 ug Cot1 had as much suppression "power" as 10 ug salmon sperm DNA).
The relative amounts of probe and competitor DNA (human Cot1 DNA) used in most experiments are depicted in Table 5.