Cosmid Pics (2025)

| | Likely Cause | Fix | |-----------------|------------------|---------| | Single bright band at well | High molecular weight gDNA contamination | Add more RNase A; increase digestion time | | “Smiling” bands (curved) | Uneven gel polymerization or overheating | Cool gel before casting; lower voltage | | Multiple bands in uncut lane | Nicked and supercoiled forms | Check handling; avoid vortexing cosmid DNA | | White “ghost” bands on autorad | Insufficient washing after probing | Increase stringency; add SDS to wash buffer | | No bands at all | Cosmid lost or degraded | Re-transform; check antibiotic selection |

A classic image from this step is a petri dish containing hundreds of bacterial colonies, some white and some blue (if using a LacZ-based screening system). This visual readout allows researchers to quickly distinguish colonies that have a cosmid with an insert (usually white) from those that have religated empty vector (blue). cosmid pics