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The precipitate is likely excess lipid or EDTA and will probablly not affect transfection efficiency. If your DNA plasmid is suspended in TE, be sure the concentration of EDTA is <0.3 mM, or suspend the DNA in sterile molecular biology grade water instead.
Yes, we recommend cells are at a low passage when being used for any application, including transfection. The reason why depends on what type of cells they are. Primary cells will undergo a finite number of divisions, and as they get closer to senesence they divide more slowly - both affecting their ability to take up DNA (transient transfection), and minimizing their abillity to incorporate the DNA into the genome (stable selection).Cultured common cell lines are often immortalized, and generally continue to aquire mutations, leading to a heterogenous population that may perform differently from cells of lower passage number - leading to results that are not reproducible.
The size of the plasmid should be considered when selecting a transfection reagent with the best efficiency. In general, larger sized plasmids should easily transfect with readily available transfection reagents, as along as the plasmid DNA is of high purity.
For many common cell lines, transfection reagent efficiency is very high and the protocols will not require any optimization. For hard-to-transfect cells or those ultimately expressing a toxic protein, the protocol should be optimized for best transfection efficiency. Taking time to optimize will give you more transfected cells with each procedure, which can mean more protein expressed and results that are reproducible.
There are many guides that help you select a transfection reagent. In general, consider:The type of cell(s) you will transfectThe type of nucleic acid or protein you will introduce to the cellThe composition of your cell culture mediumThe need for stable or transient transfectionThe equipment you have availableThe other factors important to you - cost, protocol flexibility, ease of use, etc.
The DNA needs to be good quality or it may cause the cells to lyse and/or they won't transfect efficiently. Plasmid DNA prepared with a column-based DNA purification kit is suitable for transfections. Sigma's GenElute™ Minprep, Midiprep and Maxiprep kits work well for DNA plasmid purification. After preparing the DNA, confirm the OD A260:A280 ratio is greater than 1.6 for use in plasmid transfections.
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