what are the main contaminant that can occur in plasmid DNA extration, and how we reduce it?

Question

i not sure the answer, i think is RNA, but really not sure

Answer 1

RNA can be removed by treating the sample with RNAse, which will leave the DNA untouched but degrade the RNA.

I'm pretty sure (95%) that's the right answer, based on experience. The other possibility (5%) could be chromosomal DNA, which can contaminate your plasmid prep if you're too rough with it or vortext it after adding the SDS/NaOH (solution 2). The way to reduce chromosomal contamination is to be gentle with your sample.

RNA is normally the contaminant, it's basically always popping up in your sample if you don't use RNAse. Genomic DNA contamination happens much less often, however in terms of causing trouble RNA poses little threat whereas genomic DNA can potentially screw you up later.