@revelde20 I like my way because you only enter the liquid once and only pull off the desired product. If you try to take out the bottom layer, then you are penetrating both phases, which might lead to a bit of mixing and then when you do pull out the waste phase, your product is left in a dirty tube (you can imagine the walls of the tube are covered in organic gunk). So, while you can do it both ways, I think you get a more pure product by pulling off the top layer and going into fresh tubes.
According to wikipedia, signature tagged mut. is used to infer the function of a gene through transposon insertion. It is a random process targeted at the whole genome.
Site specific targets specific residues for mutation using specialized PCR primers.
I hope that helps!
I leave my DMSO at room temp so it does not freeze. Alternatively, you can just pop it into a heat block or water bath for a minute or so and that should do the trick.