DNA types and sources

[Gabriel Akopian]

	I was just wondering if anyone out there can help me out with a  
few questions.  This might seem obvious to some of you, but I'm just  
starting to work with DNA, so any help would be appreciated.  What exactly  
are the differences between type I, II and type III DNA.  Also, why do  
most researchers use Calf Thymus DNA instead of lets say Salmon testes  
DNA...based on what I've seen in the literature, most of the structural  
studies involve Calf Thymus DNA type I.  Why?
	Another question is, how does one go about preparing a certain  
molar solution of DNA, in terms of molarity of phosphate?  Thanks in  
advance.