DNA is among the largest known. It is difficult to isolate DNA without fragmentation. The entire DNA of bacteria, such as Escherichia coli, has a molecular weight of 2.6 × 10⁹. Viral DNAs range in size from about 1 to 350 × 10⁶. The residue weight of a single nucleotide is 300 to 350. Thus, there are about three thousand nucleotides per million molecular weight of DNA.