Re. restr. digest

[Karl-Heinz Worm]

Paul N. Hengen wrote
> Normally 1 Unit of enzyme will digest 1 ug of DNA in 1 hour at 37 C, so
> for 1 mg DNA = 1000 ug you'll have to add a minimum of 1000 units to digest
> your sample in an hour....

This is what I often read but what I believe is wrong.
As far as I know 1 unit of a restriction enzyme will digest 1ug 
of LAMBDA-DNA in 1h at (in most cases) 37 C (unless no site is present in 
lambda). That means that 1 unit of a given enzyme might be sufficient for 1 ug 
of any DNA but may also be an unappropriate amount. 
For example SalI has two sites in lambda (about 50 kb). If 
you want to digest a Plasmid of 5 kb containing two SalI sites
you have to use the tenfold amount of enzyme i.e. 10 units per ug.

CORRECT ME IF I`M WRONG !! (but don` correct the english. no hope for it)

Karl-Heinz Worm
worm at mpi-muelheim.mpg.d400.de