Help! Physicist needs Chemist's help!

[Stew2]

I need a little info on DNA finger-printing.  I need to know why S35 is
used, and if possible, can it be substituted with a stronger radioactive
substance?  I would also like to know if anyone knows the smallest (on the
average) size of a "line" on the x-ray plate is.  I would appreciate it if
you could e-mail me any answers, or info on where I can find answers. 
Thanks-----Stewart 
email----Stew2 at aol.com