Radioactive probes by PCR

[Bert Gold]

Recently I prepared a radiolabeled probe from a 400 bp PCR fragment
by:

1) Purifying the fragment from an agarose gel (I use gene-clean,
   but whatever method you normally use will probably work).

2) Assess the amount of DNA fragment by loading 1/10th on a minigel.

3) Random prime the appropriate amount of the fragment (I think I
   used about 75 nanograms) using the Amersham Multiprime labeling
   kit #RPN1601C and alpha 32P-dCTP from NEN/DuPont.

4) I purified the labeled fragment from the unincorporated nucleotide
   by annealing it to gene-clean (Bio 101).  This works well, but
   you must use three washes.  I'm sure a minicolumn will do...

5) I ended up with between 75 and 100 million DPMs of label.
   It worked well on a Northern.