Southern with small probe?

[mpage at molbiol.ox.ac.uk]

In article <jpcd0-1703951652080001 at macr1-5.welc.cam.ac.uk>, jpcd0 at mole.bio.cam.ac.uk (John Dixon) writes:
> Hi, I just tried to do a southern with a 220 bp fragment. No bands at all.
> The fragment is the supF minigene which I know is included in both LTR's
> of some proviral insertions I am investigating.
> 
> Is 220 bp too small for some reason? - I have probed happily with 500bp
> fragments (random priming in both cases with alpha 32P CTP)
> 
> 220 bp may well be too small for efficient random priming. You might do 
better to label using specific primers complementary to one or both ends of the 
fragment. If you make two primers, you can generate a nice probe by PCR'ing with 
[35-S] dATP or DIG-dUTP in the reaction mix.