DNA shearing methods - (Jun/02/2006 )

Hi, everyone...
I am suffering some problems with shearing DNA and I am expecting to get some advice from anyone here...

I am comstructing a cosmid library of DNA from environmental bacteria, which need the DNA size of around 40kb. The problem is that I cannot get a proper way to shear DNA to that size. when I sheared DNA with a needle for different times up to over 40, the bigger size of DNA didn't change much, but generate some very small DNAs...

Thanks very much for helps...

try sonication

Thanks, but I've already solved the problem...Still use the needle...

So, tell us the secret.

Different types of needles (I mean the like needles of 5ul or 10ul) will cause different types of shearing, so I obtained the proper size of DNA with a different type of needle...