why do we use DIG labeled lambda DNA size standards when performing a southern hybridization?

Answer 1

Well, you know that lambda size standard (ie lambda DNA that's been cut into specific-sized fragments by some restriction enzyme) can be run on a gel with your samples, right? That lets you compare the DNA fragments in your samples with fragments of known size. When you transfer the DNA from a gel to a membrane to make a Southern blot, the standards of course go too. Then, when you label the DNA with whatever probe you use, it would be nice to see the size standards on the film in addition to your particular band of interest. The problem is, that your probe is specific to one area of DNA, and the odds are that it won't match anything on the lambda fragments, so you need some other way to light up the standards (preferably without lighting up anything else and increasing your non-specific/background labeling), Since you mention DIG, I'm figuring that you're using an ECL system with DIG-labeled probes. These trigger a chemiluminescent reaction so that you get a release of photons wherever the probe is bound. If the size standard is labeled with DIG as well, then each lambda band will also trigger that CL reaction when you put on the reaction mix, and so you'll see on the film your band(s) of interest and a set of size standard bands with which to compare.

Answer 2

To allow us to estimate the size of our fragments in the same blot.