This is an important question. The more a police officer or investigator knows about DNA, the better it will be. However, in my opinion, there are a number of points that are a must.
First, police officers and investigators must know how to get a person's DNA properly, so that they would test the right person. This point might seen simple, but in a world that is complex as ours, we need to be able to test properly.
Second, officers need to know how reliable tests are. They should know how DNA has been used in the past. What are the blind spots? Have there been controversies? Has DNA tests ever failed? These are the questions that they should be thinking about.
Finally, officers should also know something about the testing process. The method of testing can be flawed as well. In short, more knowledge never hurt anyone.
Well, you need a good enough understanding of the DNA so that you can properly collect it and then the Forensic Analysts and such can find out who it belongs to which can ultimately to the crime being solved.