DNA evidence is a really strong piece of evidence if the sample it's obtained from is in good shape. If there is a puddle of blood at the crime scene, then the dried out, oxidized, exposed to the surface portions of the blood are not the greatest sample. Bacterial action, basic chemistry, environmental chemicals will degrade it. However, the inside of a large puddle, or sample that is contained somehow in a cold area will provide a near perfect sample for DNA extraction.
In rape cases, the intravaginal fluids will actually help preserve the semen deposited by the raper so in a rape case the DNA can be a very strong sample. There is the chance, however, that if the victim was a very sexually active person that there could be many different DNA samples in there.
So as you can see, the time between when the crime was comitted and the evidence sampled is critical. The more time that passes, the less effective the DNA evidence is.
I worked in a drug-testing lab during college (kind of ironic) and learned all about the process involved in drug-testing. A similar type of confirmation needs to be done with DNA evidence. That's why any prosecution who is worth anything will send a sample off to multiple labs in order to get confirmation from two independant labs that the DNA sample is legitimate.
DNA evidence alone is not going to convict someone. However, DNA evidence can be an EXTREMELY powerful piece of evidence if used in conjunction with other evidence in a trial. By definition, it is Circumstantial evidence but 99.9% of ALL evidence is circumstantial. This is thanks to technology which can make videos, audio recordings, and photographs all circumstancial. In terms of the power of the evidence, DNA samples that are validated by multiple labs and have a high probablity of matching the suspect, is as close to direct evidence as you can get.
It still must be mentioned that in all trials that use DNA evidence, DNA evidence is not the only evidence they use. I don't know of ANY trials where only one piece of evidence is used. The sad thing about scientific evidence is that the VAAAAAAAAAAAAAAAAAAAST majority of the general public, the public that sits on the juries, has no idea how it works. Therefore, the prosecution needs to properly teach them about the evidence and what it means and most prosecuters don't know enough about it.
