I have seen countless TV shows in which the police find some DNA at the scene of a crime and match it with the DNA of a suspect. Usually the matching process takes about 30 minutes. On the other hand, I have read that because of the chemical/biological processes involved in the matching of two samples of DNA, it takes 4-5 days and that all the demands for speed can not hurry it up. What is the truth? Can DNA matching be rushed or must it proceed at its own slow pace?
The usual way you match DNA in the USA is by doing selective PCR amplification of specific STRs, doing electrophoretic separation and measurement of the size of those regions, and comparing that set of numbers (13 of them) against the CODIS database. I don’t remember the exact amount of time it takes, but it’s longer than 30 minutes and shorter than 4-5 days. The workflow looks something like what you see here, though it looks like they’re trying to sell actual full sequencing stations instead of simple electrophoretic separators. This abstract claims 6 hours for an integrated system that does the analysis in the field.
The reason it actually takes 4-5 days is because of legal (custodial) issues and resource allocation; there’s only so many people available that are legally and technically qualified to do this, in only so many facilities, and nobody is going to work around the clock on a single sample.
EDIT: Oh, and in real life, it takes longer than 4-5 days for the same reasons as mentioned above. 4-5 days would be awfully fast turnaround time.
I’m not a forensic scientist, but I do use the same sorts of techniques in an academic lab. I’d say the process would take about a full working day. Maybe less if you work fast and have good reagents. Isolating the DNA takes an hour or so, depending on the techniques. Amplifying that DNA by PCR takes minimum an hour and a half, and possibly much longer, depending on how long the piece of amplified DNA is. Then the samples are loaded either for gel electrophoresis or capillary electrophoresis, and separated by length using an electric field. Another half hour to set up, and an hour or so to run. And to that total, and another hour to account for the few minutes here and there spent setting things up or walking across lab. So… five hours at the minimum.
Often, I’ll spread this over two or three days, since I’m multitasking a lot. After each step, samples can be tossed in a freezer for the next day.
I’d hope that in a forensic setting, the technicians would work very slow and carefully. Also, there might be verification and certification steps along the way that would make it longer.
If you’re doing PCR, you’re talking a day or so actual work time. If you’re doing a Southern-based assay, like RFLP, it’s more like 3-4 days, depending on how long you incubate various steps.
But you don’t need to be hovering over the equipment every minute of the process, right? You could, for instance, do a good many samples in parallel, if you had multiple apparati. So the turnaround might be a working day or more, but the throughput would be more than one a day.
Yes, absolutely. The delay is generally because of manpower, equipment, and sample volume issues.
Actually, if you have a dedicated setup and a technician to run it available at that exact instant, and if you ignore the chain-of-evidence requirements, there are protocols for very fast PCR direct from sample that might in fact be capable of returning a result in under an hour. They’re not industry standard by any means.
For PCR it’s typical to do more than one sample per run; as long as they all take the same conditions (as with a standardized assay), you can do as many as will fit into the machine (typically 48 to 384 for the standard research model, there may be machines that can do more at once).
That time is precious, and it’s when the techs get to eat lunch and check email and post on message boards :p.
Also, again depending on how it’s done, you can get an RFLP analysis done in a day. If you PCR amplify the source, then digest it, it just adds about an hour to the whole procedure. If you’re just digesting a purified sample without amplifying it, then you need to do the Southern to detect the small quantities of DNA.