While gel electrophoresis used to be the standard way to get a “DNA fingerprint”, I think nowadays it’s more often done by sequencing, which provides more information.
To fake someone’s DNA, you need to know their DNA profile. So the scenario would have to be that you know their profile from seeing the results of a forensic analysis, but you don’t have access to their sample.
20 loci are used in the U.S. (CODIS). There are only a few different alleles in the population at each locus, the power comes from the large number of loci. So if you had samples from a bunch of random people (my guess is something like a few hundred) you would cover most of the variation present in the population. From this “library” of samples you could selectively amplify each locus from a person who matches your target at that one locus. Then mix the products together, and you’ve got 20 segments that would simulate a DNA sample from your target. If a few loci were not available in your library, that wouldn’t matter - not all loci always amplify in forensic testing, because the DNA sample is degraded.
As I mentioned above, the presence of just segments from the 20 CODIS loci (rather than genomic DNA) would work only if there were no grounds for suspicion, if the lab just ran the routine DNA profiling reaction. If they did suspect, it would be easy to show that genomic DNA was not present by simply attempting to amplify any other parts of the genome.
So can a lab actually separate out samples from multiple people? (The classic “she had sex with 3 different people”)? or does it all get processed mixed together, so if #1 has a 2 and an 8, #2 has a 3,4,and 6, #3 has a 4,12, 18 and #4 has 2 and 9… etc. then a false match would be anyone who matches all those numbers - 2,6,4,2etc., 2,4,12,2etc. 8,4,12,9etc. 8,6,18,9etc. would all be matches even though there are only 3 segments of any allele, hence 3 suspects. (I understand over 20 numbers the likelihood of match is still incredibly astronomically low)
It depends on how you do the analysis, which means it depends on how much time and money you’re willing to throw at the case.
For anybody reading along at home: It’s there at the top of the page referenced: “make millions of copies of a particular section of DNA”.
You don’t get DNA. You get sections of DNA.
You do get DNA from a PCR reaction.
I think there is some confusion in terminology here. DNA is a material - the polymer that the genome is made from.
I think what you’re trying to say is - you don’t get the entire genome. If you do PCR, you amplify short segments of the genome. Everything involved - the genome, the PCR primers, the PCR product - is made of the polymer DNA.
For anybody reading along at home, Melbourne thinks that making millions of copies of DNA isn’t making DNA.
Not necessarily.
It would make it more open to challenge by the other side’s lawyer:
- did this amateur have appropriate training to process crime scenes?
- did this amateur collect & handle the DNA sample properly, to avoid contamination?
- did this amateur maintain a proper chain of evidence, so that we know the sample sent to the lab is the same one collected at the crime scene?
Police crime-scene technicians have extensive training & experience in this; they have procedures to follow, they have numbered/recorded materials (envelopes, test tubes) to contain the collected samples, an evidence room to hold them until lab tests, etc.
An amateur would not have most of this. All that could be used by the lawyer to raise doubts in the minds of jurors about the accuracy of these specific DNA tests.
As OP subsequently explained, steepone had misconstrued the plot. The protagonist is not gathering evidence and doing forensic tests herself, she is amplifying DNA sample to plant evidence (for reasons that presumably become clear in the book).
Deliberate misquoting isn’t very nice.
I in no way whatsoever misquoted you.
Amplifying the DNA using PCR is one part. You will need, the PCR machine, centrifuges and lab environments with over 100 reagents, can’t go into all of the reagents.
You will need the enzymes that do the reaction. They are quite expensive and need freezers that go to -70 C. The specialist companies may not sell to you.
But what will be the biggest hurdle. 2 things. You will need the DNA or RNA primers and the radio isotopes that label the primers and its constituents. You will not be able to get the license for it.
When I did PCR, we didn’t have anywhere near those resources. We had the machine and a small number of reagents (I think only 2 or 3), we didn’t need any freezers, and I have no clue what we would even have done with radioisotopes even if we had had them. And all of this fit within the budget of a high school lab.
This is essentially completely wrong, aside from the fact that (as already stated upthread) some basic lab equipment is required.
Radio isotopes are not involved in any of the techniques we have discussed, you do not label primers or use radionucleotides for routine PCR. None of the reagents required for PCR or Whole Genome Amplification are dangerous, and none require a license. PCR and DNA prep/purification requires perhaps 10 reagents, not the 100 that you “can’t go into”, and standard commercial kits are available (as linked upthread) for Whole Genome Amplification.
Some reagents may last longer in low temperature freezers, but everything required for what we are discussing here is routinely kept in ordinary freezers for months. In fact, the key characteristic of TAq DNA Polymerase, the enzyme used for PCR, is that it is unusually temperature stable. I’ve never tried it, but I think you could leave it out at room temperature for days and it would retain most of its activity. It’s also cheap, as are all the other reagents involved in PCR. You can order custom DNA oligos (primers) for a few dollars, they will arrive in a couple of days via regular mail services.
If you are wanting to amplify target DNA you will need the sequence of the sample. How will you get that sequencing done without isotopes. How will you get specific primers to bind to the DNA if you don’t know the sequences.
(1) We’re talking about amplifying from human genomic DNA. Obviously the sequence of the human genome is known, so even if we were developing a new protocol we could easily design primers. But if you read the thread, you’ll see that we are talking about amplifying the 20 standard CODIS STR loci, which are extensively documented in the literature, along with validated primers and protocols.
(2) Sequencing would require much more specialized equipment; but no sequencing method now in use involves the use of radio isotopes.
You’re amplifying the DNA you put into the machine. Presumably, the amateur has some way of getting a small sample of DNA from the person they’re targeting. That’s the sample you put in the machine.
and the fourth point -
-is the amateur detective an impartial observer in the case?
Presumably, the whole story points to someone who has reason to tilt the investigation in one direction or another. Theoretically(!) police lab techs are impartial observers with no inclination to alter results to favour any particular outcome. Members of the general public, less so. Where they collected a sample, how, and whether they stored it properly are all up for discussion.
Back in the late 1800’s a Sherlock Holmes story had someone falsifying fingerprint evidence. planting forensic evidence is not unheard of.