Which cells to pick to get a pure sample of DNA without precise equipment?
Normally when conducting DNA testing, I presume techniques are utilized to ensure you're just testing one individual's DNA from a sample, even if it is possible that sample was contaminated one way or another.
However, in a thing I'm writing, there's a character that is able to produce personalized medicine for another individual after ingesting a sample of that individual's DNA.
The catch is that they can't always have laboratory equipment to isolate someone's cells (and thus DNA) from contaminants with other DNA (like from bacteria or just other people's dead skin). They might have to make due from eating hair or licking sweat, and so on, but the more contaminant DNA there is in the sample leads to reduced efficacy in the medicine.
My question, for the most part, would be which cells are good targets to collect samples from in this instance that are unlikely to have someone or something else's DNA in the sample; excluding anything that requires special equipment to get to or decontaminate the sample.
And to a lesser extent, which are the bad targets that fall under the conditions of being relatively easy to obtain a sample of, but are likely to have a high amount of contaminants?
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1 answer
Saliva is a nice clean DNA sample. Ditto cheek cells. You don't need anything more complex to get.
Since these are on the inside of the body, they won't have other DNA from touching things. You do need to make sure the mouth is reasonably clean before taking a sample (small amounts of food aren't an issue but large amounts of food or someone else's bodily fluids should be removed first.
All the genetic genealogy companies use saliva (spit into a clean collection container with preservative) or cheek cells (with a clean cotton swab put into a plastic case). This includes companies doing whole genome testing and medical DNA testing. You can use blood but it's not required.
Bacteria contamination is simply not an issue with DNA testing. Sure, you can have bacteria or other infectious substances degrade your sample but the testing equipment has no trouble separating out the bacteria (etc). Presumably your character has a way to separate out her/his own DNA from the analysis.
Your character can get a sample from a prolonged kiss with the subject (depending on how much DNA is needed; the vials of spit one gives can be used for multiple tests and the subject gives that much as a backup, even with a swab, one swab can be used for multiple tests and you can even get DNA from an old licked postage stamp). Get some tongue in there for science. A little scrape of the teeth doesn't hurt.
Another method would be to give the target a water bottle to drink from then have the character drink the backwash.
Of course, if the target wants this testing done, providing a sample is as easy as one-two-spit.
A poor sample would be from the outside of someone's body because it has a risk of being contaminated by someone else's DNA. I would especially avoid the hands and the feet (if the subject goes barefoot or wears sandals). Not to mention that outer layers of skin and hair and etc are not good sources of living DNA, no matter how clean they are (the little bulb you sometimes bring up when you pull out hair can have good DNA).
Sperm is not a viable choice for collection because it only has a random half of each chromosome other than the sex chromosomes and then either the X or the Y from that. It's good enough for a basic DNA match for forensics, but it won't give you the medical information you're looking for, though you can make predictions about the alleles if you analyze enough sperm cells from the same man.
Tears and urine only work if they have skin cells in them (like saliva does). But they and other bodily excretions (like earwax) can work some of the time. Sweat won't have usable DNA.
So make it easy. Spit and gentle scrapings of the inside of the mouth.
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