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Genetic Testing

Could they really have done all those genetic tests on Osama bin Laden so quickly?

-A curious adult from California

May 4, 2011

Yes, definitely. With the magic of a technique called PCR, they could have easily had their results in less than 24 hours.

They would need to know exactly what parts they wanted to test. And to have tested as many relatives beforehand so they would only need to do tests on Osama's DNA. As long as they had all of these ducks lined up in a row, they could compare the various DNAs and get the results quickly.

Since they presumably don't have a sample of DNA they know is from Osama bin Laden, they can't know for sure that it his DNA though. For example, if they just were comparing the DNA they got from the dead man in Pakistan to Osama's sister, they could conclude that he was probably her brother. But with just her DNA, they couldn't conclude that it was for sure Osama.

Of course, if all of her brothers are accounted for, then by the process of elimination that brother must be Osama bin Laden (as long as their isn't an unknown brother out there). If the governemnet had more relatives' DNA to look at than just his sisters', then they could be even more confident that the man they killed was Osama bin Laden.

For example, if they had DNA from one or more of his children and his sister, then scientists could make a very strong conclusion about his identity (assuming that an unknown brother didn't father his children). And the more relatives they can pile on, the better.

Luckily the government had more than DNA evidence. They had fancy facial recognition software and a bit of eyewitness testimony. So odds are they have almost certainly killed Osama bin Laden.

What I thought I'd do for the rest of the answer is briefly go over how the DNA tests were done. And give some idea about how long they might take. As you'll see, they can actually be pretty quick. 

Just a Bit of DNA

The first step is getting his DNA. This would have been a snap with any of the commercial kits available out there. They could get DNA from his blood in two hours or less.

Now that they had the DNA, they couldn't look at all of it. This kind of genome sequencing still takes way too much time. (It is expensive too but my guess is that wouldn't be a concern here.)

What the scientists would need to do is to look at just certain bits of his DNA. This is similar to what is done in a standard paternity test.

In these tests, scientists compare the DNA of the related people at multiple spots in the genome. The DNA they look at (called STRs) are repeated over and over, back to back and tend to be different in unrelated people. So if a bunch of these STRs match up in the right way, then odds are that the people are related.

Remember, DNA is made up of four chemical "letters" (or nucleotides) that are usually abbreviated as A, G, T, and C. So an STR is a string of letters that is repeated. Imagine the set of DNA letters AGTC. One person might have 10 of these in a row. And another might have 11 or 9.

Of course, many people will have 10 of these repeats. That is why scientists can't look at just one STR. Here in the U.S., forensic scientists have chosen 13 of them to tell people apart.

Since the government only had a few relatives to compare Osama's DNA to, they almost certainly looked at many more spots. The more spots you look at and the more spots that match, the more confident you can be that two people are related.

PCR Magic

The scientists can't just look at the DNA and see these STRs. No, the DNA needs to be manipulated so that the scientists can see it. This is where PCR comes in. This would probably take 3-4 hours to do.

PCR is a really fast way to make lots of copies of a bit of someone's DNA. Scientists use a special protein called a "polymerase" to copy the DNA. They tell the protein what DNA to copy by giving it starting and ending points, called "primers."

Primers are small bits of DNA that match the part of the DNA a scientist is interested in. The protein then copies everything between the two primers.

Here the government would have premade all of the primers on each side of the STRs they were interested in. They would have also worked out all of the reaction conditions so they could get the results in just a few hours. A single PCR reaction would look something like this:

 

If someone has a different number of repeats, then they'd have a different size of copied DNA. Imagine our example above. If I have 10 copies of AGTC and you have 11, then my copied piece of DNA will be 4 letters shorter.

Scientists then load the PCR reaction into a capillary electrophoresis sequencer so they can determine the sizes of DNA the reactions produced. They then compare all the DNAs from Osama with the DNAs from his relatives. Lots of the right matches mean they are related.

Osama's sister's DNA and the DNA from the man who died in the Pakistani compound must have shared lots of these STRs. So many in fact, that they were almost certainly brother and sister. And if you can add in a child's DNA, then you can find matches in the same generation and matches that are passed to the next generation.

Animation about how PCR works.

By Dr. Barry Starr, Stanford University


DNA evidence could be
obtained in less
than 24 hours.


Scientists looked at lots of little bits
of his DNA to confirm that
Osama bin Laden had been killed.