A new perspective on PCR: Amplifying Mammoth mtDNA

Joshua L. Pollack et. al.

Muchas Gracias to Dr. Allan Wilson of Cal for spending his Saturdays not in the Memorial Stadium or at Raleigh’s but at his lab. Dr. Wilson was a pioneer in using genetics to understand human origins and today his successors have extended it to beyond just humans but to anything that is alive or was once alive (like the mammoth).

In a seminar I attended, Professor Monty Slatkin and his postdoc Josh Pollack presented a paper “Multiplex amplification of the mammoth mitochondrial genome and the evolution of Elephantidae” that was published in Nature. The paper discussed a new technique involving PCR to amplify the whole mitochondrial genome of a wooly mammoth.

Usually PCR is used in labs to amplify a small piece of DNA and such amplified products are used for various purposes including evolutionary genetics, phylogenetics, and disease resistance. What happens in this process is that we use two known primers to bind to a part of DNA of interest and then amplify that fragment in multiple steps. While this process works for contemporary DNA, it poses some challenges to analyze ancient DNA. PCR is super sensitive to contamination and old DNA molecules have some degradation which is a form of contamination and thus are prone to errors. Besides the amplified products from old DNA are tiny (less than 2,000 bps). Due to this problem, there are lots of ambiguities while using genes to analyze history.

According to Pollack, his team has found a new technique to generate a much bigger fragment or even the whole mitochondrial DNA of prehistoric organisms using “multiplex polymerase chain reaction.” In this process, if I understand it right, two sets of primers are used for an initial amplification so that products of one set overlaps with those of the second set. The first set has some primers that bind to specific DNA and the second set has primers that cover the gap between the primers in the first set.

Thus these two sets can cover the entire mtDNA genome. After the initial amplification each amplified product can be used as templates for further amplifications. Thus, with samples that would be necessary for a typical PCR, the whole mtDNA genome can be recovered. They applied this technique on a wooly mammoth to isolate its whole mtDNA which was an astonishing discovery. After analyzing the mammoth mtDNA they concluded that the mammoth was more related to the Asian elephants not African elephants as previously assumed.


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