Origin: A Genetic History of the Americas

generally lost the tiniest chains of the ancient DNA fragments. If I had tried
to sample this tooth 10 years ago, it’s unlikely that I would have been able
to obtain as much DNA as I could now. It has taken some time to improve
DNA extraction methods to the point that we can recover more of these
crucial puzzle pieces.
One final spin to dry the column, and then I added an elution buffer,
changing the pH that was used to detach the DNA fragments from the
silica. The finished product looked unimpressive; holding a small tube up to
the light, I could see 100 microliters of clear fluid at the bottom, the
approximate size of the tip of my three-year-old son’s pinky finger.
I carried the tubes into the innermost room of the lab. This was the tiny
space past the garbing room and the extraction room kept at the highest
positive pressure level of the lab complex. We reserved this room for
activities involving the extracted DNA and negative controls. Inside one of
three benchtop hoods I began to add tiny quantities of chemicals into small
tubes to facilitate the next step in the process: determining whether there
was any DNA present in my extraction and negative control. The chemical
reaction that I was going to employ in this step—called the polymerase
chain reaction, or PCR—would produce millions of copies of a small region
of human mitochondrial DNA. In this case, I would target a section of the
mitochondrial genome that did not code for any proteins. I would melt the
DNA double helix into its two strands with a machine that controlled
temperature. Each strand would then serve as a template for making a new
strand within a cocktail of chemicals that imitated the DNA copying
mechanisms that cells use for reproducing their own genomes. I would
target a small region—less than 100 DNA bases long—for amplification
with two short, custom-made DNA fragments called primers. The primers
would bracket the targeted region and help direct the DNA copying
mechanisms to that site (4).
I also added another set of negative controls to this stage so that I might
be able to distinguish between contamination that occurred during this step
of the process and during the extraction process.
As I squinted at my pipette tips to verify that I was doing things
correctly, I remembered the words of James José Bonner, a kindly