Saturday, March 8, 2008

This Week's Citation Classic

Mullis K, Faloona F, Scharf S, Saiki R, Horn G, Erlich H. 1986. Specific enzymatic amplification of DNA in vitro: the polymerase chain reaction.Cold Spring Harb Symp Quant Biol. 51 Pt 1:263-73.

Kary Mullis is an interesting guy. If I had to pick one invention that most impacted biology in the second half of the 20th century, I'd pick the PCR. The PCR allows one to amplify specific fragments of DNA using small, synthesized
oligonucleotides called primers. The basic technique involves three steps: denaturing a double-stranded DNA molecule via high temperatures, reducing the temperature to allow primers to "anneal", and then increasing the temperature to induce DNA polymerase obtained from thermophilic bacteria to synthesize new complementary DNA strands. The process is repeated many times to generate billions of new DNA fragments. The applications of the basic process are various, from sequencing DNA to introducing known mutations to DNA strands.

In the old days, the process was performed using water baths and a stopwatch (see photo). Now the process has been simplified using thermalcyclers. An animation describing the PCR is available here.

Mullis supposedly thought of PCR while driving. He describes the event here.

"I was working for Cetus, making oligonucleotides. They were heady times. Biotechnology was in flower and one spring night while the California buckeyes were also in flower I came across the polymerase chain reaction. I was driving with Jennifer Barnett to a cabin I had been building in northern California. She and I had worked and lived together for two years. She was an inspiration to me during that time as only a woman with brains, in the bloom of her womanhood, can be. That morning she had no idea what had just happened. I had an inkling. It was the first day of the rest of my life."

Outside of the PCR invention, Mullis has been quite controversial. He is a HIV skeptic, a global warming skeptic, and was supposedly aided in his discoveries by LSD. The HIV skepticism is largely moot since everyone now agrees that AIDS drug therapy works. Global warming skepticism is a bit more problematic, but Mullis appropriately cites the Scientific Method in defense of his claims:

"Very little experimental verification has been done to support important societal issues in the closing years of this century. Nor does it have to be done before public policy decisions are made. It only needs to be convincing to the misinformed voter. Some of the big truths voters have accepted have little or no scientific basis. And these include the belief that AIDS is caused by human immunodeficiency virus, the belief that fossil fuel emissions are causing global warming, and the belief that the release of chlorofluorocarbons into the atmosphere has created a hole in the ozone layer. The illusions go even deeper into our everyday lives when they follow us to the grocery store." [From Dancing Naked in the Mind Fields].

One man's kook is another man's iconoclast. I won't defend his views other than his right to have them.

Mullis received the 1993 Nobel Prize in Chemistry for his work.

Here are some of his thoughts on science:

"Science, like nothing else among the institutions of mankind, grows like a weed every year. Art is subject to arbitrary fashion, religion is inwardly focused and driven only to sustain itself, law shuttles between freeing us and enslaving us. Science consistently produces a new crop of miraculous truths and dazzling devices every year, truths and devices that enrich our lives and grow up out of the graciously willing puzzles of the unknown in an orderly but unpredictable way, out of a process of observation, hypothesis, experiment, conclusion; a process that as far as we know, was first proposed and adopted, only a few hundred years ago by a number of Europeans faced with a new world to explore and some worn out scholastic tools passed down from the ancient Greeks to explore it with.... Now we each of us have things and thoughts and descriptions of an amazing universe in our possession that kings in the Seventeenth Century would have gone to war to possess. We are the recipients of scientific method. We not only can luxuriate in its weed-like growth, but we can each of us be a creative and active part of it if we so desire. And we will. There is no stopping it, nor can there be any end to it."

Growing up, Mullis was given considerable latitude to explore his interests.

"I found a Gilbert Chemistry Set. Something about tubes filled with things with exotic names intrigued me. My objective with that set was to figure out what things I might put together to cause an explosion. I discovered that whatever chemicals might be missing from the set could be bought at the local drugstore. In the 1950s in Columbia, South Carolina, it was considered okay for kids to play with weird things. We could go down to the hardware store and buy 100 feet of dynamite fuse, and the clerk would just smile and say, 'What are you kids going to do? Blow up the bank?"

In today's hypersafety conscious world, I can't even imagine obtaining dynamite fuse as an adult.

Kary Mullis maintains a website here.

Photo: An "old-school" PCR machine from wikipedia.


  1. "In the old days, the process was performed using water baths and a stopwatch (see photo)."

    Shouldn't that be "using an undergrad, water baths, and a stopwatch"? That's the joke in our lab for anything tedious!

  2. Love your site.

    1) What did Mullis actually come up with: the technique itself, or the use of a thermolabile polymerase?

    2) Was he actually on LSD when he came up with whatever he came up with - or is this just an urban legend that grad students tell each other?

    3) I recall hearing that in the earliest version of the PCR - prior to Taq - someone would replenish a reaction with fresh polymerase after each denaturation step. Was this true? And how does this relate to (1)?

  3. Dear Punk,

    1. Its clear that Mullis did not "invent" PCR because that was mostly due to work done by late Kjell Kleppe, a Norwegian scientist working at Nobel Laureate H. Gobind Khorana's famous Institute for Enzyme Research at University of Wisconsin from 1968 to 1970. He presented his results at a Gordon Conf. and published in JMB in 1971. I think Mullis should be credited with "refining" the technique in addition to the idea of TAQ. How much of this can be credited to his Cetus colleagues is debatable and probably will never be known for sure.

    2. In a Q&A interview published in the September 1994 issue of California Monthly, Mullis said, "Back in the 1960s and early '70s I took plenty of LSD. A lot of people were doing that in Berkeley back then. And I found it to be a mind-opening experience. It was certainly much more important than any courses I ever took." During a symposium held for centenarian Albert Hofmann, Hofmann revealed that he was told by Nobel-prize-winning chemist Kary Mullis that LSD had helped him develop the polymerase chain reaction that helps amplify specific DNA sequences. I'm not sure this means that Mullis was tripping when he came up with PCR, but may have found "inspiration" for the idea, whatever that means.

    3. Yes it's true. That's essentially what Kleppe did, but the technique did not catch on since you had to add more polymerase after each round.

  4. I don't know if my last comment got through.

    Let's try it again.

    Here is your gift , John.