It’s probably just as well that few us are aware that the bodies we live in are a battlefield – the cells and molecules that make us are in constant strife to ensure our survival. The lid is lifted from time to time – when we get a cold or pick up some other infection and our immune response sorts it out but not without giving us a headache or a runny nose, just to let us know it’s on the job. By and large though, we plough our furrow in glorious ignorance.
Saving our cells
Perhaps the most important of all the running battles is to save our DNA – that is, to repair the damage continuously suffered by our genetic material so we can carry on. It’s an uphill struggle. The DNA in one of our cells can take up to a million hits every day – and the bombardment comes from every direction: from radiation, air pollution and carcinogens in some of the food we eat. And, of course, we don’t need to mention cigarette smoke.
Damaged chromosomes (blue arrows)
On top of all that cells have to make a new DNA copy every time they reproduce – and we do a lot of that: recall that you set sail on the journey of life as one single, fertilized egg cell and now look at you: a clump of ten trillion (1013) cells that, just to stay as you are, has to make one million new cells every second. What’s more some of your cells deliberately break their own DNA in a process called ‘gene shuffling’ that goes to make the finished product of your aforementioned immune system. The biochemical machinery that does these jobs is mighty efficient but nobody’s perfect – except, of course, for John Eales, Australia’s most successful rugby union captain, nicknamed “Nobody” because “Nobody’s perfect”. When the three thousand million base-pairs of DNA are stuck together for a new cell there’s a mistake about once in every million units added – but a kind of quality control check (mismatch repair) then fixes most of these, so that the overall error is about one in a thousand million. That’s one example of the nifty ways evolution has come up with to fix the damage suffered by our genetic material from all this replicating, assaulting and constructing.
Keeping the show on the road
The overall upshot of the repair machinery is that less than one mutation per day becomes fixed in our genomes – and thus passed on to succeeding generations of cells. The range of things that can damage DNA – and hence the different forms that damage can take – tells you that there must be several different repair systems and indeed we now know that about 200 genes and their protein products have a hand in some repair process or another. There’s so much to know that DNA damage and repair has its own data-base called, inevitably, REPAIRtoire. Much of what we know is, to a considerable extent, thanks to the labours of Tomas Lindahl, Paul Modrich and Aziz Sancar who have just been jointly awarded this year’s Nobel Prize in Chemistry. Because damage to DNA – aka mutations – drives the development of cancers you might suppose that in these pages we will have met these gentlemen before – and indeed we have, if not by name.
Tomas Lindahl Paul Modrich Aziz Sancar
Winners of the 2015 Nobel Prize in Chemistry
Forty odd years ago much of the above would have bewildered cell biologists. Thirty years before then, in 1944, Oswald Avery, Colin MacLeod and Maclyn McCarty had shown for the first time that genes are composed of DNA, a finding confirmed in 1952 by Alfred Hershey and Martha Chase in a classic experiment using a virus that infects and replicates within a bacterium. But with the acceptance that, however improbable, our genetic material was indeed made of DNA there came the assumption that it must be very stable. After all, if it carried our most valuable possession then surely it had to be made of molecular granite, absolutely resistant to any kind of chemical change or degradation. Had the bewildered boffins been told that in the twenty-first century we would be sequencing woolly mammoth DNA from samples that are millions of years old they would have been confirmed in their view.
It was Tomas Lindahl in the early 1970s who demonstrated that, although DNA is indeed more stable than its close rello RNA (the intermediate in making proteins) it nevertheless decays quite rapidly under normal conditions – it’s only when sealed in permafrost or blobs of amber that it becomes frozen in time. Lindahl realized that for life based on DNA to have evolved there had to be repair systems that could sustain our genetic material in a functional state and he went on to resolve how one of these did it. Aziz Sancar has worked particularly on the circadian clock (discovering that CRY is a clock protein) and how cells repair ultraviolet radiation damage to DNA: people born with defects in this system develop skin cancer if they are exposed to sunlight. Paul Modrich has contributed mainly to our knowledge of mismatch repair.
Lindahl, Modrich, Sancar and their colleagues over many years haven’t come up with the philosopher’s stone – the chemists still can’t transmute base metals into gold without the aid of a particle accelerator. But what they have done is much more useful for mankind. Revealing the detail of how genome maintenance works has already lead to new cancer treatments and from this beginning will come greater benefits as time goes by. They should enjoy the proceeds of turning molecular knowledge if not to gold then into Swedish kronor (8 million of them) – for the rest of the world it’s a bargain.
References
Lindahl, T. (1993). Instability and decay of the primary structure of DNA. Nature 362, 709-715.
Yang YG, Lindahl T, Barnes DE. (2007). Trex1 exonuclease degrades ssDNA to prevent chronic checkpoint activation and autoimmune disease. Cell 131, 873-886.
Shao, H, Baitinger, C, Soderblom, EJ, Burdett, V, and Modrich, P. (2014). Hydrolytic function of Exo1 in mammalian mismatch repair. Nucleic Acids Research 42, 7104-7112.
Tan C, Liu Z, Li J, Guo X, Wang L, Sancar A, Zhong D. (2015). The molecular origin of high DNA-repair efficiency by photolyase. Nat Commun. 6, 7302.
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