Quantum Evolution, Johnjoe McFadden. Chapter 3

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Quantum Evolution - Chapter 3 - Life’s biggest action

‘…But where do mutations come from? Mostly they are generated during DNA replication. The enzyme that has the job of constructing new DNA strands is called DNA polymerase. Its basic activity is to string together units of nucleotides (the units of DNA that carry the bases) to make new DNA strands. However, the enzyme doesn’t work unless it has a template on which to build the new strand. The old DNA strands are first unwound to allow DNA polymerase to slide along each strand (it forms a kind doughnut structure around the DNA) and make the new strand. But occasionally (about one in every ten thousand bases) DNA polymerase makes a mistake and inserts the wrong base. The enzyme has a number of possible excuses for its errors. Radiation and some hazardous chemicals can cause mutations; and they do so by promoting errors during the DNA replication process. However, one of the sources of replication errors is unavoidable because it is due to the intrinsic quantum nature of the DNA code.

In his 1944 book, What is Life, Erwin Schrödinger proposed that the genes were aperiodic crystals (crystals lacking a periodic structure) and that quantum fluctuations may be a source of mutations. However, Schrödinger went along with the prevailing view at the time that genes were made of protein, and so his suggestion was generally ignored when the true nature of the genetic code became known. The Swedish geneticist Per-Olov Löwdin from the University of Uppsala revived interest in the quantum nature of the genetic code by pointing out that it could be viewed as a linear array of protons. As I described above, the coding properties of DNA are due to the hydrogen bonding between protons and electrons in the DNA bases: the position of these particles determines which hydrogen bonds can form and thereby the base-pairing that underlies the genetic code. Protons and electrons are fundamental particles and their position is subject to quantum mechanics. The genetic code becomes a quantum code.

One of the peculiar features of quantum mechanics is that in most circumstances we cannot ever know exactly where a particle exactly is: its position is uncertain (an aspect of Heisenberg’s famous uncertainty principle that we will be examining in more detail in later chapters). This uncertainty is the basis for a phenomenon called quantum tunnelling, whereby protons are said to tunnel from one position to another. In fact quantum particles don’t really tunnel anywhere. It is just that the inevitable (Heisenberg) uncertainty in their position means that they materialise in places where you wouldn’t normally expect to find them.

The coding protons in DNA can (and must) quantum tunnel within the DNA molecule. This leads to tautomeric structures for DNA bases, with coding protons tunnelling from one atom to another.

Tautomeric forms of DNA bases can pair with the incorrect base: A can pair with G and T can pair with C (rather than A pairing with T and C pairing with G). Watson and Crick proposed that if during DNA replication, either the template DNA base or the incoming base is in the tautomeric form, then the wrong base may be inserted into the new strand, resulting in a mutation. Tautomeric forms of DNA bases account for about 0.01% of natural DNA bases so incorporation of incorrect bases, due to tautomerisation, is likely to be a relatively common event. However, our DNA replication machinery has proof-reading enzymes that are able to recognise incorrectly inserted bases and clip them out of the growing strand. The inclusion of proof-reading into the system vastly reduces the error rate for DNA replication to only about one wrong base inserted for every billion correct bases. Those errors that escape the correction machinery are the source of naturally occurring mutations; and their source is quantum mechanical.

Watson and Crick’s structure was therefore the culmination of centuries of biological progress. The great mysteries were laid bare: how biological information was encoded, how it was inherited and how it was changed. But it also pointed in quite a surprising direction, towards the involvement of that other great triumph of 20^th century science – quantum mechanics – in the fundamental basis of life and the driving force of evolution. ‘