The scale of the universe is truly mind-boggling, and it’s worth dwelling on. But it’s important to keep looking in other directions. Every day your body produces millions of new cells without you even thinking about it. Each of your cells contains the same set of DNA instructions*. Your cellular DNA quota (genome) is an incredibly thin chemical chain with about 6 billion links called nucleotides, and is approximately 2 metres long** (don’t test this at home!) Each time a new cell is produced, that DNA has to be copied to an extremely high level of accuracy. It was the same for all 50 trillion cells in your body – it all started with DNA replication. When a cell is about to divide, a number of proteins recognise and bind to the DNA at specific points. Geneticist John Bryant has said that it’s harder to begin DNA replication than it is to start a nuclear war – the process is that tightly controlled. At least forty different proteins have to be in position before replication can begin.
Once initiated, DNA replication happens relatively quickly. The clue to how this works is in the iconic DNA double helix image that is represented in art and architecture the world over. DNA consists of two complementary strands twined together: one is a mirror image of the other. This helix is unwound, and each chain is used as a template to build a new complementary strand of DNA. When you were conceived, you received a copy of each of your parents’ DNA. Making DNA is like writing: without proper editing mistakes will undoubtedly slip in. For DNA replication, multiple layers of proofreading ensure a high level of accuracy. So out of your 6 billion inherited DNA chain-links, only 30-70 of the links are wrongly copied. That’s a maximum of one mistake in every 100,000,000***. If I could do anything that accurately I’d be very happy! And this is all happening at great speed: 6 billion chemical reactions often in less than 24 hours. I remember writing about DNA replication in great detail during an exam at University. At the end of my essay I waxed lyrical about how this process was happening incredibly fast, at such a high level of accuracy, and without any conscious effort on our part. I’m not sure what the person marking my exam thought about my reverie, but I was impressed! I often find that looking in detail at the universe – even just standing outside on a dark night – gives me a feeling of smallness. Staring at the stars, or studying cosmology in depth, has given some people an awareness that there might be a God out there after all. What does looking at the very small and complex make people think? Tiny packages like cells or atoms can contain surprisingly complex systems, and immense power. Nothing is as simple as it seems. Perhaps as biology proceeds over the next few decades we’ll hit up against similar philosophical questions to those raised by the older sciences of physics and astronomy.*Apart from the lenses in your eyes and your red blood cells, in which the DNA is broken down to make way for crystallins and haemoglobin, respectively. ** This 2m of DNA is not a single unbroken strand but comes in 46 chunks, packaged into chromosomes. *** The vast majority of those mistakes don’t cause any problems, mostly thanks to the large proportion of noncoding DNA (which is often more flexible in terms of nucleotide sequence than sections of the DNA that code for proteins) in our genome. There is also redundancy in the genetic code, so that mutations in ‘coding’ DNA are not always destructive. References for the exceptionally keen DNA replication: http://www.ncbi.nlm.nih.gov/books/NBK21113/ Human mutation rates: http://www.nature.com/nrg/journal/v13/n4/full/nrg3206.html http://www.nature.com/ng/journal/v43/n7/full/ng.862.html http://www.nature.com/news/2009/090827/full/news.2009.864.html http://www.ncbi.nlm.nih.gov/pubmed/22345605