Posts Tagged ‘double helix’
The young researcher Matt Meselson must have been very excited when he pulled a photograph showing a series of grey stripes out of his wallet and passed it round at breakfast on New Year’s day 1958. Most of us might have a limited understanding of what he was celebrating, but his work has since been hailed as ‘the most beautiful experiment in biology’.
Last week marked the 60th anniversary of the publication of Watson and Crick’s famous Nature paper describing the structure of DNA. The now iconic helix was a bold idea based on data from the biophysicist Rosalind Franklin, and kick-started a revolution in biology. From the 1960s onwards, molecular biologists, including Matt Meselson, have been unravelling the secrets of the genome.
As a student in genetics I was taught the key experiments that helped scientists to accept that DNA was the molecule of inheritance, understand its information-carrying properties, and figure out how that information is passed on. I’m glad we didn’t have to reproduce this work in the laboratory because it was highly technical, rather tedious, and often involved the use of radioactive chemicals. With my impressive track record of spilling liquids, I’m not sure I would have survived! The resulting data, however, are beautifully simple and satisfyingly visual.
Perhaps the fuzzy grey bands that Meselson pushed under his friends’ noses that day would not look beautiful or simple to most people. To a biologist, however, the clear and visible demonstration of the ‘semiconservative’ replication of DNA by Meselson and his co-worker Frank Stahl is beauty itself. Something that looked rather boring – a series of grey stripes representing DNA with different chemical labels – has changed the way we see ourselves in a fundamental way. Read the rest of this entry »
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
I’ve been getting some useful feedback on these posts, and I would particularly appreciate your input on this one. It’s an illustration that I’ve been using in a number of talks recently.
This is a cartoon of the molecular structure of DNA. Obviously no one has ever seen it – it’s too small even for the most powerful of microscopes. It was discovered by James Watson and Francis Crick (and several others) using x-ray crystallography in crystals of DNA. After their famous Nature paper was published, the science of molecular biology flourished and Watson and Crick’s model for the structure of DNA was tested in many ways. Now we have so much evidence for the double helical structure of DNA that no biologist would doubt it.
The structure of DNA hasn’t been ‘proved’. You can only prove things using maths. Science is about disproving things – narrowing down the options and getting nearer the truth. So in a way we believe that DNA is a double helix by faith – faith that is well founded on the ‘laws’ of physics, and lots and lots and lots of reliable evidence. I’d be fairly confident to stake my life on it (though I can’t imagine why I’d need to…)
What does that have to do with Christianity? This is something that I do stake my life on, so I’d better be sure it’s worth believing. Like science, none of the interesting questions in life – ‘Is that a great painting?’ ‘Does that person love me?’ ‘Is there a God?’ – can be solved mathematically. But I think there’s plenty of evidence to back up the claims of Christianity in history, archaeology, answers to prayer, the study of the Bible, and so the list goes on. For me the most important evidence is the way in which God changes lives. When someone you know well becomes a Christian you have the opportunity to see whether Jesus’ claims are true – first hand. I’ve seen it happen over and over again. That’s the reason why I’m a Christian, and why I think science and Christianity have something in common.