The marble of science

C2 Proficiency · Gapped Text · Objective Proficiency 2nd Edition, Cambridge University Press

You are going to read an extract from a 'popular science' book that gives information about living cells and DNA. Seven paragraphs have been removed from the extract. Choose from the paragraphs A–H the one which fits each gap (1–7). There is one extra paragraph which you do not need to use.

Can such a magnificently self-orchestrating process be explained or might the mystery of life be, in the end, impenetrable? In 1933, the physicist Niels Bohr, one of the founders of quantum mechanics, concluded that life hides its secrets from us in the same way as an atom does.

It is this templating that is the basis for the replication process and ultimately, the recipe for life. If a DNA molecule is pulled apart and if there is a supply of free base molecules – As, Gs, Cs and Ts – floating around, they will tend to slot in and stick to these exposed stumps and thereby automatically reconstruct a new strand.

Near the top of my list of its defining properties is reproduction. Without it, and in the absence of immortality, all life would sooner or later cease. For a long time, scientists had very little idea how organisms reproduce themselves. Vague notions of invisible genes conveying biological messages from one generation to the next revealed little.

Of course, there's more to it than just a bag of gadgets. The various components fit together to form a smoothly functioning whole, like an elaborate factory production line. The miracle of life is not that it is made of nanotubes, but that these tiny diverse parts are integrated in a highly organised way.

Boiled down to its essentials, this secret can in fact be explained by molecular replication. The idea of a molecule making a copy of itself may seem rather magical, but it actually turns out to be quite straightforward. The underlying principle is in fact an exercise in elementary geometry.

I beg to differ. Over the past few decades, molecular biology has made gigantic strides in determining which molecules do what to which. Always it is found that nature's nanomachines operate according to perfectly ordinary physical forces and laws. No weird goings-on have been discovered.

The former perform a purely scaffolding role, holding the molecule together. The business part of DNA lies with the latter, which are constructed from four different varieties of molecules or bases, with the chemical names adenine, guanine, cytosine and thiamine – let's use their initials for simplicity's sake.

As a simple-minded physicist, when I think about life at the molecular level, the question I keep asking is: How do all these mindless atoms know what to do? The complexity of the living cell is immense, resembling a city in the degree of its elaborate activity. Each molecule has a specified function and a designated place in the overall scheme so that the correct objects get manufactured.

Some years ago, scientists at Cornell University released photographs of a guitar no larger than a human blood cell, its strings just one hundred atoms thick. This Lilliputian instrument was sculpted from crystalline silicon, using an etching technique involving a beam of electrons. The implications of being able to develop machines that are too small to be seen with the naked eye are breathtaking, but we should not lose sight of the fact that nature got there first. The world is already full of nanomachines: they are called living cells. Each cell is packed with tiny structures that might have come straight out of an engineer's manual. Minuscule tweezers, scissors, pumps, motors, levers, valves, pipes, chains and even vehicles abound.

Individually, atoms can only jostle their neighbours and bond to them if the circumstances are right. Yet collectively, they accomplish ingenious marvels of construction and control, unmatched by any human engineering. Somehow nature discovered how to build the intricate machine we call the living cell, using only the raw materials to hand, all jumbled up. Even more remarkable is that nature built the first cell from scratch.

Like any urban environment, there is much commuting going on. Molecules have to travel across the cell to meet others at the right place and the right time in order to carry out their jobs properly. No overseer supervises their activities – they simply do what they have to do. While at the level of individual atoms life is anarchy, at this higher level, the dance of life is performed with exquisite precision.

Even nowadays, some people flatly deny that science alone can give a convincing explanation for the origin of life, believing that the living cell is just too elaborate, too contrived, to be the product of blind physical forces alone. Science may give a good account of this or that individual feature, they say, but it will never explain how the original cell was assembled in the first place.

It would be wrong, however, to suppose this is all there is to life. To use the cliché, the whole is more than the sum of its parts. The very word 'organism' implies cooperation at a global level that cannot be captured in the study of the components alone. Without understanding its collective activity, the job of explaining life is only partly done.

With the discovery of DNA, however, this mystery was finally solved. Its structure is the famous double helix, discovered by Crick and Watson in the early 1950s. The two helical strands are attached by cross-links and we can imagine the whole shape unwound and laid out to make a ladder, where the handrails are the two unwound helices and the rungs the cross-links.

Each rung is actually a pair of bases joined end to end and it is here that geometry comes in. A and T stand together – a table mat to butt neatly with T, while C and G similarly slot together snugly, though the forces that bind these base pairs in their lock-and-key fit are in fact rather weak. Imagine the two handrails being pulled apart, breaking all the base pairs, as if the ladder had been sawn up the middle. Each would be left with a row of complementary projecting arms.

So long as the base-pairing rules work correctly, this is guaranteed to be identical to the original. However, no copying process is perfect, and it is inevitable that errors will creep in from time to time, altering the sequence of bases – scrambling up the letters. If the message gets a bit garbled during replication, the resulting organism may suffer a mutation. Viewed like this, life is just a string of four-letter words, for we are defined as individuals by these minuscule variations in DNA.

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