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to calculate the positions of atoms within the DNA.
    After many weeks spent building different models with rods and sheets of cardboard and metal to represent the atoms within DNA, Watson and Crick suddenly found one which fitted exactly with the X-ray pattern. It was simple, yet at the same time utterly marvellous, and it had a structure that immediately suggested how it might work as the genetic material. As they put it with engaging self-confidence in the scientific paper that announced the discovery: ‘It has not escaped our notice that the specific pairings we have postulated immediately suggest a possible copying mechanism for the genetic material.’ They were absolutely right, and were rewarded by the Nobel Prize for Medicine and Physiology in 1962.
    One of the essential requirements for the genetic material had to be that it could be faithfully copied time and again, so that when a cell divides, both of the two new cells – the ‘daughter cells’, as they are called – each receive an equal share of the chromosomes in the nucleus. Unless the genetic material in the chromosomes could be copied every time a cell divided it would very soon run out. And the copying had to be very high quality or the cells just wouldn’t work. Watson and Crick had discovered that each molecule of DNA is made up of two very long coils, like two intertwined spiral staircases – a ‘double helix’. When the time comes for copies to be made, the two spiral staircases of the double helix disengage. DNA has just four key components, which are always known by the first letters of their chemical names: A for adenine, C for cytosine, G for guanine and T for thymine. Formally they are known as nucleotide bases – ‘bases’ for short. You can now forget the chemicals and just remember the four symbols ‘A’, ‘C’, ‘G’ and ‘T’.
    The breakthrough in solving the DNA structure came when Watson and Crick realized that the only way the two strands of the double helix could fit together properly was if every ‘A’ on one strand is interlocked with a ‘T’ directly opposite it on the other strand. Just like two jigsaw pieces, ‘A’ will fit perfectly with ‘T’ but not with ‘G’ or ‘C’ or with another ‘A’. In exactly the same way, ‘C’ and ‘G’ on opposite strands can fit only with each other, not with ‘A’ or ‘T’. This way both strands retain the complementary coded sequence information. For example, the sequence ‘ATTCAG’ on one strand has to be matched by the sequence ‘TAAGTC’ on the other. When the double helix unravels this section, the cell machinery constructs a new sequence ‘ TAAGTC ’ opposite ‘ATTCAG’ on one of the old strands and builds up ‘ ATTCAG ’ opposite ‘TAAGTC’ on the other. The result is two new double helices identical to the original. Two perfect copies every time. Preserved during all this copying is the sequence of the four chemical letters. And what is the sequence? It is information pure and simple. DNA doesn’t actually do anything itself. It doesn’t help you breathe or digest your food. It just instructs other things how to do it. The cellular middle managers which receive the instructions and do the work are, it turns out, the proteins. They might look sophisticated, and they are; but they operate under strict directions from the boardroom, the DNA itself.
    Although the complexity of cells, tissues and whole organisms is breathtaking, the way in which the basic DNA instructions are written is astonishingly simple. Like more familiar instruction systems such as language, numbers or computer binary code, what matters is not so much the symbols themselves but the order in which they appear. Anagrams, for example ‘derail’ and ‘redial’, contain exactly the same