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The description of cancer given at the beginning of this chapter might have been written in 1950 or i960. It represents an understanding of how cancer cells behave in terms of what was then known about cellular biology. Very little was understood shout what controlled events within cells and determined (among other things) their proliferation and differentiation-Things began to change in 1953 when Francis Crick and James Watson created their now famous model of the structure of DNA, the double helix. Today we can create computer images of DNA - Crick and Watson built models of wire and cardboard.

Chemical substances are made up of molecules. The discovery by Crick and Watson that DN A, a large molecule found in all cells, was shaped like two intertwined strands presented the solution to a problem that had puzzled scientists for years.

The DN A molecule is constructed from four types of smaller and simpler molecules, known as bases, strung out along each strand. Some people like to visualize the double helix as a spiral staircase, with the bases as the steps, and this is quite a good way of thinking about it.

Crick and Watson realized that this double-stranded structure could explain how the biological information in a cell could be copied exactly and transmitted to the two new cells which result when a cell divides. Each step of the spiral staircase consists of a pair of bases bonded in the middle and, because the bases do not pair up at random to form 'steps', but always pair up with a complementary base, each strand in the DNA molecule has a sequence of bases that is exactly complementary to the sequence of bases on la partner strand. When a cell divides, the two strands separate so that the DSA is split along its length. Each strand then becomes a blueprint for making a new partner strand with complementary bases, and the biological information in each new pair of strands is identical to that in the original DN A molecule.

What exactly is this biological information? As we see, there are four types of bases (actually representing four different chemical substances designated by the letters A, T, G and C). These can be thought of as a four-letter code, with the sequence in which the bases are strung out along the strands providing a coded message. Different pieces of the same DNA molecule can each have a unique sequence of bases so that each piece carries its own coded message. The number of different possible sequences using a genetic code of four letters is enormous, especially when we consider that a single typical animal cell contains one metre of DNA. This huge potential for different coded messages is the basis of the great variety that we find in the Jiving world and is, of course, the reason why one species k different from another and why each individual is unique.

Genes are made of DNA. They represent a section of DNA which carries enough information in its coded sequence to instruct the cell to make a particular protein. Each protein will then make up a part of the cell's structure or control an aspect of its function. The unique structure and function of each cell will be determined by the genes carried within that cell.



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