THE FINAL ADJUSTMENTS 129 



built up from smaller molecules which are attached 

 to one another end to end. In this case the smaller 

 molecules are not nucleotides but are another variety 

 of chemical configuration known as amino acids. It 

 is natural to suppose that the sequence of nucleotides 

 along the thread of DNA in the chromosome deter- 

 mines a sequence of amino acids which will be built 

 together to form the protein produced by that gene. 

 This idea receives some confirmation when one looks 

 at the proteins produced by abnormal genes. Proteins 

 are such complicated substances that only in a few 

 cases have they been fully analysed. One case in which 

 the analysis has been carried very far is that of haemo- 

 globin, the red colouring matter of the blood. Certain 

 genes are known in man which produce abnormal 

 types of haemoglobin. It has been shown that several 

 of these abnormal haemoglobins differ from the nor- 

 mal one only by a change in a single amino acid, out 

 of the whole sequence of several hundred. This is just 

 what we should expect if the abnormality in the gene 

 was due, in the first place, to an alteration of one ol 

 its nucleotides, and this altered sequence of nucleo- 

 tides determined the appearance of an altered sequence 

 of amino acids. 



We still do not know exactly how this sequence of 

 nucleotides in DNA determines the sequence of amino 

 acids in protein. It is certain that the process is not a 

 very simple one. For instance, the DNA is normally 

 found in the chromosomes inside the nucleus, and it is 

 doubtful if it ever moves far away from that location. 



