64 Inside the Living Cell 



X-ray burns, and this suggested the possibility of these substances 

 causing mutations. 



Since the original experiments of Dr Auerbach and her colleagues, 

 a great many substances have been discovered which are capable of 

 producing mutations. They are not usually very highly reactive sub- 

 stances. One of the necessary features of such a substance will be its 

 ability to reach the nucleus and react there with the chromosomal 

 filaments. If it is too reactive it will react with the first proteins and 

 other substances it meets when it enters the cell. One property of the 

 chemical mutagens, as they are called, is their ability to deliver a de- 

 layed punch in the cell nucleus. But some of the mutagens are appar- 

 ently only very weakly reactive (e.g. certain hydrocarbons) and the 

 nature of their action is still unknown. But they have one feature 

 which is shared by all or nearly all of them; they are also *carcino- 

 genic' agents — they are capable of causing some types of cancer (see 

 p. 95). 



They cause changes in the chromosomes which are very similar 

 to those produced by ionizing radiations. Not only are chromosome 

 breaks observed; but a 'stickiness' similar to that caused by X-rays, 

 i.e. in cell division the two sets of chromosomes do not pull away 

 from each other cleanly but leaves 'bridges' of chromosome material 

 between them. In fact it appears that the two sets of chromosomes 

 are joined together at one or more places. It is significant that many 

 of the substances which give rise to these effects are known as 'cross- 

 linking' agents, i.e. they can join two protein molecules together. It 

 is likely that they do join in this way the two dividing strands of the 

 chromosome fibre. 



STUDYING MUTATIONS WITH MICRO-ORGANISMS 



Many genetic experiments have also been carried out with micro- 

 organisms — fungi or bacteria. Work with these organisms has the 

 great advantage that the time taken to pass through one generation is 

 very short so that very large numbers of individuals can be dealt with, 

 and it is possible to make a thorough analysis of the frequency of 

 mutations and mutations which only occur very infrequently can be 

 discovered. 



A favourite material for experiments of this kind is the fungus 

 Neurospora. Beadle and Tatum exposed the spores of this fungus to 

 X-rays. Many of them are killed by this treatment, but among those 

 which survive mutants are found. These have lost the ability to per- 

 form one or more of the chemical reactions which the original 'wild 

 type' neurospora could perform. 



