Chapter *23 



GENE AND POINT MUTATIONS 



WHAT have we learned, begin- 

 ning with Chapter 18, about 

 the mutational units of the 

 genetic material? We have seen that the 

 unit of mutation in the genotype may be a 

 whole genome, single chromosomes, and 

 parts of chromosomes. Even though each of 

 these units involves more than one gene, it is 

 possible that our study of these larger units 

 can tell us something about the mutational 

 characteristics of a single gene. It is also 

 possible that our knowledge concerning the 

 recombinational properties of individual 

 genes will shed light in this direction. Ac- 

 cordingly, we shall begin the present discus- 

 sion by considering what can be revealed 

 about single gene mutation by what we have 

 already learned. 



It has been demonstrated that the genes 

 in a chromosome are arranged linearly. This 

 linear order could be formed in two ways, 

 that is, either by having the genes attach to 

 each other directly, or by having some non- 

 genic material serve to connect adjacent 

 genes. In either event, the fact remains that 

 a chromosome is invariably linear, being 

 either a rod or a ring, but never branched. 

 For not only are all ordinarily observed 

 chromosomes linear, but, even when chro- 

 mosomes have been observed immediately 

 after crossing over, or after they have been 

 broken more than once and the pieces have 

 joined together, no case has ever been ob- 

 served of an authentic branched chromo- 

 some. This is almost conclusive evidence 

 that the gene cannot be joined, directly or 

 197 



indirectly, to other genes at more than two 

 places, and that a mutation of this kind 

 cannot occur either spontaneously or induc- 

 tively in genie material. The fact that this 

 change is never observed, regardless of the 

 organism studied, can be interpreted to 

 mean either that the gene never had this 

 property, or that it is lost to all presently 

 existing genes. We are led to conclude, 

 therefore, that all interstitial (nonterminal) 

 genes are bipolar, and that mutation is in- 

 capable of causing the gene to be more than 

 bipolar. 



Almost all mutations retain the bipolarity 

 of genes, as evidenced by the "stickiness" of 

 both ends produced by a break in a chromo- 

 some. However, in some relatively rare 

 cases broken ends are known to become per- 

 manently healed, so that mutation from bi- 

 polarity to unipolarity does occur. That 

 mutation must possess this property of chang- 

 ing genes from a bipolar to a unipolar type, 

 or the reverse, is evidenced also by the pres- 

 ence of telomeres, unipolar genes that serve 

 to seal off the normal ends of chromosomes. 



You may be acquainted with the fact that a 

 change from genie bipolarity to unipolarity 

 is a regular phenomenon in the life history 

 of certain animals, as in certain species of the 

 roundworm Ascaris. Here, in nuclei which 

 remain in the germ line there is a single pair 

 of chromosomes, but in those nuclei which 

 enter the somatic line these chromosomes 

 break up into a number of smaller linear frag- 

 ments, each of which has sealed-off ends 

 and behaves normally during mitosis. (The 

 latter behavior is made possible by the fact 

 that, in these cases, the germ line chromo- 

 some has a number of centromeres along 

 its length, and each fragment of the chromo- 

 some in a somatic cell has one of these centro- 

 meres. This is an exception to our state- 

 ment, on page 19, that normal chromosomes 

 are unicentric, and involves a polycentric 

 chromosome which in the germ line has the 

 action of all but one centromere suppressed.) 



