Chapter *14 



POINT MUTATIONS 



W: 



e have already found that 

 the mutational unit of the 

 genotype may be a whole 

 genome, a single chromosome, or a part of 

 a chromosome. Perhaps a study of these 

 units will reveal more about the mutational 

 characteristics of a single gene; perhaps 

 the recombinational properties of individual 

 genes will illuminate this area of investiga- 

 tion. Let us consider what we already know 

 about the mutation of single genes — the class 

 of mutation that is probably the most impor- 

 tant in evolution because it causes the small- 

 est shift in gene balance. 



All chromosomes are linear and un- 

 branched whether or not they have under- 

 gone segmental rearrangement by crossing 

 over or breakage. This linear arrangement 

 could be due to the linkage of gene to gene 

 directly, or indirectly by a nongenetic ma- 

 terial which connects adjacent genes. In 

 either case, the fact remains that a chromo- 

 some is invariably either a rod or a ring, 

 providing almost conclusive evidence that a 

 gene cannot be joined to other genes at more 

 than two places, and that a mutation which 

 permits a gene to be joined to more than two 

 others cannot occur spontaneously or be in- 

 duced. That this type of mutation is never 

 observed regardless of the organism studied 

 can be interpreted to mean that genes never 

 had this property or that all existing genes 

 have lost this property. We are led to con- 

 clude, therefore, that all interstitial genes are 

 bipolar, and that mutation is incapable of 

 189 



causing the gene to be more than bipolar. 

 After chromosome breakage, the "stick- 

 iness" of the new ends is evidence that al- 

 most all mutations retain the bipolarity of 

 genes. In some relatively rare cases, how- 

 ever, break-produced ends (broken ends) are 

 known to become permanently unsticky or 

 healed, so that mutation from bipolarity to 

 unipolarity does occur. That mutation can 

 change genes from a bipolar to a unipolar 

 type, or vice versa, is evidenced also by the 

 presence of telomeres — unipolar genes which 

 seal off the normal ends of chromosomes. 



The chromosomal change from bipolarity 

 to unipolarity occurs regularly in the life 

 history of certain animals. In particular 

 species of the roundworm Ascaris, for ex- 

 ample, nuclei which remain in the germ line 

 have a single pair of chromosomes. When 

 the nuclei first enter the somatic line, how- 

 ever, these chromosomes break up into a 

 number of small linear fragments whose ends 

 are sealed off and behavior during mitosis 

 is normal — normal mitotic behavior being 

 possible because a germ line chromosome 

 has numerous centromeres along its length 

 (each surviving fragment of the chromosome 

 in a somatic cell has at least one). In the 

 germ-line polycentric chromosome all centro- 

 meres but one are suppressed. Because 

 chromosome fragmentation in Ascaris takes 

 place only in somatic cells, these polarity 

 changes can be attributed to some physiolog- 

 ical difference between cells entering the so- 

 matic line and cells remaining in the germ 

 line. These polarity changes should be con- 

 sidered recombinational rather than muta- 

 tional events because the changes from 

 bipolarity to unipolarity are numerous, si- 

 multaneous, and normal — therefore lacking 

 the novelty of mutations. 



Although mutations which change polarity 

 from bipolarity to unipolarity apparently oc- 

 cur, no unambiguous case has ever been 

 reported of the reverse, that is, of a muta- 



