108 Nature of the Genetic Material 



gene), which has been called the target area (see Timof^eff, Zimmer, 

 and Delbriick). If the number of atoms within a cubic centimeter of 

 organic substance is known, and also the number of ionizations pro- 

 duced by one r dose of X rays in organic material, and, further, the 

 probability of producing mutation by one dosage unit in the radiation 

 experiments, then the radius of a target area can be calculated in which 

 one ionization must take place to produce a mutation with the proba- 

 bility of 1. The result calculated from experiments with many radia- 

 tions was a radius of 1.39 X 10""^ in one set and 1.77 X 10~' in an- 

 other set ( see TimofeefiF and Zimmer, 1947 ) . These are supposed to be 

 minimum numbers and, in addition, it is assumed that the gene itself 

 is larger than the target area. The conclusion is that the target area 

 and also the gene has the size of known large organic molecules. 

 Another calculation by Lea (1947), using the same principles, comes 

 to the conclusion that the molecular weight of the gene is 10,000 — 

 100,000, with a diameter, if spherical, of 2-6 m/x. A similar value was 

 obtained by a calculation for lethal mutations. 



These are the data from which the number of genes (capable of 

 lethal mutation, which is assumed to mean all genes) can be calcu- 

 lated. Following Lea (1947), the gene diameter applied is 4 mfx; the 

 dose of X rays required for an average of one mutation per gene is 

 2.9 X 10~'^r. The dose required for an average of one lethal mutation 

 per X-chromosome is 3.46 X lO^r. Therefore the number of genes in 

 one X is 2.9 X 1073.46 X 10* = 838 and, within the possible different 

 values of target area size found, varies from 280 to 1,000. This result 

 can be compared with calculations made from the structure of the 

 salivary gland chromosomes. The number of bands in an X-chromo- 

 some is near 650, and one-band deficiencies are known to act as if a 

 single locus were involved. If 1,000 genes are assumed to be present 

 in a salivary X-chromosome of 200 ix length, about 200 mp, would 

 be available for one gene, or according to Muller's measurements, 

 for two genes. Taking the estimated thickness of the chromosome into 

 accovmt, the upper limit for the size of a gene would be 100 X 20 m/i 

 with a molecular weight of about 25 million, which compares well 

 with some virus weights of 11-42 million (see Lea), which are known 

 to be nucleoproteins. The conclusion from these deliberations is that 

 genes are autonomous nucleoprotein molecules "regimented in chro- 

 mosomes to facilitate synchronization in division" (quoted from Lea). 



To return now to the counting and measuring of genes, it is clear 

 that the entire argument is based upon the correctness of the target 

 theory, with the premise that the gene is a definable unit. Without this 



