584 GENETICS AND EVOLUTION 



the amount of DNA per nucleus, has led to the conclusion that DNA 

 is an integral part oi the gene. Microchemical analyses have shown that 

 the amount ot DNA, like the number ot genes, is the same in all of 

 the somatic cells of a given species, and that there is only half as much 

 DNA in an egg or sperm as there is in a somatic cell of the same 

 species. There is other evidence that DNA is responsible for the trans- 

 mission of genetic information from one generation to the next. "Trans- 

 forming agents" can be isolated from certain strains of bacteria, such 

 as the one causing pneumonia, which will transform one strain of bac- 

 teria into another. These agents, with j^roperties quite similar to those 

 of genes, are composed solely of DNA. DNA is the carrier of genetic 

 information in bacterial viruses (bacteriophages). W^hen a bacteriophage 

 enters a bacterium, its protein coat remains outside; only the core of 

 nucleic acid enters. This nucleic acid core produces many additional 

 bacteriophage particles, both their nucleic acid cores and their specific 

 protein coats. When the infected bacterial cell finally bursts, many bac- 

 teriophage particles, complete with protein coats, are released. 



The separation of the nucleic acid part from the protein part of a 

 plant virus has been achieved by W. M. Stanley. The nucleic acid part, 

 but not the protein part, has some weak viral activity alone; viral activity 

 returns to normal when the two parts are recombined. Stanley then 

 added nucleic acid isolated from one virus to protein obtained from an- 

 other kind and found that the new "hybrid" virus had the genetic prop- 

 erties only of the strain which contributed the nucleic acid, and did 

 not resemble the strain which had contributed the protein. He believes 

 that the nucleic acid determines the biologic properties of the virus and 

 the protein forms a protective coat which stabilizes the nucleic acid. 

 Evidence from the experimental production of gene mutations also 

 favors the concept that DNA is an essential component of the gene, 

 for the physicochemical properties of the substance which mutates and 

 those of DNA are very similar. 



Estimates of the Number and Size of Genes. We have fairly reli- 

 able estimates of the number of genes per unit length of chromosome 

 in organisms such as corn and fruit Hies. If we assume that the number 

 of genes per chromosome in man is comparable, then man has about 

 25,000 pairs of genes in the nucleus of each cell. The error in this 

 estimate is probably no more than five-fold, and the true number of 

 genes lies between 5,000 and 125,000. 



Early estimates of the size of a gene suggested that it was a very 

 large particle, with a molecular weight in the range of 40,000,000 to 

 60,000,000. Hemoglobin, an average-sized protein, has a molecular weight 

 of 68,000. More recently, estimates of gene size have been revised down- 

 ward to perhaps half of the original value. At one time it was believed 

 that a gene was a true, indivisible unit, and it was enthusiastically 

 hailed as the "basic unit of life." It was believed that the unit of cross- 

 ing over in the chromosome, the unit which undergoes mutation to form 

 new types of genes, and the functional unit which regulates the pheno- 

 typic appearance of the character are all the same unit, the gene. It is 

 now clear, however, that these units have quite different sizes, the muta- 



