Cleland • Cytology: the Study of the Cell 



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ture constitutes the gene? Genes show 

 great diversity in their functional ac- 

 tivities. Not only are there thousands 

 of different kinds of genes in a single 

 organism, doing thousands of different 

 things, but the genes in different organ- 

 isms differ, at least in part, from each 

 other. The total number of different 

 genes in all of the species of plants, ani- 

 mals and microorganisms is undoubt- 

 edly very great. What part of the chro- 

 mosome is capable of existing in such 

 myriad forms? Apparently not the his- 

 tones, for they are relatively simple 

 structures, probablv not the globular 

 proteins, which constitute such a small 

 proportion of the chromosome. The 

 suspicion falls on the nucleic acids 

 which recent students have shown 

 are capable of an enormous variety 

 in their structure. This surmise is 

 strengthened by the finding that self- 

 duplicating bodies in the cell seem 

 all to contain nucleic acid: since one 

 of the chief characteristics of the gene 

 is its ability to reproduce itself, this 

 suggests that nucleic acid is tied in with 

 its structure in some way. It is prob- 

 able, however, that not all nucleic acid 

 is genie in character. Some organisms, 

 such as members of the lily family, for 

 instance, have relatively enormous 

 chromosomes, and the chromosome 

 set has a relatively huge amount of nu- 

 cleic acid compared with such organ- 

 isms as the fruit fly or man. It is not 

 likely, however, that the lily has more 

 genie material than the latter organ- 

 isms. The quantity of nucleic acid 

 present in a set of chromosomes, there- 

 fore, is probably not a measure of the 

 number of genes present. We are not 

 justified in exactly equating genes with 

 nucleic acid. It is likely that genes are 

 not pure nucleic acid but a combina- 

 tion of nucleic acid and protein, the 

 nucleic acid imparting to the gene its 

 specificity. 



Another focus of interest which is 

 occupying the attention of many cy- 



tologists at the present time is the 

 cytoplasm— that portion of the cell 

 which lies outside of, and surrounds, 

 the nucleus with its chromosomes and 

 genes. For a long time cytologists 

 tended to place relatively little empha- 

 sis on the c\toplasm, largely because 

 there seemed to be so little that could 

 be obser^'cd microscopically in this 

 portion of the cell. Bodies as striking 

 as chromosomes were seldom present, 

 very few definitive activities seemed to 

 accompany cell division, in contrast 

 with nuclear division. Some bodies, to 

 be sure, were to be seen, but except for 

 plastids (chloroplasts, etc.) none of 

 these were large enough to show much 

 structure under the ordinary micro- 

 scope, and none seemed to undergo 

 any sort of marked change or cyclical 

 modification. There seemed little, 

 therefore, that the cytologist could 

 learn about this portion of the cell. 



This situation is rapidlv changing, 

 however, in view of the discoveries by 

 both the geneticists and the biochem- 

 ists. The geneticists have discovered 

 that the cytoplasm has a role in hered- 

 ity which is far larger than had origi- 

 nally been suspected. The plastids, for 

 instance, have been shown to have 

 hereditan' characteristics of their own 

 which to a degree are independent of 

 those of the genes. To be sure, the 

 genes set up the conditions under 

 which the plastids operate, and if the 

 wrong genes are present, a given kind 

 of plastid may not be able to function 

 successfully. There are also cases where 

 a gene may succeed in bringing about 

 a more or less permanent change in 

 the structure or function of a plastid. 

 On the other hand, cases are known 

 where plastids remain uninfluenced by 

 foreign genes, in whose presence they 

 are unable to function, but to whose 

 influence they fail to yield— so that if 

 they are removed from the presence of 

 the uncooperative gene and find them- 

 selves again in a congenial genie en- 



