244 CELL HEREDITY 



each of the above hypotheses which, of course, are not mutally exchisive 

 (except as appMcd to individual cases). 



We shall review some systems among which may be examples of each 

 type. First it is appropriate to consider what is known about the organ- 

 ization of cell structure, especially in relation to replicating mechanisms 

 other than that for DNA. 



A REVIEW OF CELL STRUCTURE 



The precision of mitosis and meiosis implies a precise copying mechan- 

 ism; consequently, observations of these events led some cytologists to 

 predict an essential role for chromosomes in heredity before any sup- 

 porting evidence was available from genetics. No other cell structures 

 have been found which exhibit similar regularities in pairing and segrega- 

 tion at the level of the light microscope. Therefore, no obvious candidate 

 comes to mind as the carrier of nonchromosomal heredity. On the other 

 hand, many cell organelles can be seen to divide, suggesting the existence 

 of a submicroscopic copying mechanism. 



When cell structures are observed with the electron microscope, at the 

 level of their macromolecular architecture, one still does not find clear 

 evidence of replication. However, a basic uniformity of structural 

 organization is found in most or all cell types, involving membranes, 

 particles, and organelles such as mitochondria, so well defined as to merit 

 evaluation as possible autonomous hereditary systems. Furthermore, 

 since RNA may carry hereditary information, evidence on the localization 

 of RNA s within particular cell structures may provide some clues for the 

 identification of nonchromosomal determinants. 



In this brief survey we shall consider: (1) the membranous systems of 

 the cytoplasm; (2) the particulate components which contain most of the 

 high molecular weight RNA of the cell, namely, the ribosomes of the cy- 

 toplasm and the nucleolus of the nucleus; and (3) organelles such as 

 mitochondria, chloroplasts, and kinetosomes. 



Membranous Systems of the Cytoplasm 



As the techniques for electron microscopy of sectioned cells developed 

 over the last several years, new membranous systems of the cytoplasm 

 were observed, described, and named. Subsequently, as connections 

 were found between these systems, it became more and more likely that 

 they represented regional differentiations of a single continuous network. 



