The role of the cytoplasm in heredity.

Abstract

This paper is an attempt to review in a rather general way the role of the cyto­plasm in genetics. In spite of more than fifty years’ work there is still much uncertainty over this topic, even extending in the opinion of some to such basic questions as whether hereditary cytoplasmic factors exist at all. By comparison with the chromosomally located genes, knowledge of which has grown amazingly both in the ‘classical’ chromosome era and also in the present ‘molecular’ one, our understanding of cytoplasmic factors is poor indeed. Nevertheless, there are of course many indications that cytoplasmic elements do play important roles in cellular development and heredity. My intention here is to consider—with perhaps a rather severe degree of scepticism—the more obvious non-nuclear components of the cell in relation to two questions: First, is there any evidence for the existence in the cytoplasm of elements which have all the properties of genes, save only those imposed on them by their chromosomal location? In other words, does the cytoplasm contain merely an auxiliary set of genes which have for some unknown reason become detached from the main nuclear headquarters, or are possibly relics of some ancient system of cellular organization which has in the main been abandoned in the course of evolution? Secondly, whether or not the answer to the first question is ‘yes’, are there non­nuclear elements which have hereditary properties but which do not consist of gene­ like units (presumably small strands of DNA ) and have a role different in principle from that of the nuclear genes? Without attempting to speculate in detail what such a role might be, we can imagine that the cytoplasmic elements could be responsible for the assembly of gene products into more complex structures.