DIFFERENTIATION AND PROTEIN SYNTHESIS 109 



Glandular cells, such as those of the pancreas or thyroid, have a high 

 rate of protein synthesis and it has long been known that their cytoplasm 

 stains deeply with basic dyes (Caspersson, 1950 and Brachet, 1957). When 

 it was shown that this basophilia was due to nucleic acids a connexion 

 between these acids and synthesis seemed certain. There are two kinds of 

 nucleic acids distinguished, among other things, by the sugars they 

 contain: desoxyribonucleic acid (DNA) containing the sugar desoxy- 

 ribose, and ribonucleic acid (RNA) containing ribose. The view that 

 nucleic acids were somehow related to synthesis was argued some years ago 

 by Caspersson (1950) who used ultra-violet absorption methods to detect 

 them and Brachet (1957) who used specific dyes. Brachet more definitely 

 urged the participation of RNA. Since that time direct chemical analysis 

 has confirmed the presence of nucleic acid and an enormously diverse 

 amount of cytochemical evidence based on all sorts of organisms has been 

 accumulated to show that DNA is invariably present in the nucleus of all 

 cells, and that RNA is always present in the cytoplasm of proteogenic cells 

 (Brachet, 1957). 



Caspersson at first suggested a scheme, based largely on the distribution 

 of ultra-violet absorption material, in which it was supposed that genes 

 (DNA) located on the chromosomes controlled the synthesis of histone-like 

 (basic) proteins which accumulated first as a nucleolus, and later passed 

 through the nuclear membrane and, entering the cytoplasm, provoked the 

 formation of RNA and the specific proteins. This view cannot now be 

 sustained in full. The presence of basic proteins is questioned and the 

 movements of RNA are thought to be different. 



Current research is centred on the interrelations of the genetic DNA, 

 RNA and protein and both experiment and speculation are very active. 

 Almost all authors accept the view that genetic information is carried by 

 molecules of DNA (Crick, 1958) and that these molecules must therefore 

 be duplicated at each cell division. The DNA of the nucleus in its genetic 

 role is said to contain all the information to ensure its own replication and 

 to form the various other materials of the cell. It is a basic assumption (a 

 " central dogma " according to Crick) that only nucleic acids possess this 

 peculiar property of conserving information and using it to guide synthesis. 



Isolated DNA has been shown by physicochemical methods and by 

 electron microscopy to be an extremely long molecule (several microns) 

 with a molecular weight of several million. A combination of X-ray 

 crystallography and chemical analysis shows that it consists of two helically- 

 intertwined chains (Watson and Crick, 1953) which are complementary in 

 shape to each other. If the two unwind and separate each might serve as a 

 " template " for the assembly of a new complement or for the formation 

 of other nucleic acids (RNA) as " copies." Current literature abounds 

 with hypothetical schemes for this replication process. In the chromosome 



