184 CELL HEREDITY 



fibers of molecular dimensions, 100-200 A in diameter for chromosomes, 

 30 A for extracted DNA-histone, 20 A for DNA fibers alone. 



Let us turn now to a consideration of replication and duplication 

 of these genetic materials, processes which occur when the chromo- 

 somes are uncoiled. 



DNA AND CHROMOSOME DUPLICATION 



Here we shall distinguish between duplication and replication. Dupli- 

 cation is a general term, referring to any process by which a single struc- 

 ture becomes two identical ones. The structure in question may be a 

 molecule, an aggregation of molecules, an organelle, a cell, or an organ- 

 ism; and the mechanisms involved may be equally diverse. Replication, 

 on the other hand, refers to a unique process in which a molecule or 

 aggregate is duplicated, with full conservation of specificity, by copying 

 from a pre-existing molecule of the same kind. The only, materials 

 within the cell which are replicated are the hereditary materials; and, 

 conversely, the demonstration that a particular material is replicated 

 represents evidence that it is hereditary material. (For further discus- 

 sion see Chapter 9). 



We shall refer to the doubling of DNA as replication in view of the 

 evidence to be presented below that it does replicate, as well as the 

 inference from its role as hereditary material that it should do so. 

 Regarding chromosomes as a whole, however, we can speak only of 

 duplication, since the processes by which the non-DNA components are 

 synthesized and assembled are not yet known. 



Time of Chromosome Duplication 



Although cytologists studied the chromosomes of mitosis and meiosis 

 very systematically, they learned little about the duplication process 

 for the excellent reason, unknown until recently, that it occurs during 

 interphase when the chromosomes cannot be seen with the light micro- 

 scope. Recent cytochemical measurements of DNA per nucleus in cells 

 at recognizable division stages of mitosis and meiosis have shown that 

 the DNA content doubles during interphase. Nuclei in prophase and 

 metaphase stages have the double amount, which is reduced back to 

 the basal level by chromatid separation (Figure 7.2). 



In the analysis of genetic recombination, the time of chromosome pair- 

 ing in relation to the time of DNA replication is an important question. 

 Clearly, homologous chromosomes must be paired for exchanges to 



