II. KKPLICATIOX OF DNA IN CHROMOSOMES 67 



dem with non-DXA linkers, or alternately attached by their ends to 

 a shorter central axis; and (3) one long continuous DNA double helix 

 with its associated protein folded and coiled to form the visible chro- 

 inonema of a typical chromosome. 



First a morphological picture of a chromosome will be presented 

 briefly. Tiien the evidence on replication at the chromosomal and molec- 

 ular levels will be summarized. After examining the information avail- 

 able on the jiroperties of the subunit*:, their exchanges and reunion dur- 

 ing genetic recombination, and their repair following radiation-induced 

 breakage, the results will l)e interpreted in terms of a molecular model 

 of a chromosome. 



II. A NIoKl HDl.OGICAL PlCTlKE OF THE ChKO.MOSOME 



A. MITOTIC CHROMO.^^OMES 



Chromosomes present a changing pattern in relation to the cell divi- 

 sion cycle, and during interphase when some of the most important and 

 significant changes are occurring they present little structure which has 

 yielded to analysis with either the light or the electron microscope. 

 Perhaps it will be appropriate to begin a consideration of the chromosome 

 at anaphase when it is composed of a single chromatid. In some favor- 

 able preparations these rather flexible rods can be seen to consist of 

 helically coiled chromoneraata. Usually they appear to have a single 

 chromonema, but in some fixed preparations, probably due to treatment 

 with acid fixatives, the chromatids appear double. In these there may 

 be two helically coiled chromonemata (half-chromatids) usually ver\' 

 poorly separated as if the coils were interlocked and perhaps the axes 

 otherwise bonded together. Fixation in hot water may also reveal the 

 doubleness, but in vivo a doubleness has never been demonstrated. In 

 the living state anaphase chromatids appear to be solid cylinders. An 

 exception may occur in certain coccids which have chromosomes with a 

 multiple or diffuse spindle attachment (Hughes-Schrader. 19401 and in 

 certain symbiotic protozoa of termites (Cleveland, 1949 •. In the sec- 

 ondary' spermatocytes of the coccids the half-chromatids may be com- 

 pletely separated. In the symbiotic protozoa mentioned above half- 

 chromatids are reported to be separated in some cells and not in others 

 of the same species. Whether these move apart during fixation or actually 

 separate in the living cell is not clear. In addition, it is not clear that 

 the half-chromatids in these cases are comparable to the half-chromatids 

 of other organisms. However, it may be noted that the agents (fixatives) 

 which supposedly produce this partial separation would break hydrogen 



