et al, 1956). That the amount of DNA per nucleus may be affected by 

 changes in metabolic activity is suggested by several studies in which 

 it has been shown that reduction occurs in root tip cells following ex- 

 posure to low temperatures (Chayen, 1959; Evans, 1956; Heyes and 

 Shaw, 1958; LaCour, et al, 1956). There is no evidence of a similar 

 relationship in animal cells nor in bacteria (Allfrey, et al, 1955; Evans, 

 1956). It should be noted in passing that the phrase "constant amount 



(b) 



Figure 4-23. (a) Autoradiograph of Nucleus of a Cell from Intestinal 

 Wall of Fully Grown Larva of Rhynchosciara angelae, Injected with Triti- 

 ated Thymidine 24 Hours Prior to Fixation. Film developed after seven days' 

 exposure to the radiation from the chromosomes. The entire set of chromo- 

 somes (A, B, C, and X) is clearly shown, (b) Autoradiograph of Salivary 

 Chromosome from Intestinal Wall Cell of a Larva of R. angelae, showing 

 incorporation of tritiated thymidine only in specific regions of the chromo- 

 some, mainly in the heterochromatic chromocenter and a few of the euchro- 

 matic bands. (Fig. (a) from Ficq, A. and Pavan, C, unpublished results. 

 Fig. (b), from Pavan, C, 1958. "Morphological and Physiological Aspects 

 of Chromosomal Activities," Proc. X. Intern. Congress Genetics, 1, Fig. 2, 

 p. 324; courtesy of Dr. C. Pavan, Universidade de Sao Paulo, Sao Paulo, 

 Brazil.) 



of DNA" is by no means an absolute since the recorded variations 

 between cells appear to be considerably greater than can be accounted 

 for either by random errors in measurement or changes in chromosome 

 number. 



The results of short-term labeling of plant cells with tritiated thymi- 

 dine (Figure 4-24 (a) and (b)) by Taylor, et al. (1957), indicate that 

 chromosomal DNA replicates as if it were a double-stranded macro- 

 molecule of the type originally proposed for DNA by Watson and Crick 



STRUCTURE AND FUNCTION OF THE NUCLEUS / 105 



