. Cytology. Cytology. 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 Fu
. Cytology. Cytology. 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 (a) from Ficq, A. and Pavan, C, unpublished results.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. Please note that these images are extracted
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