BIOLOGICAL ROLE OF DEOXYPENTOSE NUCLEIC ACIDS 471 



enormous range of variation in the degree of replication of the "unit" 

 haploid amount of DNA. One gains the impression that the DNA might, 

 on occasion, be accumulated nonspecifically and stored in the chromo- 

 somes in addition to being replicated in specific functional form. 



One place in which DNA clearly seems to be accumulated in excess of 

 immediate needs is in the unfertilized egg or earlj^ embryo of the sea 

 urchin^2o-222 q^j^^ gf ^j^g fj.Qg 223,224 gince in the frog embryo the total 

 amount of deoxyribosides does not increase until a late blastula stage,^^' it 

 appears probable that the large amount present in the egg is serving as a 

 storage reserve for early growth, as do other components. The quantity of 

 deoxyriboside present is great enough to more than occupy the whole egg 

 nucleus, and in fact can be demonstrated to be present in the cytoplasm 

 in about 5,000 times the amount present in the haploid sperm nucleus, 

 and yet remains so widely dispersed that it is virtually unrecognizable by 

 staining or photometric assay. Once their own DNA synthesis is well es- 

 tablished, embryonic tissues do not show wide departures from the "con- 

 stant" amount of DNA per cell or per nucleus, except that, as already 

 noted, these rapidly growing tissues may tend to have an average content 

 near the double quantitj' needed for division. What may be exceptional 

 about the egg DNA is perhaps not so much its reasonable accumulation in 

 high amount, but that it is present in the cytoplasm, and will very likely 

 prove to be in a precursor form, not yet converted into genetically specific 

 DNA. 



Preparation of hen's-egg avidin, the biotin-binding protein, usually gives 

 a product containing nondialyzable DNA of typical composition.^" There 

 is some evidence that the complex of avidin and nucleic acid is not readily 

 dissociable and may exist as such in the egg white. 



The kinetoplast of the Trypanosoma, a chromatin body lying at the base 

 of the flagellum, takes the Feulgen stain and apparently contains DNA. 

 This body, although not connected with the nucleus, undergoes regular 

 division of its own when the cell divides; it is altogether possible that it 

 may yet prove to have a function in principle not unlike that of the nuclear 

 chromatin. Lwoff includes the kinetoplast among the biological entities 

 endowed with genetic continuity, and states that the Feulgen-positive 

 material appears to be in an annular ring or shell.^-® 



"0 C. Vendrely and R. Vendrely, Compt. rend. soc. biol. 143, 1386 (1949). 

 "1 D. Elson and E. Chargaff, Experientia 8, 143 (1952). 

 "2 E. Zeuthen, Pubbl. staz. zool. Napoli 23, 47 (1951). 

 «3 E. Hoff-J0rgensen and E. Zeuthen, Nature 169, 245 (1952). 

 "* L. C. Sze, J. Exptl. Zool. 122, 577 (1953). 



"6 H. Fraenkel-Conrat and E. D. Ducay, Biochem. J. 49, xxxix (1951). 

 "6 A. Lwoff, Colloq. intern, centre nail, recherche sci. (Paris) 8, Unites biol. donees 

 contin. genet. 1 (1949). 



