430 R. M. S. SMELLIE 



will not undergo further division, Malkin assumes that there is some syn- 

 thesis of DNA in resting cells. The extent of C^^ incorporation found in the 

 sperm DNA was however small, and, bearing in mind the relatively high 

 initial concentrations of labeled precursor used, the absolute amount of 

 DNA synthesized must have been very minute. Taylor and McMaster-"^ 

 have followed the formation of DNA during microgametogenesis in Lilium 

 longiflorum using autoradiographic methods combined with microphoto- 

 metric measurements of Feulgen-stained preparations. Measurements of 

 the DNA content of the cells studied showed it to increase in bursts at 

 definite stages. Autoradiography of preparations grown in solutions con- 

 taining P^' showed that the incorporation of isotope into the DNA also 

 occurred in bursts corresponding to the stages at which the DNA content 

 increased. 



Further work has been carried out by Hokin and Hokin^"'' on the uptake 

 of P^- by pigeon pancreas slices in vitro. The assimilation of isotope by the 

 slices was inhibited anaerobically, but stimulation of amylase synthesis 

 by the addition of amino acid mixtures led to increased labeling of the 

 PNA nucleotides. Stimulation of secretion of the enzyme by various agents 

 was without effect on the renewal of the PNA. 



The in vitro incorporation of P^- into the PNA nucleotides of cat brain 

 slices has been the subject of attention by Rossiter and his co-workers.^"^ •^'^^ 

 It has been shown that the isotope is assimilated in such conditions, and 

 various factors affecting the rate of uptake of P^- have been studied. Thus 

 anaerobiosis, cyanide, and malononitrile effectively inhibit uptake of iso- 

 tope by the nucleotides, while the addition of pyruvate, lactate, glucose, 

 or mannose stimulates incorporation. 



Lu and Winnick-" have studied the incorporation of adenine-8-C''* and 

 guanine-8-C'* into the nucleic acids of chick heart fibroblasts in tissue cul- 

 ture and have found that adenine is well utilized in the synthesis of PNA 

 and DNA adenine and guanine. Guanine, however, was not an efficient 

 precursor of PNA or DNA guanine and there was scarcely any conversion 

 of guanine to polynucleotide adenine. When the cells were cultured in a 

 medium supplemented by labeled PNA or DNA, both the PNA and DNA 

 isolated from the cells were found to contain the isotope. 



The metabolism of mouse liver nuclear and cytoplasmic PNA has been 

 examined by Fresco and Marshak-"^ using uniformly labeled adenine-C^*. 



203 J. H. Taylor and R. D. McMaster, Chromosoma 6, 489 (1954). 



204 L. E. Hokin and M. R. Hokin, Biochim. et Biophys. Acta 13, 401 (1954). 



205 H. A. Deluca, R. J. Rossiter, and K. P. Strickland, Biochem. J. 55, 193 (1953). 



206 J. M. Findlay, R. J. Rossiter, and K. P. Strickland, Biochem. J. 55, 200 (1953). 



207 K. H. Lu and T. Winnick, Ezptl. Cell Research 6, 345 (1954). 



208 J. R. Fresco and A. Marshak, J. Biol. Chem. 205, 585 (1953). 



