POLYNUCLEOTID1-: SYNTHESIS IN NUCLEOLUS AND CHROMOSOMES 1 75 



pattern of the chromatid exchanges. The analysis indicates that the two units 

 of DNA are structurally different, which supjiorts the concept that lluv repre- 

 sent complementary units (15, 14). 



Recently Meselson and Stahl (5) have obtained evidence that the first particles 

 of DNA replicated in N^''-labeled E. coli, which have been transferred to a 

 medium containing only N' ^-labeled nutrients, are hybrid with respect to their 

 content of N^'^ and N^^. At the next replication these hybritl particles, with a 

 molecular weight of about seven million, produce one hybrid particle and one 

 particle free of N^ '. These results, which demonstrate the conservation and seg- 

 regation of DNA at the molecular level similar to that indicated for the whole 

 chromosome by the tritium experiments, make the concept of replication from 

 a duplex type of template almost a certainty. 



SYNTHESIS OF RNA 



Since RNA (ribonucleic acid) is a similar type of molecule, the question arises 

 concerning a template mechanism for its synthesis. The duplex nature of the 

 molecule is not established and it has not been possible to build a precise model 

 for its duplication. In the absence of large units which contain a permanent 

 component of RNA, such as the chromosome provided for DNA, there does not 

 appear to be a situation in which the hypothesis of replication can be tested by 

 autoradiography. Chloroplasts provide a possible object for testing the idea, but 

 so far a suitable biological material and a selective method of labeling have not 

 been found. Another approach to the mechanism of synthesis is an indirect one, 

 in which evidence concerning the site of synthesis or correlations in RNA and 

 DNA synthesis within intracellular dimensions are determined. If RNA acts 

 as a code for determining the sequence of amino acid in proteins, this information 

 initially resides in the genetic material, which is most probably DNA. Therefore, 

 many investigators have considered the idea that some of the RNA may be syn- 

 thesized in the chromosomes on the DNA templates. 



Is there a sequence in the synthesis of RNA and DNA? In other words, does 

 one process stop when the other begins? If DNA were the template for both its 

 duplication and RNA synthesis could the two processes proceed simultaneously 



Fig. I. Autoradiograph of chromosomes after one duplication in tritium-labeled thymidine. 

 Root tip cell of Bellevalia followint; a short treatment in colchicine. [After (14).] X MiO- 



Fig. 2. Autoradiograph of a similar cell after one duplication in tritium-labeled thymidine 

 and one duplication in a medium free of labeled precursors. [.After (13).] X Mi"- 



Fig. 3. Phase photograph of salivary gland cell of DrosophiLi. The dark object near the 

 center of the nucleus (lighter region) is the nucleolus. X I3<'<'. 



Fig. 4. Autoradiograph of the cell in figure 3 fixed 3<i minutes after the larva began eating 

 tritium-labeled cytidine. Most of the grains are over the nucleolus, but a few are already over 

 the chromosomes of the lower nucleus ami also over the nucleus in the upper center of picture. 

 The radioactivit\' is all removable with ribonuclease. X 1300. 



