176 SUBCELLULAR PARTICLES 



in the same nucleus? Evidence cited above indicates that in some instances syn- 

 thesis of DNA proceeds in a zipper-Hke manner along the chromosomes, but in 

 others it apparently occurs nearly simultaneously in many parts of the chromo- 

 somes. In those in which asynchronous synthesis occurs in the various parts of 

 chromosomes, DNA synthesis might be occurring on some molecules while 

 others are acting as templates for RNA synthesis. However, if most or all of the 

 molecules are replicating simultaneously, the template function for RNA syn- 

 thesis might be temporarily interrupted. The information available on correla- 

 tions of synthesis is still somewhat confusing. In several instances cells with DNA 

 synthesis in progress show relatively little incorporation of label in RNA. For 

 example, in the microspores of Tulbaghia, an onion-like plant, synthesis of DNA 

 for the next division occurs immediately after meiosis is completed. During this 

 period no synthesis of RNA in any part of the cell is indicated by incorporation 

 of P^" (15)- Iri addition, RNA synthesis is absent or at a low level during actual 

 division stages and during the zygotene pairing stages when the chromosomes 

 are perhaps in a non-metabolic condition. Sisken(9) also reports an inverse rela- 

 tionship between incorporation of orotic acid-C^* into RNA and DNA of root 

 cells of Tradescantia. Cells with high incorporation in DNA show relatively little 

 label in RNA. In premeiotic microsporocytes of lilies the incorporation of gly- 

 cine-C^^ into RNA of nucleus and cytoplasm appears to stop during DNA syn- 

 thesis. However, when orotic acid-C^* is used as the precursor the two types of 

 nucleic acid appear to be labeled simultaneously (16). In Drosophila salivary 

 gland nuclei, RNA and DNA synthesis proceed at the same time. 



In experiments carried out by one of us (Dr. Woods), tritium-labeled cytidine 

 was used to follow RNA and DNA synthesis in root tip cells of Vicia jaba. Dur- 

 ing interphase in these cells very little incorporation into RNA of chromosomes 

 (chromatin) occurred. Incorporation did take place in the nucleolus and cyto- 

 plasm. Incorporation into the nucleolus occurred during all stages of interphase 

 and during prophase. As soon as the nucleolus appeared in late telophase nuclei, 

 some isotope was incorporated. Therefore, DNA synthesis did not interfere 

 with RNA metabolism in the nucleolus of these cells. Not only was incorpora- 

 tion occurring at all of these stages, but at all stages the rate of incorporation into 

 nucleolar RNA was much higher than in any other part of the cell. This ob- 

 servation shows that meristematic cells of plants are similar to salivary gland 

 cells of Drosophila and several other types of cells in this respect. 



SITE OF SYNTHESIS OF NUCLEOLAR RNA 



What is the source of this nucleolar RNA? Is it derived from the chromosomes 

 or is it synthesized in the nucleolus? The answer should help in deciding 

 whether RNA can be synthesized without a DNA template, for typical nucleoli 

 contain almost no DNA. The example used in this discussion will be the large 



