Organization, Replication, and Types of DNA in Vivo 



317 



material from the nucleus has been described 

 in Artemia oocytes; extrusions of DNA from 

 the nuclei of ovarian cells of an insect have 

 been reported, and this DNA has been associ- 

 ated with the transference of nutritive sub- 

 stances to the developing eggs. Finally, the 

 high DNA content in the salivary gland cells 

 of Helix progressively decreases as the secre- 

 tion product is manufactured. 



In almost all of the cases where chromo- 

 somal DNA is produced in excess, either by 

 proportionate or disproportionate replica- 

 tion, it occurs in cells which themselves do not 

 give rise to future cell generations. Thus, 

 dipteran cells containing polytene chromo- 

 somes do not divide again, and Artemia 

 oocytes that extrude DNA die. While we 

 may conclude that DNA is sometimes re- 

 leased to the cytoplasm to perform certain 

 functions, we have no data from the studies 

 mentioned which indicate that this material 

 shows any of the other known or assumed 

 properties (replication, mutation, recombina- 

 tion) of genetic material once it has left the 

 chromosome. Accordingly, whether or not 

 this material is genetic remains an open 

 question. 



It should be realized that the DNA which 

 leaves the nucleus might serve an extra- 

 nuclear function which is quite different from 

 that which DNA performs as a nuclear 

 cistron. For example, nucleus-derived, cyto- 

 plasmically located, DNA may serve as raw 



material for future synthesis of nuclear DNA. 

 This may be the fate of the DNA contained 

 in the nonfertilizing sperm which degenerate 

 in the cytoplasm of insect eggs fertilized by 

 polyspermy (in which more than one sperm 

 enters the egg although only one fertilizes). 

 The DNA in the nurse cells of Drosophila is 

 reported to enter the cytoplasm of the 

 developing oocyte, presumably to perform 

 this function even before maturation. It has 

 been reported also that extracted DNA is 

 phagocytosed by fibroblasts and white blood 

 cells in vivo, and that phagocytosis of extracted 

 DNA by mammalian cells in tissue cultures is 

 followed by the appearance of this DNA in 

 the nucleus. We should note again that these 

 cases furnish no evidence that such DNA has 

 any of the characteristics we have described 

 for genetic material. Moreover, with the 

 exception of the special case of chromosome 

 elimination, we do not know if there is a flow 

 of DNA from the nuclei of more typical 

 cells — those that subsequently undergo cell 

 division. Nevertheless, it is clear from the 

 preceding discussion that all the DNA which 

 occurs in a nucleus may not always remain 

 there to perform the usual functions we would 

 expect from nuclear genetic material. In this 

 respect, then, there are two kinds of DNA, one 

 that is conserved ^s a part of the chromosome, 

 which remains our candidate for nuclear 

 genetic material, and one that is not conserved, 

 and which may or may not be genetic material. 



SUMMARY AND CONCLUSIONS 



DNA /// vivo usually exists in the Watson-Crick double helix configuration and usually 

 replicates, after the chains separate, by the formation of complementary chains. 



In certain viruses, (/)X174 and SI 3, the DNA is single-stranded. 



From the standpoint of behavior there are two kinds of DNA. One type is conserved 

 as a regular part of the chromosome and is presumably genetic. The other type is not 

 conserved, is nucleus-derived, and may be found in the cytoplasm and/or the nucleus. The 

 examples mentioned off"er no clear evidence that nonconserved DNA has genetic properties. 



REFERENCES 



Bensch, K. G., and King, D. W., "Incorporation of Heterologous Deoxyribonucleic Acid 

 into Mammalian Cells," Science, 133:381-382, 1961. 



