(94 



CHAPTER 40 



(replication, mutation, recombination) once 

 it has left the chromosome. As a result, 

 whether this DNA is genetic material re- 

 mains an open question. 



It should be realized that DNA which 

 leaves the nucleus may serve an extranuclear 

 function quite different from the function 

 DNA performs within the nucleus. For 

 example, nucleus-derived, cytoplasmically 

 located DNA may serve as raw material for 

 synthesis of nuclear DNA Such may be 

 the fate of the DNA in nonfertilizing sperm 

 which disintegrate in the cytoplasm of in- 

 sect eggs fertilized by polyspermy (more 

 than one sperm entering but only one ferti- 

 lizing the egg). In Drosophila. the DNA 

 of the nurse cells which surround the de- 

 veloping oocyte enters the cytoplasm of the 

 oocyte and presumably serves as raw ma- 

 terial for future DNA synthesis. The same 

 fate is suggested for DNA phagocytosed by 

 fibroblasts and white blood cells in vivo, 

 since phagocytosis of DNA by mammalian 

 cells in tissue cultures is followed by the 

 appearance of this DNA in the nucleus. 

 That DNA loss may be associated with dif- 

 ferentiation is indicated by: the loss of some 

 chromosomal material during chromosome 

 diminution in Ascaris; the differential elim- 

 ination of chromosomes by the two sexes 

 of Sciara; and the decrease in DNA in the 

 salivary gland cells of the snail Helix as the 

 secretion product is manufactured. It has 

 also been suggested ' that cytoplasmic DNA 

 may act as a messenger. 



It is clear from the preceding discussion 

 that all the DNA in a nucleus may not al- 

 ways remain there to perform the usual 

 functions of nuclear genetic material. In 

 this respect, then, there are two kinds of 

 DNA: the one conserved as part of the chro- 

 mosome (which serves as genetic material); 

 the one not conserved (which may or may 

 not be genetic material). In bacteria, the 



•See P. B. Gahan (1962). and J. J. Holland and 

 B. J. McCarthy (1964). 



nonconservation of nonintegrating or deinte- 

 grating DNA involved in transformation, 

 conjugation, or transduction has already 

 been mentioned. 



Lampbrush Chromosomes M 



Amphibian oocytes have giant "lampbrush" 

 chromosomes (Figure 40-2), whose appear- 

 ance is due to the lateral projection of 

 numerous pairs of loops from the main chro- 

 mosomal axis. Each loop is asymmetric — 

 one end being thicker than the other. In 

 addition to normal pairs of loops, some 

 giant granular loops are found which con- 

 tain a thin axial thread continuous with the 

 main chromosomal axis and having a dense, 

 contorted region at the thinner end of the 

 loop; a coarsely granular matrix at the 

 thicker end. 



When newt lampbrush chromosomes are 

 exposed to tritium-labeled (H' A ) uridine, 

 autoradiographs reveal that incorporation 

 into a pair of giant granular loops occurs 

 in a definite sequence, starting at the thin 

 end of the loop and proceeding around the 

 loop in about 10 days. These results demon- 

 strate the sequential synthesis of RNA by 

 different portions of the loop. Other evi- 

 dence proves that: 



1 . The loops contain DNA 



2. The RNA synthesis observed is DNA- 

 dependent 



3. Agents which inhibit nuclear RNA 

 synthesis (such as actinomycin D) 

 lead to disappearance of the loops 

 (and inhibition of puffing in insect 

 polynemic chromosomes ) . 



Such results lead to the hypothesis that a 

 loop (like a puff) is a temporarily unwound 

 portion of the chromosome thread. As the 

 thick portion of a loop completes its syn- 

 thetic activity, it presumably winds up and 



" Based upon work of W. R. Duryee, of J. G. Gall 

 and H. G. Callan ( 1962), and of M. Izawa. V. G. 

 Allfrey. and A. E. Mirsky (1963). 



