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CHAPTER 40 



pkism and nucleoli of the oocyte contains 

 an amount of aonconserved DNA equal to 

 that present in the chromosomes. Most ol 

 the RNA present in the mature amphibian 

 oocyte is synthesized in the lampbrush stage. 

 over 90 per cent being ribosomal RNA. The 

 lampbrush type of chromosome structure has 

 also been reported in the growing oocytes 

 oi animals ranging from mollusks to mam- 

 mals, in the pigeon, in the onion.' and in 

 the Drosophila spermatocyte Y chromo- 

 some. 10 It may well be more widespread 

 than previoush suspected. 



Histones 



The evidence presented strongly suggests 

 that in a large variety of organisms, chro- 

 mosome uncoiling and consequent DNA un- 

 masking are requisites for the utilization of 

 DNA as a template. The DNA of a mature 

 T phage is highly coiled in the phage head. 

 After T-phage attachment the DNA unwinds 

 and enters the host, where it is immediately 

 available for use as a template. Thus, phage 

 DNA when functional is probably uncoiled 

 and not complexed with protein. These 

 conditions probably also hold true for func- 

 tional chromosomal DNA in E. coli. In 

 contrast to bacterial and phage DNA, the 

 chromosomal DNA of most types of cells is 

 usually joined with basic proteins (such as 

 histone or protamine) to form deoxyribonu- 

 cleoproteins (p. 253). According to the 

 view already presented, perhaps the union 

 of DNA with histone causes coiling, which, 

 in turn, results in gene inactivation. The 

 influence of histones on gene action will be 

 considered further after we have explored 

 some of their properties. 



When DNA has been removed from nu- 

 cleohistone, the histone can be separated 

 by electrophoresis, ultracentrifugation, and 

 chromatography into numerous subfractions, 



; 'See B. R. Nebel and E. M. Coulon (1962). 



10 See W. Beermann. O. Hess, and G. F. Meyer 



(1963). 



indicating that a given type of cell probabl) 

 contains a heterogeneous population of hun- 

 dreds of kinds of histone molecules. These 

 molecules are relatively small — having a 

 molecular weight of 3,500 to 74. 000 — and 

 differ in amino acid composition. One class 

 is relatively rich in lysine (and proline) and 

 poor in arginine; another is relatively rich 

 in arginine and poor in lysine. DNA can 

 combine with these histone fractions to re- 

 constitute nucleohistone whose DNA seems 

 to be as fully complexed with histone as the 

 DNA in native nucleohistone. 



The deoxyribonuclcohistone reconstituted 

 by combining pure DNA and purified chro- 

 mosomal histones is about 35A in diameter. 

 The histone in such a nucleohistone seems 

 to cover the DNA uniformly; it may be 

 bound spirally around the DNA and occupy 

 one if not both the grooves of the DNA 

 double helix. Other arrangements, how- 

 ever, have not yet been ruled out. 



Interphase chromatin of somatic cells 

 seems to contain a uniform structural unit. 

 As seen through the electron microscope, 

 this unit is a rod approximately 160A in 

 diameter containing two double helices of 

 DNA (each 20A in diameter). The two 

 double helices are coiled about each other 

 paranemically (p. 268) and the histone 

 seems to occupy the space between and 

 around them. 11 Histones are probably syn- 

 thesized in the nucleolus, which is reported 

 to contain ribosomes. 



When DNA is complexed with histones 

 its melting or denaturation temperature rises. 

 In this respect, then, histones stabilize 

 DNA. 1 - Lysine-rich histones increase the 

 temperature required to melt half the DNA 

 of a pea from 70 C to 81 C; arginine-rich 

 nucleohistones half-melt their DNA at 71°C. 

 An approximately linear relation exists be- 



11 See V. Luzzati, and A. Nicolai'eff (1963). and 

 J. Bonner and P. O. P. Ts'o (1964). 

 ] -See also S. Felsenfeld. G. Sandeen. and P. H. 

 von Hippel (1963). 



