72 J. llKHUKRr TAYLOR 



as those of the amj^hibian oocyte were called lainphiush ciiromosomes 

 because some early cytologists who studiccl them thought the extensions 

 from the axis were not loops hut bristles like llio.se of a brush. Actually 

 they may be bristles instead of loops at the earlier stages, during 

 pachytene, for example (Tayloi-, 1962). This point has not been estab- 

 lished. However, Moses (1956) demonstrated that an electron-dense 

 axial element exists in the meiotic chromosomes and that when fixed in 

 neutral osmium tetroxide the lateral extensions were fine filaments 50- 

 100 A in diameter which are often paired where they radiate from the 

 axis. That they contain DNA could be demonstrated by staining adjacent 

 thick sections of the same chromosome by thj Feulgen reaction. Moses 

 (1960) proposetl a model which is consistent with this structure as well 

 as that observed in the nuclei of amphibian oocytes described above. 



The axial element cannot be demonstrated after diplotcne by which 

 time the bristles, if that is what they are rather than loops, are being 

 folded or otherwise packaged in the condensing chromosome. Further 

 details of the chromosome's organization is not discernible with the elec- 

 tron microscope as it approaches the metaphase state. 



III. Replication of DNA 



A. CHROMOSOMAL LEVEL 



Until AVatson and Crick (1953a) pi-oposed a specific model for DNA, 

 ideas concerning its mechanism for transmission of genetic information 

 were rather vague. However, following the demonstration by Avery 

 et al. (1944) that the transforming principle of Pneumococcus was DNA, 

 the idea that DNA was the genetic material began to influence the 

 design of experiments. The finding of a constancy in amount of DNA 

 per genome (Boivin et al, 1948; ]\Iirsky and Ris, 1949) and finally the 

 experiments by Hershey and Chase (1952) which indicated that only 

 the DNA of phage entered the bacterial cell made the case more con- 

 vincing. Watson and Crick (1953a) took the information available on 

 bond distances and bond angles, on the most likely tautomeric fonns 

 for the bases, on the X-ray diffraction pictures which suggested a helix, 

 and the chemical analyses, which indicated that DNA contained a 1:1 

 ratio of adenine to thymine and of guanine to cytosine, and constructed 

 a model in which the purine and pyrimidine bases were directed toward 

 the axis of the helix. By allowing two hydrogen bonds between each of 

 these base pairs in opposing antiparallcl chains which rotated to form 

 a double helix with one gyre per 10 nucleotide pairs, a stable rather stiff 

 molecular model was obtained with the phosphoric acid residues at the 

 periphery of the structure. 



