Stanford Medical Bulletin 



ever, this difference may be illusory. In 

 special situations, F does have preferential 

 Mits of fixation (77). and generally, trans 

 locations of F t<> different sites are more 

 readil) discovered than those ol >■ would 

 be. 

 (5) Induction. Exposure ol lysogenic bacteria 

 to small doses of ultraviolet light causes the 

 prophage to initiate a lytic cycle with the 

 appearance first ol vegetative, then of ma- 

 ture phage (62). Hfr bacteria make no 

 analogous response. However, the kinetics 

 of the reversion, Hfr ~~ * F+, has not been 

 carefully studied. 



The genetic function of bacteriophages is 

 further exemplified by transduction whereby 

 genes are transferred from cell to cell by the 

 intervention of phage particles (42,91). In our 

 first studies we concluded that the bacterial 

 genes were adventitiously carried in normal 

 phage particles (92, 66, 83). Further studies 

 favor the view that the transducing particle has 

 a normal phage coat but a defective phage 

 nucleus. This correlation has suggested that a 

 gene becomes transducible when a prophage 

 segment is translocated to its vicinity (2, 9, 60). 



Transduction focuses special attention on 

 the phenomenon of specific pairing of homol- 

 ogous chromosome segments. Howsoever a 

 transduced gene is finally integrated into the 

 bacterial genome, at some stage it must locate 

 the homologous gene in the recipient chromo- 

 some. For in transduction, as in sexual recom- 

 bination, new information is not merely added 

 to the complement; it must also replace the 

 old. This must involve the confrontation of 

 the two homologues prior to the decision 

 which one is to be retained. Synapsis is even 

 more puzzling as between chromosomes 

 whose DNA is in the stabilized double helix 

 and then further contracted by supercoiling. 

 Conceivably gene products rather than DNA 

 are the agency of synaptic pairing. 



The integration of a transduced fragment 

 raises further issues (41). The competing hy- 

 potheses are the physical incorporation of the 

 fragment in the recipient chromosome, or the 

 use of its information when new DNA is repli- 

 cated. The same issues still confound models 

 of crossing over at meiosis in higher forms; 



s- 



once again the fundamentals of chromosome 

 structure are needed for a resolution. 



VIRUS VERSUS GENE 



The homology of gene and virus in their 

 fundamental aspects makes their overt differ- 

 ences even more puzzling. According to the 

 simplest nucleic doctrine, DNA plays no active 

 role in its own replication other than furnish- 

 ing a useful pattern. Various nucleotide se- 

 quences should then be equally replicable. 

 What then distinguishes virus DNA, which 

 replicates itself at the expense of the other path- 

 ways of cellular anabolism? For the T-even 

 phages, the presence of the unique glucos- 

 ylated hydroxymethylcytosine furnishes a 

 partial answer (12). However, other viruses 

 such as a display no unique constituents; fur- 

 thermore, as prophage they replicate coordi- 

 nate^ with bacterial DNA. Does the virus 

 have a unique element of structure, either 

 chemical or physical, so far undetected? Or 

 does it instruct its own preferential synthesis 

 by a code for supporting enzymes? 



THE CREATION OF LIFE 



The mutualism of DNA, RNA, and pro- 

 teins as just reviewed is fundamental to all 

 contemporary life. Viruses are simpler as in- 

 fective particles but must, of course, parasitize 

 the metabolic machinery of the host cell. What 

 would be the least requirements of a primeval 

 organism, the simplest starting point for pro- 

 gressive replication of DNA in terms of pres- 

 ently known or conjectured mechanisms? 

 They include at least: 



(1) DNA. 



(2) The four deoxyribotide pyrophosphates 

 in abundance. 



(3) One molecule of the protein, DNA po- 

 lymerase. 



(4) Ribotide phosphates as precursors for 

 RNA. 



(5) One molecule of the protein RNA po- 

 lymerase. 



(6) A supply of the twenty amino acyl nucle- 

 otidates. 



(a) Failing these, each of the twenty en- 

 zymes which catalyze the condensa- 

 tion of an amino acid and correspond- 



106 



