502 



CHAPTER 41 



earl) region as expected. Because the total 

 Dumber of genes in phage is apparently Ear 



greater than the sum of structural proteins 

 in the mature phage plus the enzymes needed 

 to make phage DNA (Chapter 35), it has 

 been suggested that many of the genes that 

 do not specify the manufacture of phage 

 DNA or protein nevertheless play roles in 

 particle morphogenesis. 



Developmental Genetics of Amphibia 



The zygotic nucleus of a fertilized frog egg 

 can be removed by microsurgery.' 1 The 

 enucleated cell cannot normally perform the 

 functions of maintenance, growth, division, 

 and differentiation; lacking the normal chem- 

 ical reactions to carry on such functions, it 

 eventually undergoes degeneration. That 

 the cell's metabolic failure is attributable to 

 the loss of the nucleus rather than to the 

 operation itself, is proved by the normal be- 

 havior zygotes show after undergoing sim- 

 ilar operations without being enucleated. 

 More important, however, is the observation 

 that when the same (or a similar) nucleus 

 is replaced in a second operation, normal 

 zygotic activity resumes. Nuclei from blas- 

 tula. gastrula, and later embryological stages 

 can be transplanted into enucleated zygotes. 

 Such experiments reveal that the later the 

 stage supplying the nucleus, the more ab- 

 normal the development, demonstrating that 

 in the course of embryogenesis, nuclei are 

 progressively less able to promote complete, 

 normal development. 



Nuclei can be transplanted between dif- 

 ferent species of frog. Rana pipiens nuclei 

 which have multiplied in the cytoplasm of 

 R. sylvatica eggs are unable to promote gas- 

 trulation when retransferred to eggs of their 

 own species. Since this limitation persists 

 through repeated transfers to enucleated 

 eggs, 4 we can conclude that upon exposure 



• Based upon work of R. W. Briggs and J. T. King. 

 4 See J. A. Moore (1960). 



to cytoplasmic factors the chromosomes' 

 ability to function can become permanently 

 fixed (p. 487). 



Injection of small amounts of various pro- 

 tein fractions (albumin or his tone) from 

 adult frog liver cells into zygotes of the same 

 species stops cell division and arrests de- 

 velopment at the late blastula stage." At 

 about the same time, chromosomes become 

 essential for further development. Although 

 new cytoplasmic ribosomes do not appear 

 until the later tail-bud stage and new RNA 

 is first detected at the gastrula stage, ,; protein 

 synthesis — using messenger RNA and ribo- 

 somes synthesized before fertilization — be- 

 gins with fertilization. 



Differentiation and Transcription 



The similarities and differences among the 

 populations of nucleic acids in various tis- 

 sues of the mouse can be assessed by the 

 formation of double-stranded structures from 

 single-stranded DNA or RNA complexed 

 with single-stranded DNA entrapped in agar. 

 Competition reactions among radioactively 

 labeled and unlabeled sets of molecules fail 

 to show any differences in DNA polynucleo- 

 tide sequences, providing additional evidence 

 for the same DNA content in all somatic 

 cells. On the other hand, large differences 

 are found among rapidly labeled RNA mole- 

 cules isolated from different organs, as ex- 

 pected if differentiation is associated with 

 the production of different populations of 

 mRNA in different kinds of differentiated 

 tissues. 7 Other RNA-DNA hybridization 

 experiments s show that the mRNAs from 

 three growth phases of Bacillus subiilis are 

 derived from distinctly different groups of 

 loci, supporting the concept that differential 

 transcription of the genome occurs during 

 morphogenesis. 



■See C. L. Markert and H. Ursprung (1963). 

 ,; According to D. D. Brown and J. D. Caston. 

 "See B. J. McCarthy and B. H. Hoyer (1964). 

 "See R. H. Doi and R. T. Igarashi (1964). 



