176 CELL HEREDITY 



the present status of knowledge in this field. At the end of the chapter 

 we shall return to the analysis of genetic recombination. 



COMPARATIVE ORGANIZATION OF GENETIC MATERIAL 



Studies of meiosis and genetic recombination in a great variety of 

 sexual organisms from algae, fungi, and yeast to insects, higher plants, 

 and mammals ha\e underscored the fundamental uniformity of the proc- 

 ess. Findings established in studies with one species have wide ap- 

 plicabilitv to others. In contrast, processes of recombination in viruses 

 and bacteria have some common features with the sexual ones, and some 

 important differences. Because of the differences, it is important to dis- 

 tinguish carefully between genetic observations made in sexual and in 

 other systems. The similarities and differences which we observe as 

 particular patterns of recombination are the expression of specific struc- 

 tural features. Before proceeding to the details, a brief comparison of 

 bacteria and higher forms with respect to the organization and duplica- 

 tion of the genetic material may be helpful. 



The Nucleus 



The nucleus of sexual organisms is a highly developed organelle, as 

 seen in Plate IX, containing not only the chromosomes but also a special 

 RNA-containing body, the nucleolus, which has not been found in bac- 

 teria. Also, the nucleus is surrounded by a porous double envelope, 

 studded with RNA-containing granules on its outer cytoplasmic surface. 

 No evidence has been found in electron micrographs of bacteria of any 

 membrane between the DNA and the surrounding protoplasm (Plate 

 II). 



Organization of the DNA 



The DNA of sexual organisms occurs in chromosomes which are com- 

 plex nucleoprotein rods, consisting of DNA bound to a histone-type 

 protein, and of other proteins and RNA. In bacteria, the DNA seems to 

 exist in a relatively free form, both in uninfected and in phage-infected 

 bacteria. In viruses, too, the nucleic acids seem to be free of specific 

 attachment to the protein coats which surround them. 



In sexual organisms, the interphase chromosomes exist as fibers below 

 the resolving power of the light microscope, as does the DNA of bacteria. 



