Pyrimidine Moieties in Animals, Plants, and Bacteria 101 



Another point of interest is the fact that the small heterogeneity 

 of base composition among the DNA molecules of an organism 

 seems to be true for smaller regions within molecules (72). This 

 indicates that the intramolecular distribution of (G+C) and 

 (A+T) pairs is fairly unifoim, although in short regions (for 

 example, tri- or tetranucleotides) nonrandomness has been demon- 

 strated. 



The Formation of Hybrid DNA Molecules and Their Use in 

 Studies of DNA Homologies 



DNA molecules having the same average molar nucleotide com- 

 position do not necessarily have the same nucleotide sequence along 

 the DNA chains. Since techniques for measuring the nucleotide 

 sequence are not currently available, possible sequence homologies 

 between DNA molecules from different sources must be investi- 

 gated by indirect methods. There have been two general approaches 

 to this problem so far: 1) an analysis of the distribution of oligo- 

 nucleotides in partial hydrolysates of DNA; and 2) a study of 

 DNA hybrids. 



Burton (6) has measured the distribution of short chain oligo- 

 nucleotides in acid degradation products of DNA. Differences 

 could be detected in the distribution of dinucleotides and tri- 

 nucleotides of four animal and four bacterial species. 



The formation of hybrid DNA molecules has been investigated 

 by Schildkraut and co-workers (61). These studies depend upon 

 the fact that each DNA molecule consists of two complementary 

 strands which can be separated in solution. Strand separation can 

 be accomplished by heating the DNA to a temperature which 

 will "melt" the hydrogen bonds which hold together the double 

 stranded helix. One of the DNA preparations to be tested is 

 labeled with N^^ C''\ or deuterium, so that it will form a heavy 

 band when it is centrifuged in a density gradient. The second 

 DNA preparation is of normal density. The heavy and the light 

 DNA molecules are mixed and the strands are separated by 

 heating. Wlien DNA is slowly cooled, the complementary strands 

 attract each other and the hydrogen bonds are reformed {renatura- 

 tion). Thus, the DNA duplexes are reconstituted. 



Let us consider the situation when two DNA molecules from 

 the same species are renatured, but where one molecule is labeled 



