Chargaff ratios

Pairing A with T and G with C gives the proper Chargaff ratios (A = T and G = C). Each pair contains one purine and one pyrimidine base. This makes the A—T and G—C pairs approximately the same size and ensures a consistent distance between the two DNA strands. 

Avery s paper prompted other biochemists to rethink their ideas about DNA One of them Erwin Chargaff of Columbia University soon discovered that the distribution of adenine thymine cytosine and guanine differed from species to species but was the same within a species and within all the cells of a species Perhaps DNA did have the capacity to carry genetic information after all Chargaff also found that regardless of the source of the DNA half the bases were purines and the other half were pyrimidines Significantly the ratio of the purine adenine (A) to the pyrimidine thymine (T) was always close to 1 1 Likewise the ratio of the purine guanine (G) to the pyrimidine cyto sine (C) was also close to 1 1 For human DNA the values are... 

Molar Ratios Leading to the Formulation of Chargaff s Rules ... 

C13-0102. hi the 1950s, Edwin Chargaff of Columbia University studied the composition of DNA from a variety of plants and animals. He found that the relative amounts of different bases changed from one species to another. However, in every species studied, the molar ratios of guanine to cytosine and of adenine to thymine were found to be very close to 1.0. Explain Chargaff s observations in terms of the Watson-Crick model of DNA structure. 

A most important clue to the structure of DNA came from the work of Erwin Chargaff and his colleagues in the late 1940s. They found that the four nucleotide bases of DNA occur in different ratios in the DNAs of different organisms and that the amounts of certain bases are closely related. These data, collected from DNAs of a great many different species, led Chargaff to the following conclusions ... 

In an early work on the compositions of DNA, Chargaff (1955) noted that the ratios of adenine to thymine and of guanine to cytosine are very close to unity in a very large number of DNA samples. This observation has been used to support the helical structure of DNA proposed by Watson and Crick (1953). From the base compositions of DNA and RNA given in the following tables, deduce the statistical significance for the statement that the base ratios (A/T(U) and G/C are unity for DNA but vary for RNA (Note Calculate the ratios first and then perform statistical analysis on the ratios). 

Erwin Chargaff found that the ratios of adenine to thymine and of guanine to cytosine were always 1 1, suggesting that these bases form pairs. The fact that the ratios are 1 1 is referred to as Chargaff s rules. 

In one paper, Chargaff (1950) described his analytical methods and some early results. Briefly, he treated DNA samples with acid to remove the bases, separated the bases by paper chromatography, and measured the amount of each base with UV spectroscopy. His results are shown in the three tables below. The molar ratio is the ratio of the number of moles of each base in the sample to the number of moles of phosphate in the sample—this gives the fraction of the total number of bases represented by each particular base. The recovery is the sum of all four bases (the sum of the molar ratios) full recovery of all bases in the DNA would give a recovery of 1.0. 

As you might expect, Chargaff s data were not completely convincing. He went on to improve his techniques, as described in a later paper (Chargaff, 1951), in which he reported molar ratios of bases in DNA from a variety of organisms ... 

Part of Chargaffs intent was to disprove the tetranucleotide hypothesis this was the idea that DNA was a monotonous tetranucleotide polymer (AGCT)re and therefore not capable of containing sequence information. Although the data presented above show that DNA cannot be simply a tetranucleotide—if so, all samples would have molar ratios of 0.25 for each base—it was still possible that the DNA from different organisms was a slightly more complex, but still monotonous, repeating sequence. 

Not very well, but he was my big competitor when I was a graduate student. My thesis research at UCLA was to develop methods to analyze purines and pyrimidines, which are the basic constituents of nucleic acids. The old methods were just terrible, not quantitative and very slow. Finally, I realized that I could use microbiological methods and determine the three pyrimidine bases. I did that and it worked. I never could find bacteria that were specific for the purines. If I had gotten that to work, I would have beaten Chargaff, but just after I had developed this methodology, he came out with his chromatographic methods. Then he was able to determine the ratio of the bases and show that they were not all present... 

The 1 1 ratios of adenine Thymine and guaninexytosine in the DNA isolated from a wide variety of species investigated by Erwin Chargaff between 1948 and 1952. (This relationship is sometimes referred to as Chargaff s rules.)... 

The structure of RNA differs from that of DNA in several respects. First, as shown in Fignre 25.17, the fonr bases fonnd in RNA molecules are adenine, cytosine, guanine, and nracil. Second, RNA contains the sugar ribose rather than the 2-deoxyribose of DNA. Third, chemical analysis shows that the composition of RNA does not obey Chargaff s rnles. In other words, the pnrine-to-pyrimidine ratio is not equal to 1 as in the case of DNA. This and other evidence rule out a double-heUcal structure. In fact, the RNA molecnle exists as a single-strand polynucleotide. There are actually three types of RNA molecules—messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (fRNA). These RNAs have similar nucleotides but differ from one another in molar mass, overall strnctnre, and biological functions. 

Chargaff also noted that the ratio which varies from species to species is the ratio (%A + %T)/(%G -h %C). He noted, moreover, that whereas this ratio is characteristic of the DNA of a given species, it is the same for DNA obtained from different tissues of the same animal and does not vary appreciably with the age or conditions of growth of individual organisms within the same species. 

At one time, it was thought that, in all species, the four principal bases occurred in the same ratios and perhaps repeated in a regular pattern along the pentose-phosphodiester backbone of DNA. However, more precise determinations of their composition by Erwin Chargaff revealed that bases do not occur in the same ratios (Table 20.2). 

---