Chargaff’s Rule

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

The Watson-Crick double helix is the outcome of three lines of work. The first is the discovery by Erwin Chargaff of Chargaff s rules." Specifically, for all normal DNAs, A = T, G = C and A + G = C + T. The actual content of each base in DNA varies from species to species over a wide range. Despite this variation, the content... 

Chargaff s rules were first defined in E. Chargaff, Experientia 6 201-209 (1950). 

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. 

Chargaff s Studies of DNA Structure The chapter section DNA Is a Double Helix that Stores Genetic Information includes a summary of the main findings of Erwin Chargaff and his coworkers, listed as four conclusions ( Chargaff s rules p. 278). In this problem, you will examine the data Chargaff collected in support of these conclusions. 

The explanation for Chargaff s rules is provided by die selective hydrogen bonds, termed complementary base pairing, that form between adenine and thymine molecules and between guanine and cytosine. Adenine and thymine form two hydrogen bonds with an overall strength of about 10 kcal/mol (42 kJ/mol) ... 

Erwin Chargaff s discovery that DNA contains equimolar amounts of guanine and cytosine and also equimolar amounts of adenine and thymine has come to be known as Chargaff s rule ... 

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.)... 

According to Chargaff s rules, if a DNA sample contains 21% adenine then it also contains 21% thymine. If the A-T content is 42%, then the G-C content is 58%. Consequently the guanine and cytosine percentages are both 29% in the DNA sample. 

In the 1940s, Erwin Chargaff " studied DNA molecules obtained from various sources and observed certain regularities. Chargaff s rules, as his findings are now known, describe these patterns ... 

Does Chargaff s rule imply that equal amounts of guanine and adenine are present in DNA That is, does G = A Does Chargaff s rule imply that the sum of the purine residues equals the sum of the pyrimidine residues That is, does A -I- G = C -t- T ... 

Does Chargaff s rule apply only to double-stranded DNA. or would it also apply to each individual strand if the double helical strand were separated into its two complementary strands ... 

The key to the double-helical structure of DNA is the formation of hydrogen bonds between bases in the two strands of a molecule. Although hydrogen bonds can form between any two bases, called base pairs, Watson and Crick found that the most favorable couplings are between adenine and thymine and between cytosine and guanine (Figure 25.19). Note that this scheme is consistent with Chargaff s rules, because... 

The structure of RNA differs from that of DNA in several respects. First, as shown in Figure 25.17, the four bases found in RNA molecules are adenine, cytosine, guanine, and uracil. Second, RNA contains the sngar ribose rather than the 2-deoxyribose of DNA. Third, chemical analysis shows that the composition of RNA does not obey Chargaff s rules. In other words, the purine-to-pyrimidine ratio is not equal to 1 as in the case of DNA. This and other evidence rule out a double-helical structure. In fact. 

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