Purine/pyrimidine ratio

A + G) = (T + C) This is the purine pyrimidine ratio, which also is very close to 1. 

The oligonucleotides in a pancreatic ribonuclease digest of RNA have the general formula (purine nucleotide) i pyrimidine nucleotide, n = 1, 2,3,... Thus for each value of the chain length, n nucleotides, the charge at neutral pH and the purine pyrimidine ratio are functions of n only and both increase monotonically with n. This... 

Deoxyribonucleic acids readily undergo hydrolysis whereby purine bases are removed to give a derived polynucleotide originally named thymic acid, but now often called apurinic acid. Hydrolysis may be carried out with dilute mineral acid, but recently apurinic acids have been prepared by fission at room temperature with an acidic, ion-exchange resin.236 Under carefully controlled conditions, removal of the purines can be performed quantitatively without destroying the polynucleotide nature of the material and without altering the inter-pyrimidine ratios of the original material.23 ... 

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. 

The size of DNA varies according to its source (Table 7.2). Chemical analysis of the base composition of various DNA molecules revealed that the relative amounts of the heterocyclic bases varied between species (Table 7.3) but were constant within a species irrespective of age and tissue of origin. The ratios A/T, G/C and purine/pyrimidine approximate to 1 so that the number of adenine residues equals thymine residues (i.e. A = T), the number of guanine residues equals cytosine residues (i.e. G = C) and the number of purine residues... 

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... 

The presence of conjugated double-bond systems in the purine and pyrimidine bases means that DNA and RNA absorb light in the ultraviolet region at 260 nm. For approximate determinations it can be assumed that a 50 /xg ml-1 solution of double-stranded DNA (dsDNA) has an absorbance of 1 at 260 nm. More exact quantitation can be obtained by comparing the ratio of the absorbance of the sample at 260 and 280 nm. The term optical density (OD) is often used in place of absorbance. Pure DNA preparations should have an OD 260/OD 280 of 1.8. Ratios less than this may indicate protein contamination while higher ratios may indicate the presence of RNA. 

The first evidence of the special structure of DNA was the observation that the amounts of adenine and thymine are almost equal in every type of DNA. The same applies to guanine and cytosine. The model of DNA structure formulated in 1953 explains these constant base ratios intact DNA consists of two polydeoxynucleotide molecules ( strands ). Each base in one strand is linked to a complementary base in the other strand by H-bonds. Adenine is complementary to thymine, and guanine is complementary to cytosine. One purine base and one pyrimidine base are thus involved in each base pair. 

The less basic purines generate different adducts. Both a C-8 adduct 107 and an 0-6 adduct 108 are produced in the presence of I, while the exclusive product of the reaction of A with 75n and 75o is the unique benzene imine 109. ° These purines also exhibit lower selectivity for trapping of the nitre-nium ions (Table 3). The pyrimidine nucleosides thymidine (T), uridine (U), and cytosine (C) showed negligible reactivity with these two nitrenium ions. ° The selectivity ratios for T, U, and C given in Table 3 are upper limits based on the decrease in the yield of the hydrolysis products at high nucleoside concentration (ca. 50mM). ° Since no adducts were isolated it is not clear that these selectivities represent nucleophilic trapping by the pyrimidines. 

G = C. The bases of DNA therefore are half purines and half pyrimidines. Furthermore, although the ratios of A to G and T to C are constant for a given species, they vary widely from one species to another. 

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