Guanine cytosine ratio

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. 

MeT as one base, and neutral cytosine, guanine or adenine as second nucleobase proceeds without formation of undesired side products only if the anionic ligand is attached to Pt first. On the other hand, reaction of cis-Pt(II) with uracil or thymine in 1 1 ratio results in formation of the complicated platinum blues... 

Other classification methods, such as the analysis of guanine and cytosine (GG) ratios in DNA, and DNA homology have no place as yet in the routine testing of the analytical laboratory. 

Quantitative analysis revealed a molar ratio of 1 1 for adenine and thymine, and the same ratio for guanine and cytosine (hence there always are equal proportions of purine and pyrimidine bases). In most animals there is more adenine plus thymine than guanine plus cytosine (molar ratios of the pairs range from 1.3 to 1.5) in bacteria the opposite may be found. 

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... 

After 1900, genetic research—but not research on nucleic acids—blossomed. Nucleic acids were difficult to work with, hard to purify, and, even though they were present in all cells, did not seem to be very interesting. Early analyses, later shown to be inconect, were interpreted to mean that nucleic acids were polymers consisting of repeats of some sequence of adenine (A), thymine (T), guanine (G), and cytosine (C) in a 1 1 1 1 ratio. Nucleic acids didn t seem to offer a rich enough alphabet from which to build a genetic dictionary. Most workers in the field believed proteins to be better-candidates. 

The secondary structure of DNA is shown in Figure B. This "double helix" model was first proposed in 1953 by James Watson and Francis Crick, who used the x-ray crystallographic data of Rosalind Franklin and Maurice Wilkins. Beyond that, they were intrigued by the results of analyses that showed that in DNA the ratio of adenine to thymine molecules is almost exactly 1 1, as is the ratio of cytosine to guanine ... 

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. 

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. 

Razskazovskiy et al. employ ESR spectroscopy at low temperatures to investigate electron transfer within brominated DNA [8]. The brominated DNA base electron traps were introduced by bromination of DNA in ice-cooled aqueous solution. The procedure is shown by NMR and GC/MS techniques to modify thymine, cytosine, and guanine, transforming them into 5-bro-mo-6-hydroxy-5,6-dihydrothymine, T(OH)Br, 5-bromocytosine, CBr, and 8-bromoguanine, GBr, derivatives. The bromination products formed in molar ratio close to T(OH)Br/CBr/GBr 0.2 1 0.23 and serve as internal electron scavengers on y-irradiation. Structurally the CBr and GBr are planar, but T(OH)Br is quite nonplanar with the bromine directly above the molecular plane. This disrupts the DNA base stack. Paramagnetic products that result... 

An interesting feature was discovered by Sharma and co-workers494,495 in the crystal structure of isocytosine. Two tautomers of isocytosine (42 and 43) exist in an exact 1 1 ratio in the crystal. The tautomers are hydrogen-bonded to each other in a manner analogous to that proposed by Watson and Crick496,497 for the guanine-cytosine pair in DNA. It is worth underlining that the base pair of isocytosine was not obtained by expedient cocrystallization of the two components. It seems therefore probable that both forms 42 and 43 of isocytosine are of approximately equal stability and are present in comparable amounts in solution. 

A complete understanding of the biochemical functions of DNA requires a clear picture of its structural and physical characteristics. DNA has significant absorption in the UV range because of the presence of the aromatic bases adenine, guanine, cytosine, and thymine. This provides a useful probe into DNA structure because structural changes such as helix unwinding affect the extent of absorption. In addition, absorption measurements are used as an indication of DNA purity. The major absorption band for purified DNA peaks at about 260 nm. Protein material, the primary contaminant in DNA, has a peak absorption at 280 nm. The ratio A26(j/A2m is often used as a relative measure of the nucleic acid/protein content of a DNA sample. The typical A260/Am for isolated DNA is about 1.8. A smaller ratio indicates increased contamination by protein. 

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. 

Oligonucleotides containing intrastrand or interstrand cross-links are easily separated by this technique. Over a period of 50 h, in the platinated duplex 1, only interstrand cross-links between complementary guanine and cytosine residues in agreement with previous results [64], are detected. In the platinated duplex 2, which contains a [Pt(dien)(dG)]2+ adduct in the lower strand three base pairs away from the trans-[Pt(NH3)2(dG)Cl]+ adduct and on its 5 -side, both intrastrand and interstrand cross-links are detected. In the platinated duplex 3, which contains a [Pt(dien)(dG)]2+ adduct in the lower strand two base pairs away from the /ra v-[Pt(NH3)2(dG)Cl]+ adduct and on its 3 -side, mainly two kinds of interstrand cross-links are detected. In the platinated duplex 4, which contains two [Pt(dien)(dG)]2+ adducts, two kinds of interstrand crosslinks are formed, but in a different ratio to that in duplex 3. In duplexes 3 and 4, there is one interstrand cross-link between the complementary guanine and cytosine residues, and the other is between the guanine and an adenine residue located four or five base pairs away on the 3 -side of the guanine residue. 

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