Guanine G

The DNA base pairs guanine (G), cytosine (C), adenine (A) and thymine (T). The uracil-2,6-diaminopyridine pair can also form three hydrogen bonds but has a much lower association constant than G-C. 

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... 

Base pair (Section 28 7) Term given to the punne of a nu cleotide and its complementary pyrimidine Adenine (A) is complementary to thymine (T) and guanine (G) is comple mentary to cytosine (C)... 

The primary stmcture of DNA is based on repeating nucleotide units, where each nucleotide is made up of the sugar, ie, 2 -deoxyribose, a phosphate, and a heterocycHc base, N. The most common DNA bases are the purines, adenine (A) and guanine (G), and the pyrimidines, thymine (T) and cytosine (C) (Fig. 1). The base, N, is bound at the I -position of the ribose unit through a heterocycHc nitrogen. 

The sugars are typically ribose (ribonucleic acids, RNA), or 2-deoxyribose (deoxyribonucleic acids, DNA). There are five common bases in nucleic acids adenine (A) thymine (T) uracil (U) cytosine (C) and guanine (G). DNA polymers incorporate the four bases. A, T, C, and G, and RNA, the set A, U, C, and G. 

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. 

C with G the pyrimidine cytosine (C) always pairs with the purine guanine (G). 

Attached by a covalent bond to carbon atom 1 of the deoxyribose ring is an amine (and therefore a base), which may be adenine, A (22) guanine, G (23) cytosine, C (24) or thymine, T (25). In RNA, uracil, U (26), replaces thymine. The base bonds to carbon atom 1 of deoxyribose through the nitrogen of the —NH— group (printed in red) and the compound so formed is called a nucleoside. All nucleosides have a similar structure, which we can summarize as the shape shown in (27) the lens-shaped object represents the attached amine. 

The DNA structure involves two polyanionic phosphodiester strands linked together by hydrogen bonding of base pairs. The strands can be separated by a denaturation process (melting). The melting temperatnre increases with an increase in guanine (G)-cytosine (C) content, since this base pair possess three hydrogen bonds as compared to just two for the adenine (A)-thymine (T) pair. 

Figure 35-1. A segment of one strand of a DNA molecule in which the purine and pyrimidine bases guanine (G), cytosine (C), thymine (T), and adenine (A) are held together by a phosphodiester backbone between 2 -de-oxyribosyl moieties attached to the nucleobases by an W-glycosidic bond. Note that the backbone has a polarity (ie,a direction). Convention dictates that a single-stranded DNA sequence is written in the 5 to 3 direction (ie, pGpCpTpA, where G, C,T, and A represent the four bases and p represents the interconnecting phosphates).

Scheme 3 Charge transfer from the sugar radical cation to guanine (G)... 

Fig. 6 Hole injection into a guanine (G) in single strands, which is separated from the enol ether radical cation by various numbers of thymidines [T]n...

Adenine (A) Cytosine (C) Guanine (G) Thymine (T) Adenine (A) Cytosine (C) Guanine (G) Uracil (U) Deoxyribose Ribose Phosphate groups... 

A sequence, in general, is the relative order of base pairs, whether in a fragment of a protein, DNA, a gene, a chromosome, or an entire genome. DNA is composed of two antiparallel strands of deoxynucleotides held together by hydrogen bonds between purine (adenine, A and guanine, G) and pyrimidine (thymidine, T uracil, U and cytosine, C) bases. 

Base pair (bp) The four nucleotides in the DNA contain the bases adenine (A), guanine (G), cytosine (C), and thymine (T). Two bases (adenine and thymine or guanine and cytosine) are held together by weak bonds to form base pairs. The two strands of human DNA are held together in the shape of a double helix by those bonds between base pairs. For example, the complementary nucleic acid base sequence to G-T-A-C that forms a double-stranded structure with the matching bases is C-A-T-G. 

Cytosine (C) A nitrogenous pyrimidine base, which bonds with guanine (G) to form the G-C base pair. 

The interstrand cross-link also induces DNA bending.72 X-ray and NMR studies on this adduct show that platinum is located in the minor groove and the cytosines of the d(GC) base pair involved in interstrand cross-link formation are flipped out of the helix stack and a localized Z-form DNA is observed.83-85 This is a highly unusual structure and very distorting—implications for differential repair of the two adducts have been addressed. Alternatively, the interstrand cross-link of the antitumor inactive trans-DDP is formed between a guanine (G) and its complementary cytosine (C) on the same base p a i r.86,87/ nms- D D P is sterically incapable of producing 1,2-intrastrand adducts and this feature has been cited as a dominant structural reason for its lack of antitumor efficacy. It is clear that the structural distortions induced on the DNA are very different and likely to induce distinctly different biological consequences. 

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