Guanine pairs

As indicated in Chapter 11, the base pairing in DNA is very specific the purine adenine pairs with the pyrimidine thymine the purine guanine pairs with the pyrimidine cytosine. Further, the A T pair and G C pair have virtually identical dimensions (Figure 12.10). Watson and Crick realized that units of such similarity could serve as spatially invariant substructures to build a polymer whose exterior dimensions would be uniform along its length, regardless of the sequence of bases. 

Figure 3.10 Stniciural details of the bridging units between pairs of bases in separate strands of the double helix of DNA (a) the thymine-adenine pair (b) the cytosine-guanine pair.

The two strands which make up DNA are held together by hydrogen bonds between complementary pairs of bases adenine paired with thymine and guanine paired with cytosine. The integrity of the genetic code (and of life as we know it) depends on error-free transmission of base-pairing information. 

As with proteins, the nucleic acid polymers can denature, and they have secondary structure. In DNA, two nucleic acid polymer chains are twisted together with their bases facing inward to form a double helix. In doing so, the bases shield their hydrophobic components from the solvent, and they form hydrogen bonds in one of only two specific patterns, called base pairs. Adenine hydrogen bonds only with thymine (or uracil in RNA), and guanine pairs only with cytosine. Essentially every base is part of a base pair in DNA, but only some of the bases in RNA are paired. The double-helix structure... 

Fig. 2. Schematic of DNA molecule showing repeating sequences of deoxyri-bose (white pentagons) and phosphodtester units that provide stiuclural support. The varying sequences of pyrimidine and purine bases encode genetic information. The purines are guanine and adenine the pyrimidines arc thymine and cysine. Note that guanine pairs with cytosine adenine pans with thymine...

It is very likely that similar variations in pA"a values as found in the 9-MeA/ Ptn system, namely, 4 log units attributable to hydrogen-bonding effects, will eventually also be seen with other nucleobases, provided such compounds can be prepared. We predict that, for example, the acidification of the guanine-N 1 position by Pt(II) coordination at N7 will exceed the typical value of ApAia 1.5 seen in many closely related Pt(II) am(m)ine complexes, if it is possible to generate complexes in which an appropriate microenvironment for a stabilization of the deprotonated N1 position is generated. It appears that the ready formation of hemiprotonated, N7-platinated guanine pairs (cf. Fig. 6) at pH values substantially lower than the pA"a for guanine deprotonation, in fact may be a consequence of this principle. 

EXCITED-STATE HYDROGEN TRANSFER FOR THE CYTOSINE-GUANINE PAIR... 

Similarly to proteins, both DNA and RNA have a secondary and a tertiary structure. The secondary structure of DNA shows two chains running in opposite directions, coiled in a left-handed (double) helix about the same axis. All the bases are inside the helix, and the sugar phosphate backbone is on the outside (see e.g. [1]). The chains are held together by hydrogen bonds between the bases with adenine always paired with thymine and guanine paired with cytosine. The base pairing in DNA is shown below ... 

In DNA, two antiparallel polynucleotide chains are joined by pairing between their bases and are twisted to form a double helix. Adenine pairs with thymine, and guanine pairs with cytosine. One chain runs in a 5 to 3 direction and the other runs 3 to 5. ... 

The structure relies crucially on the pairing up of nucleic acid bases between the two chains. Adenine pairs only with thymine via two hydrogen bonds, whereas guanine pairs only with cytosine via three hydrogen bonds. Thus, a bicyclic purine base is always linked with a smaller monocyclic pyrimidine base to allow the constant diameter of the double helix. The double helix is further stabilized by the fact that the base pairs are stacked one on top of each other, allowing hydrophobic interactions... 

The normal base pairs in DNA. Adenine in one polynucleotide chain pairs with thymine in the complementary chain guanine pairs with cytosine. A-T base pairs are joined by two hydrogen bonds G-C base pairs are Joined by three hydrogen bonds. 

The most stable DNA stracture is formed when two polynucleotide chains are joined by hydrogen bonding between the side chain bases. The base pairing is specific in that adenine pairs with thymine and guanine pairs with cytosine (A-T G-C)... 

Figure 1 G-quartet and G4 DNA. Left G-quartet, a planar array of four guanines, in which each guanine pairs with two neighbors by hydrogen bonding. Right G4 DNA. G-quartets can promote intra- or intermolecular interactions between DNA strands, as shown in the figure. Parallelograms represent G-quartets...

Purines are bases found in the nucleosides and nucleotides that make up nucleic acids. In nucleic acids, the purines match up with specific pyrimidine bases. The matching between purines and pyrimidines forms "base-pairs" in which adenine pairs with thymine (in DNA) or uracil (in RNA). Guanine pairs with the base cytosine in either nucleic acid. 

DNA Structure. Genetic information is encoded by the sequence of different nucleotide bases in DNA. DNA is double-stranded it contains two antiparallel polynucleotide strands The two strands are joined by hydrogen bonding between their bases to form base-pairs Adenine pairs with thymine, and guanine pairs with cytosine The two DNA strands run in opposite directions. One strand runs 5 to 3, and the other strand runs 3 to 5. The two DNA strands wind around each other, forming a double helix... 

The (a) thymine-adenine and (b) cytosine-guanine pairs show complementarity. The hydrogen-bonding interactions are shown by dotted lines. 

The X-ray diffraction pattern of DNA demonstrated the helical structure and the diameter. The combination of evidence from X-ray diffraction and chemical analysis led to the conclusion that the base pairing is complementary, meaning that adenine pairs with thymine and that guanine pairs with cytosine. Because complementary base pairing occurs along the entire double helix, the two chains are also referred to as complementary strands. By 1953, studies of the base composition of DNA from many species had already shown that, to within experimental... 

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