Base Pairing in DNA The Watson-Crick Model

Samples of DNA isolated from different tissues of the same species have the same proportions of heterocyclic bases, but samples from different species often have greatly different proportions of bases. Human DNA, for example, contains about 30% each of adenine and thymine and about 20% each of guanine and cytosine. The bacterium Clostridium perfringens, however, contains about 37% each of adenine and thymine and only 13% each of guanine and cytosine. Note that in both examples the bases occur in pairs. Adenine and thymine are present in equal amounts, as are cytosine and guanine. Why  

CHAPTER 24 biomolecules NUCLEIC ACIDS AND THEIR METABOLISM 

What Crick called the central dogma of molecular genetics says that the function of DNA is to store information and pass it on to RNA. The function of RNA, in turn, is to read, decode, and use the information received from DNA to make proteins. Three fundamental processes take place  

What sequence of bases on one strand of DNA is complementary to the sequence TATGCAT on another strand  

Remember that A and G form complementary pairs with T and C, respectively, and then go through the sequence replacing A by T, G by C, T by A, and C by G. Remember also that the 5 end is on the left and the 3 end is on the right in the original strand. 

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Five-Membered Unsaturated Heterocycles 1151 Structures of Pyrrole, Furan, and Thiophene 1152 Electrophilic Substitution Reactions of Pyrrole, Furan, and Thiophene 1153 Pyridine, a Six-Membered Heterocycle Electrophilic Substitution of Pyridine Nucleophilic Substitution of Pyridine Fused-Ring Heterocycles 1158 Nucleic Acids and Nucleotides 1160 Structure of Nucleic Acids 1163 Base Pairing in DNA The Watson-Crick Model Nucleic Acids and Heredity 1166 Replication of DNA 1167... 

A -DNA The Watson-Crick model of DNA is based on the x-ray diffraction patterns of B-DNA. Most DNA is B-DNA however, DNA may take on two other conformations, A-DNA and Z-DNA. These conformations are greatly favored by the base sequence or by bound proteins. When B-DNA is slightly dehydrated in the laboratory, it takes on the A conformation. A-DNA is very similar to B-DNA except that the base pairs are not stacked perpendicular to the helix axis rather, they are tilted because the deoxyribose moiety puckers differently. An A-DNA helix is wider and shorter than the B-DNA helix. 

Low resolution Fourier syntheses of electron density with phases calculated from an approximate but erroneous model may nevertheless indicate another model and thus provide an unusually compelling indication that the phasing model is imperfect. This strategy has been used to reject Hoogsteen in favor of Watson-Crick base-pairing in DNA (60,61), to eliminate 10-fold in favor of 11-fold helicies in A-RNA (62), to favor nested chains rather than double helices in the case of one form of sodium hyaluronate (40). 

The incorporation into anti-parallel DNA of a cross-linked Watson-Crick base pair (120) has been described, " though no data for the duplexes formed are provided. A dissociable covalently bonded base pair (121), modelled to impose minimal distortion of the duplex, was used to replace a Watson-Crick base pair in ODNs. The second (complementary) strand was introduced by ligation of appropriate ODNs. The un-natural base pair between 2-amino-AT -dimethyl-purine and pyridin-2-one has been developed for specific transcription. " The ribo-triphosphate of pyridin-2-one and the 5-methyl derivative are selectively incorporated opposite 2-amino-Ar -dimethyl-purine by T7 RNA polymerase. The bulky dimethyl group prevents recognition and pairing by the natural bases. 

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