G-tetrad DNA

Lorieau J, Yao L, Bax A (2008) Liquid crystalline phase of G-tetrad DNA for NMR study of detergent-solubilized proteins. J Am Chem Soc 130 7536-7537... 

Fig. 4 Schematic representation of parallel A and antiparallel B quadruplex form of DNA with the chemical structure of a G-tetrad C...

The chromosomes of eukaryotes are linear, and replication of the free ends of these linear DNA molecules presents particular problems. The sequencing of the ends of chromosomes revealed that they consist of telomeres, hundreds of tandem repeats of a hexanucleotide sequence, which in all vertebrates is d(TTAGGG). These G-rich telomeric sequences can fold into a G-quadruplex, a DNA secondary structure consisting of stacked G-tetrad planes, or G-quartets (Figure 9.18), connected by a network of Hoogsteen hydrogen bonds the cavity in the centre... 

The smallest unimolecular G-quadruplex belongs to the family d(G2NnG2NnG2NnG2), and forms two stacked G-tetrads. The solution structure of such a G-quadruplex in which the first loops are diagonal has been solved. ° A similar structure of a guanosine- and adenosine-rich sequence, but which forms a dimeric structure with two G-tetrads has also been solved by NMR. ° In determining the solution structure of a DNA 3-way junction, van Buuren et... 

Much less is known about non-G-tetrads. Even though file telomeric sequences are G-rich, they also include other bases. Therefore, the existence of non-G-tetrads cannot be excluded. Indeed, A-, T- and C-tetrads have recently been foimd in DNA tetraplex structures. Moreover, an interesting U-tetrad linked by C-H...O contacts only occurs in an exceptionally stable RNA tetraplex with G- and U-tetrads. Very recently a high-resolution x-ray structure for the same sequence has become available, that allows for a comparison between the x-ray and NMR results. ... 

Fig. 6. Parallel-stranded DNA tetraplex formed from the Tetrahymena telomeric sequence d(TTGGGGT) solved by NMR spectroscopy (PDB code 139d). The structure contains four stacked G-tetrads in e center and additional T-tetrads. The latter ones are underdefined, however. The G-tetrads have a cyclic topology IV.2(24).

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