The later (Wang et al., 1979). More

The inquiry about non-canonical structures is initiated after the discovery of three-stranded complexes (H-DNA), configured from W?1 the interconnection of ribonucleic poly-A and poly-U polymers (Felsenfeld & Rich, 1957). In 1979, left handed W?2 conformation with modified helical constraints with respect toW?3  Watson-Crick model was observed within the crystalline structure of d(CpGpCpGpCpG) DNA fragments. Those DNA fragments were termed as Z-DNA later (Wang et al., 1979). More efforts on finding a novel non-canonical structure revealed the disclosure of two more non-canonical structures: cruciform and quadraplex.

The cruciform construct forged by inverted repeats was pinpointed by S1 nuclease probing  (Panayotatos & Wells, 1981) and two-dimensional gel electrophoresis (Lyamichev et al., 1983). Next W?4 a tetraplex or G-quadruplex DNA (parallel four-stranded complexes) formed by guanine-rich DNA stretches (Sen & Gilbert, 1988) was discovered. Further search for new non-canonical conformations resulted in the discovery of more than 20 different non-canonical conformationsW?5  untillW?6  now (Ghosh & Bansal, 2003).In fact W?7 B-DNA conformations are low energy states while non-canonical DNA conformations that are formed transiently at the sites of repeat sequence are high energy levels. Energy W?8 required for the transition from low to high energy conformation or stabilizing the high-energy conformation is provided by various biophysical factors such as negative supercoiling, high salt andW?9  positively charged ligands. Separation of Watson-Crick base pairs in the B-form DNA, a prerequisite for the transition of B-DNA to other non-canonical DNA, is an energy-consuming process. In accordance with the laws thermodynamicsW?10 , the genomic DNA has propensity W?11 to remain in low energy form (B DNA) except for the process of “Central Dogma”.

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In the “Central Dogma” of molecular biology, energy W?12 required for DNA unwrapping from nucleosome cores or the DNA duplex split-up is provided by the positive free energy linked with the negative super-helicity of a closed circular DNA molecule. This unwinding generates the single strandedW?13  DNA which is favorable for non-canonical structure formation during replication in the lagging strand.


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