( m why are they optimal why are they b...)Uncovering DNA's 'sweet' secret

Although homo-DNA was first synthesized in 1992, a detailed picture ofthe molecule's structure had been lacking. Egli's high resolution structure is now able to provide answers to some of the lingering questions about why DNA is made the way it is.

While the homo-DNA structure shows a number of similarities with DNA, it is much more stable than DNA. However, it has a more haphazard appearance than normal DNA, looking more like a "slowly writhing ribbon" than the tightly twisted ladder of DNA.

"The reason that DNA was 'picked' is not because it's thermodynamically extremely stable," Egli said. "There are others - including homo-DNA - that are actually superior in that regard."

Egli's structure also shows that homo-DNA has more flexibility in how the bases (rungs of the ladder) bind. The bases in normal DNA adhere to a somewhat strict binding scheme - guanine (G) binds with cytosine (C) and adenine (A) binds with thymine (T). In this "Watson-Crick" basepairing, the G:C bonds are much stronger than A:T or any other bonds.

"In homo-DNA, the Watson-Crick base pairing rules are changed," Eglisaid. "For example, G:C is similar to G:G or A:A, so you have a much more versatile pairing system in homo-DNA. Therefore, the nature of the sugar in the backbone affects the pairing rules."

But despite homo-DNA's apparent versatility in base pairing and its thermodynamic stability, other features of the molecule's architecture probably preclude it from being a viable genetic system

For example, it cannot pair with other nucleic acids - unlike DNA and RNA which can and must pair with each other. Also the steep angle, or inclination, between the sugar backbone and the bases of homo-DNA requires that the pairing strands align strictly in an antiparallel fashion - unlike DNA which can adopt a parallel orientation. Finally,the irregular spaces between the "run
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Source:Vanderbilt University Medical Center