Laika was a stray found on the streets of Moscow which literally for once rocketed to fame aboard a

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Laika was a stray found on the streets of Moscow, which literally (for once) rocketed to fame aboard a Soviet space ship on 3 November 1957, just a month after the Soviet Union began the space race by putting Sputnik, the first man-made satellite, into orbit. Maurice Wilkins, who shared the 1962 Nobel Prize for the structure of DNA with Crick and Watson, says Curtis's proposal should not be dismissed out of hand.Although he does not think the geometrical model is correct, Wilkins welcomes the challenge. Such a structure may be sustainable, says Curtis, but that does not necessarily mean it is correct. "Unless they can show me an image from natural DNA confirming Crick and Watson's hypothesis, I will not be convinced".And he may have an ally. He points out that the images published in the early Eighties were of synthetic DNA, built up in the laboratory from just 12 individual base pairs, with the sugar phosphate groups preattached to each base pair so that they were constrained to bond in the Crick and Watson configuration. The hollow centre found in this formation ensures that the bases remain side by side within the structure, allowing access for replication and repair from any point within the helix.Unfortunately, coming up with an interesting idea is only the first part of any battle to have it accepted.

Some scientists who have seen his model say it is plain wrong."He has got carried away with the beauty of what he's doing," says Professor Richard Henderson, director of the Medical Research Council's Laboratory of Molecular Biology in Cambridge. He says the idea is "nice, but it doesn't help science at all".The structure of DNA has been conclusively and objectively determined using X-ray crystallography, he maintains. Images published in 1981 swept away any lingering doubts: every detail of the structure was clearly visible, and Crick and Watson's hypothesis was shown to be correct, he says.Nevertheless, Curtis is undeterred. This modification to the base pairing could exist in both the enol form (where a hydrogen atom is attached to an oxygen) and the keto form (in which the hydrogen is attached to a nitrogen atom).The pentagonal geometry provides the infrastructure required to make a consistent, stable and uniform helical structure, and also establishes why there should consistently be 10 bases within a single turn of the helix. But Curtis suggests that it is the base pairs themselves that provide that integrity, and he uses geometry to show how it could be done.Curtis's work shows how 10 joined pairs of regular prismatic pentagons stacked stepwise on top of each other, and turning by one face at each step, produces a single, full turn of a helix in 10 steps.

With 10 base pairs per turn of the helix, he was puzzled that the accepted model joins the purine (guanine and adenine) and pyrimidine (thymine and cytosine) base pairs with the pentagonal ring of the purine base oriented away from the central axis.If, instead, he theorised, the base pairs are reoriented, then two pentamers could be placed at the core of each pair. Why shouldn't something similar be true of DNA?In the accepted model, the double helix depends on sugar phosphate groups attached on the outside of the base pairs to form an external "backbone", which provides the super-molecule with an overall structural integrity. Using the standard textbook dimensions of the double helix, Curtis says, it struck him that it did not conform to any set of geometrical principles. But a series of relatively simple geometrical relationships underlies every element of the alternative he suggests. And surely nature loves geometrical simplicity? After all, the mathematical Fibonacci ratio is found in the positioning of plant leaves on a growing stem.