Why the symbol of life is a loop not a helix

Jamie Davies writes: Here is a remarkable fact about identical twins: they have the same DNA, and therefore the same ‘genetic fingerprint’, yet their actual fingerprints (such as they might leave behind on a murder weapon) are different, and can be told apart in standard police observations. Fingerprints are, of course, produced by the pattern of tiny ridges in skin. So, it would appear that certain fine-scale details of our anatomy cannot be determined by a precise ‘genetic blueprint’.

It isn’t only fine details that seem open to negotiation in this way: anyone who has seen Bonsai cultivation knows how the very genes that would normally build a large tree can instead build a miniature-scale model, given a suitable environment. Bonsai trees aren’t completely scaled down, of course: their cells are normal-sized – it’s just that each component is made with fewer of them.

In the 1950 and ’60s, many children were affected by their mothers taking the drug thalidomide while pregnant, when the drug blocked growth of the internal parts of their limbs. Even though growth of the skin is not directly affected by thalidomide, the very short limbs of affected children were covered by an appropriate amount of skin, not the much larger amount that would be needed to cover a normal limb. The growth of the skin cannot, therefore, just be in response to the command of a hard-wired internal blueprint: something much more adaptive must be going on.

Such observations are not troubling for biological science as such. But they are troubling for a certain picture of how biology works. The symbol for this worldview might be the DNA double helix, its complementary twisting strands evoking other interdependent pairs in life: male and female, form and function, living and non-living. DNA on its own is just a chemical polymer, after all, essential for life but not itself alive. Yet it holds out the promise that we can explain living processes purely in terms of the interactions between simple molecules. [Continue reading…]

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