Biological complexity: Feedback loop importance (Introduction)

by David Turell , Saturday, April 23, 2016, 22:06 (3136 days ago) @ David Turell

Without feedback loops in cells function and even embryonic formation of organs would not take place. DNA makes proteins, but the controls for the uses of those proteins lie in feedback loops present in cells. This is where the 'intelligent' reaction of cells is managed. This is a long essay, which should be fully read, because it shows how DNA is just a starting pint for life to form itself and work at living:- https://aeon.co/essays/the-feedback-loop-is-a-better-symbol-of-life-than-the-helix?utm_... "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.-***-" The concept of ‘the gene for feature x' is giving way to a much more complicated story. Think something like: ‘the gene for protein a, that interacts with proteins b, c and d to allow a cell to undertake process p, that allows that cell to co?ordinate with other cells to make body feature x'. The very length of the above phrase, and the weakness of the blueprint metaphor, emphasises a conceptual distance that is opening up between the molecular-scale, mechanical function of genes and the interesting large-scale features of bodies.-***-"When cells first meet, in normal development or in a culture dish, neither can ‘know' in advance precisely where contact will first be made. The internal cytoskeletons of the cells cannot, therefore, be built to an advanced plan; they must develop adaptively to suit the precise conditions at the time.-***-"Thus the cytoskeleton's anatomy organises itself according to its environment and adapts continuously to changing mechanical loads.-This is just a very small-scale example of adaptive self-organisation. But the same principles work at larger scales, too. Organs consist of vast numbers of cells of different types, intricately assembled to perform whatever the organ's function might be. -***-"In some cases, we are starting to understand how this self-organisation happens, and how it relies on cell-to-cell communication. The growth of blood capillaries provides a good illustration. Tissues need blood capillaries to bring them oxygen and nutrients and to take away waste products, but a cell that is too far from the nearest blood capillary will find itself short of oxygen. At this point, a protein called HIF1A, which is normally destroyed by an oxygen-dependent process almost as soon as it is made, starts to accumulate. HIF1A puts a temporary brake on further cell proliferation and also causes the cell to secrete a protein called VEGF, which spreads out through the surrounding tissue. The cells that make blood capillary walls are sensitive to VEGF: if they detect it, they begin to proliferate and extend new capillary branches towards its source, and so the tissue cells that were short of oxygen will receive a supply from the new blood vessels. When capillary growth is adequate, there will be enough oxygen to make HIF1A unstable again, so the brake on tissue proliferation is released, VEGF production will cease and so will capillary growth. In a growing organ, this sort of thing happens again and again to make sure that growth doesn't outstrip blood supply. (Comment: perfect feedback loop)-***-"In the case of the blood capillaries, the extent to which present growth has been adequate to bring enough oxygen into the tissues is fed back, via VEGF, to control whether the capillaries continue to grow or remain as they are. -***-"Genes are therefore essential to self-organisation at all the scales of life - just not in a deterministic way. Rather, the genes are needed to make the machines that mediate feedback-driven self-organisation: the self-organisation is a high-level property that emerges from the underlying network, not a feature of any of the individual components.-***-"The DNA helix is important, of course...Unlike the helix, loops also operate at scales far above the molecular, covering a range of sizes from bacterial colonies to the vast ecosystems of the rainforest - perhaps to the ecosystem of the entire Earth."-Comment: Why Shapiro is wrong.