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decades aerwards, apparently contradicted by a faulty experiment that seemed to show that cells could continue to divide and multiply indefinitely if supplied with warmth and nutrition.⁴⁸ But modern biology once more considers his description ageing at the cellular level to be correct…as far as it goes. Weismann recognised that there is a difference in the mortality of germ cells (sperm, ova) and somatic cells (everything else) in the body. Each of us carries at birth our personal genetic code preserved in female egg cells or (the precursors o) male sperm cells. ese cells are able, from the moment of conception to reproduce vigorously by cell division, transferring our genetic information with extraordinary fidelity to our children, who in turn carry that transcription to the next generation. It’s possible to imagine the nuclear “payload” in these cells enjoying a kind of immortality, reproducing by cloning and mixing its distinctive genetic information with other genetic lines, but never ageing and never dying out.⁴⁹ Somatic cell lines do, however, grow old and die. eir evolutionary purpose is only to ensure the organism survives to and, as we have seen, beyond reproduction in order to give the genes in the germ cells the best chance of “immortality”. Cells reproduce by cell-division during the life of the individual as the structures of the body develop, fight infection, repair themselves and deal with the insults of the environment. But eventually their reproductive rate falls; aer no more than to divisions, the individual cells of the muscles, connective tissue, bone, organs and skin are unable to divide further. e “Hayflick Limit” , foreseen by Weismann and named for the biologist Leonard Hayflick who confirmed it experimentally in , has had a powerful influence on the imagination of researchers, philosophers, science journalists, and the religious enthusiasts of various ’life-extension’ faiths. It seems to import death into the very base of life’s pyramid by seing a short and certain term for every building block of our bodies. But that image, it turns out, is misleading. Death at the Hayflick Limit is an unusual sacrifice cells make to the survival of the organism. In one of Nature’s sharp ironies, somatic cell ageing and death results from the way our chromosomes carry out their role in creating new life. When it is time for the cell to reproduce by division into two ‘daughter’ cells, enzymes in the nucleus of the cell start to prise apart each of the twisted double-strands of DNA that make up our chromosomes, working at many points along the ribbon simultaneously to speed up the duplication.⁵⁰ In the gaps of the unravelling DNA ribbon, an enzyme called DNA polymerase travels along each of the unwound strands — designated the “leading” and “lagging” strands — using the paern of DNA bases as a template to create a new strand. Each base in the original strand is matched with its chemical partner to create a new strand that is the chemical mirror of the template strand in the same sense as ⁴⁸Alexis Carrel’s experiment with cells taken from chicken hearts was probably contaminated by the reintroduction of new cells, intentionally or not, with the culture nutrient. It was never reproduced. Carrel, who was awarded a Nobel prize for inventing new techniques of vascular surgery, was a vigorous publicist who co-authored a book with the aviator Charles Lindburgh on the culture of organs outside the body and a book on eugenics that advocated, among other punishments, the euthanising of criminals and the criminally insane ☞ J W Shay and W E Wright. “Hayflick, his limit, and cellular ageing.” In: Nature reviews. Molecular cell biology . (Oct. ), pp. –. : - ⁴⁹Of course, unfertilized eggs do age and die. It is only the genetic information in the fertilzed egg that could be considered “immortal” ⁵⁰Otherwise it would take days, not seconds to reproduce all . billion base-pairs in the human genome, even at speeds of base-pairs a second