What is sex? For reproduction (and pleasure) of those who practice it, they say. We know that they can do without most of the lower organisms, many plants and about 43 000 known species of vertebrates only a few fish, reptiles, amphibians, but not mammals. Why? Two explanations compared: the first that sex is used to shuffle genes and to promote the development and preservation of the species, the second - more recently - is one that sees sex as a time to "repair" of DNA. According to the model so far accepted, that of Lucretius-Weismann-Fisher-Muller, reproduction by sexual favors, and then mixing the genes of the features which are encoded, that increases biodiversity (the ability to 'mix' genes so detract from the lethal risks to the species). In other words, changes unfavorable to the parents may not be true for some of the children selected according to the mixing of genes. That is the secret of evolution?
Today, however, dominated a second model: insight from Plato, taken by Freud, developed by Maynard Smith and disseminated by Michod in Eros and evolution suggests that sex is a. .. DNA repair (the effects on the evolution are all there, but indirect). This is a "cutting" the DNA, such that a 'healthy' reproduction, which occurs every time they enter the field in the sex cells (gametes). For every sexual act is "reprograms" the DNA, is healthy, can repair itself with respect to environmental risks and, if everything works, give birth to a son with a DNA 'harder'. With this new model sanerebbero several paradoxes, such as disseminating del sesso contro i suoi costi (la ricerca del partner è solo l’inizio!); l’insensatezza di scompaginare genomi (Dna) di successo in omaggio a biodiversificazioni (combinazioni di geni diverse e vincenti) vantaggiose in nuovi ambienti tanto ipotetici quanto generazionalmente lontani; la continuità evolutiva delle specie asessuate contro la discontinuità delle sessuate (persino Darwin era turbato dagli «anelli mancanti»).
Anni fa il Nobel Jacob notò che la natura fa bricolage, ma più che riparare, ricicla. L’uomo è fatto da miliardi di cellule, differenziate in circa 200 tipi riconducibili a due linee: la somatica (sangue, cuore, cervello, pelle, eccetera) e la sessuale (gameti: sperm in males and ova in females). Somatic cells used in the organism, the sexual (gametes) to its reproduction. Each cell is full of protein, RNA, etc., but a single DNA: usually starts after starting work at rest and makes copies of its stock, which shall function. The DNA is completed and lasts for a longer life, at least in somatic cells. In
gametes instead passes from one generation to another: it is perpetual.
the DNA of every human cell contains three billion "letters" combined into different words: four nitrogenous bases, A, C, G and T, in countless combinations. They are all sensitive sites: radiation and chemical substances will hit a thousand a day. Damage, or alteration of structure, and mutations or alterations in sequence, accumulate, cause genetic diseases and accelerate aging. Mistakes that the somatic cell repair without overdoing it, if they are few. If there are many, and disrupts repair, activates a cell suicide program, but the body is saved: the cells are abundant. If there are too many mistakes, it's chaos: the suicide program jumps well and you risk cancer. Ben
other care to the DNA in the sexual line. Not for the sake of posterity, but only because otherwise the DNA does not replicate. Thanks to the sex because the DNA rewards his narcissism: it perpetuates, is discontinuous and serves the development. In general, cells make their life to the replication of DNA, is possible only if intact, but to keep it that it takes time and energy. That is why nature has made the service so attractive that some people enjoy it is ready to die: of 'accidents' victims are praying mantis, the little mouse with the carrier, the centenary of cactus, the man does not stop even in the face of AIDS. Our culture has sublimated into an ideal (love) and debased into an obsession (sex) in whose name do wonders and crimes.
Here we only summarize the logic of the role of sex in the maintenance of DNA. To correct a text should be a good copy of Commons: better if, as in a film, there is also a negative. DNA is made up of two intertwined helices: one is the negative (or complement) each other. The base sequence of a propeller, that determines the other: the mistakes are corrected for a comparison with the other. Here take the first repair.
somatic cells have two DNA double helices, a mother and a father. This allows you to repair them both, provided that the damage was different in the second repair the double helix right to back-up to the wrong. But
gametes (sex cells) is not enough. They only have a double helix of DNA and are passing it on to descendants must be protected to the fullest. And so, through third coupon progenitor cells detach before becoming gametes: they, like the somatic, have two double helices, but not replicate once, but twice. This highlights all the errors on each single-stranded and optimizes the correction. That does not work in rain: a masterpiece of engineering reparative double helices aligns the maternal and paternal and earn the right parts in one, the wrong ones in the other. The new double helices, mixed maternal / paternal, each ending in a gamete: the best repaired will have a greater chance of success in natural fertilization. The gametes are mobilized by the millions and even review (especially on eggs) and selection (of sperm) are severe, four hundred out of a hundred births registered abortions and four congenital malformations. In vitro
the risk rises: there is revision, not selection. And even more so with the cloning lacks the revision. Reproduction remains a lot to learn, for example because it is easy to clone plants, but not animals. "Knowledge is power," warned F. Bacon.
These transactions also explain why the children reappear strokes in the grandparents and the parents (and vice versa). I noticed Lucretius in De rerum natura, but perhaps misled by his poetic lacked the right model. Too bad, because he had already sketched out four centuries before Plato's Symposium. Sex is
tabù e lo si esorcizza in favole. Dopo cavoli e cicogne archiviamo lotta a parassiti, biodiversità, etc, come sottoprodotti del sesso: della vita toccano l’hardware (cellule, organismi) più che il software (Dna).
Infine: perché il Dna? Se siamo strumenti di un disegno divino, amen. Alcuni cercano ragioni scientifiche: il Nobel Monod, e prima Democrito, ci vedono figli di caso e necessità. Si sa ancora ben poco della termodinamica di un processo che mira essenzialmente al Dna.
Ma su Marte, o altrove, con chimica e fisica simili alle nostre, c’è una molecola così egoista da asservire la biosfera e bella da rispettare la sezione aurea?
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