{"id":664,"date":"2019-06-04T10:17:43","date_gmt":"2019-06-04T08:17:43","guid":{"rendered":"https:\/\/blogs.futura-sciences.com\/lehning\/?p=664"},"modified":"2019-06-29T14:41:28","modified_gmt":"2019-06-29T12:41:28","slug":"des-noeuds-dans-ladn","status":"publish","type":"post","link":"https:\/\/blogs.futura-sciences.com\/lehning\/2019\/06\/04\/des-noeuds-dans-ladn\/","title":{"rendered":"Des n\u0153uds dans l’ADN"},"content":{"rendered":"

L\u2019ADN (ou acide d\u00e9soxyribonucl\u00e9ique) est le support de l\u2019h\u00e9r\u00e9dit\u00e9. Cette mol\u00e9cule, pr\u00e9sente dans chaque cellule, prend la forme d\u2019une double h\u00e9lice, qui s\u2019enroule sur elle-m\u00eame, formant ainsi un n\u0153ud.<\/p>\n

\"\"
Mol\u00e9cule d\u2019ADN formant un n\u0153ud. Sa r\u00e9plication demande de le d\u00e9nouer. Image r\u00e9alis\u00e9e au moyen d\u2019un microscope \u00e9lectronique.<\/figcaption><\/figure>\n

Duplication des mol\u00e9cules<\/h2>\n

La duplication des informations contenues dans une mol\u00e9cule d\u2019ADN se fait au moyen d\u2019enzymes. Pour \u00ab\u00a0voir\u00a0\u00bb le processus, imaginez une longue fermeture \u00e9clair qu\u2019on ouvre avant de la s\u00e9parer en deux. Cela n\u2019est possible que si le n\u0153ud peut \u00eatre d\u00e9nou\u00e9. Certains virus attaquent les mol\u00e9cules d\u2019ADN en les coupant et en les recollant de sorte qu\u2019ils soient impossibles \u00e0 d\u00e9nouer. Le type de n\u0153ud obtenu apr\u00e8s l\u2019attaque virale est caract\u00e9ristique de chaque virus. La signature de ces virus est de nature topologique\u00a0!<\/p>\n

Par ailleurs, cette question du d\u00e9nouement est au c\u0153ur de la th\u00e9orie math\u00e9matique des n\u0153uds. Certains sont faciles \u00e0 d\u00e9nouer, d\u2019autres bien plus compliqu\u00e9s, voire impossible (voir la figure ci-dessous). \u00c0 l\u2019envers de celle des virus, la seule m\u00e9thode est celle qu\u2019Alexandre le Grand employa pour d\u00e9nouer le n\u0153ud gordien\u00a0: couper la corde\u00a0!<\/p>\n

\"\"
Deux n\u0153uds. Pour d\u00e9faire le vert, il suffit de faire glisser la boucle de gauche. Le second requi\u00e8re la m\u00e9thode d\u2019Alexandre et des virus, non autoris\u00e9e en th\u00e9orie des n\u0153uds.<\/figcaption><\/figure>\n

N\u0153uds et math\u00e9matiques<\/h2>\n

Math\u00e9matiquement, les n\u0153uds sont des courbes ferm\u00e9es de l\u2019espace de dimension trois, que l\u2019on repr\u00e9sente souvent comme une courbe plane. Elle a alors des points doubles, o\u00f9 il faut distinguer la branche \u00ab\u00a0au-dessus\u00a0\u00bb de celle \u00ab\u00a0en-dessous\u00a0\u00bb. Si en essayant de d\u00e9m\u00ealer un n\u0153ud, on passe \u00e0 un autre, les deux n\u0153uds sont dits \u00e9quivalents. La th\u00e9orie des n\u0153uds consiste donc \u00e0 \u00e9tudier si un n\u0153ud est \u00e9quivalent \u00e0 une courbe non nou\u00e9e, comme le cercle, et plus g\u00e9n\u00e9ralement si deux n\u0153uds sont \u00e9quivalents. Pour \u00e9tudier ce type de probl\u00e8me, on essaye d\u2019introduire des invariants, c\u2019est-\u00e0-dire des objets math\u00e9matiques invariants quand on passe d\u2019un n\u0153ud \u00e0 un n\u0153ud \u00e9quivalent. Henri Poincar\u00e9 (1854 \u2013 1912) en a trouv\u00e9 un particuli\u00e8rement subtil, que l\u2019on appelle le groupe du n\u0153ud, malheureusement son \u00e9tude est d\u00e9licate.<\/p>\n

\"\"
Stephen Smale (n\u00e9 en 1930), William Thurston (1946 – 2012) et Mikha\u00efl Gromov (n\u00e9 en 1943) r\u00e9unis lors de la conf\u00e9rence Clay sur la r\u00e9solution de la conjecture de Poincar\u00e9, en 2010.<\/figcaption><\/figure>\n

William Thurston a d\u00e9couvert une r\u00e9alisation concr\u00e8te de ce groupe, li\u00e9e \u00e0 la g\u00e9om\u00e9trie des espaces de dimension trois, ce qui lui a valu la m\u00e9daille Field en 1982, et explique son implication en biologie ainsi que celles de Stephen Smale ou de Mikhail Gromov, sp\u00e9cialistes de ce domaine, souvent pr\u00e9sent\u00e9 tr\u00e8s loin de toute application.<\/p>\n","protected":false},"excerpt":{"rendered":"

L\u2019ADN (ou acide d\u00e9soxyribonucl\u00e9ique) est le support de l\u2019h\u00e9r\u00e9dit\u00e9. Cette mol\u00e9cule, pr\u00e9sente dans chaque cellule, prend la forme d\u2019une double h\u00e9lice, qui s\u2019enroule sur elle-m\u00eame, formant ainsi un n\u0153ud. Duplication des mol\u00e9cules La duplication des informations contenues dans une mol\u00e9cule d\u2019ADN se fait au moyen d\u2019enzymes. Pour \u00ab\u00a0voir\u00a0\u00bb le processus, imaginez une longue fermeture \u00e9clair … Continuer la lecture de Des n\u0153uds dans l’ADN<\/span> →<\/span><\/a><\/p>\n","protected":false},"author":12,"featured_media":665,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":"","_links_to":"","_links_to_target":""},"categories":[248,63,29,3],"tags":[249,251,252,250],"class_list":["post-664","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-biologie","category-histoire","category-maths-2","category-sciences","tag-adn","tag-alexandre-le-grand","tag-duplication","tag-noeuds"],"yoast_head":"\nDes n\u0153uds dans l'ADN, par Herv\u00e9 Lehning<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/blogs.futura-sciences.com\/lehning\/2019\/06\/04\/des-noeuds-dans-ladn\/\" \/>\n<meta property=\"og:locale\" content=\"fr_FR\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Des n\u0153uds dans l'ADN, par Herv\u00e9 Lehning\" \/>\n<meta property=\"og:description\" content=\"L\u2019ADN (ou acide d\u00e9soxyribonucl\u00e9ique) est le support de l\u2019h\u00e9r\u00e9dit\u00e9. 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