Avui dia es coneixen mĂ©s de 5.000 tipus diferents de virus, uns agents infecciosos submicroscòpics que infecten tots tipus de vida cel·lular, des de la dels humans i altres animals, com la de les plantes i fins i tot la dels bacteris. Tots els virus tenen el seu propi material genètic, que pot ser ADN o ARN, aixĂ com una capa proteĂŻca anomenada cĂ pside que protegeix aquests gens. Alguns, a mĂ©s, tenen un embolcall de lĂpids que els resguarda quan no es troben dins d'una cèl·lula.
Aquests organismes, causants de malalties tan comuns com el refredat o la grip i tan greus com l’ébola o la sida, sĂłn incapaços de reproduir-se per si mateixos. Es poden transmetre a travĂ©s de la sang o saliva, de l’aigua contaminada, de partĂcules d’aerosols o d’insectes vectors. Un cop dins de l’organisme, els virus penetren dins la cèl·lula hoste mitjançant la fusiĂł del seu embolcall amb la membrana de la cèl•lula. Els que no tenen embolcall ho fan per un procĂ©s anomenat endocitosi a travĂ©s del qual proteĂŻnes vĂriques de la cĂ pside reconeixen receptors a la membrana de la cèl·lula hoste i s’hi uneixen com una clau a un pany, permetent l’entrada del virus dins la cèl·lula. AllĂ , el virus es desfĂ de la seva cĂ pside, el material genètic es replica i aleshores es sintetitzen noves proteĂŻnes vĂriques, tot utilitzant la maquinĂ ria molecular de la cèl·lula. Les noves proteĂŻnes i el nou material genètic formen nous virus, que sĂłn alliberats de la cèl·lula hoste per lisi, un procĂ©s que mata la cèl·lula rebentant-ne la membrana. Si es tracta d’un virus amb embolcall lipĂdic, els nous virus s’alliberen per gemmaciĂł tot agafant part de la membrana plasmĂ tica de l'hoste com a nou embolcall. A vegades, l’ADN del virus pot integrar-se dins el genome de l’hoste, i aleshores resta latent sense activar-se durant molt de temps.
El fet que es sintetitzin amb material de l’hoste implica que, a diferència dels bacteris, els virus sĂłn difĂcils de reconèixer com a agents externs i, per tant, d’atacar. A mĂ©s, els virus co-evolucionen amb l’hoste, de forma que, per exemple, les proteĂŻnes de la seva cĂ pside estan perfectament adaptades per encaixar amb els receptors de la cèl·lula hoste.
Finalment, existeixen diferents soques d’alguns virus, com el cas del de la grip. Això explica que la vacuna contra la grip es modifiqui cada any segons les estimacions dels cientĂfics sobre quines soques seran mĂ©s freqĂĽents.
The attack of the viruses
More than 5,000 different types of viruses are known nowadays. They are sub microscopic infectious agents that attack all kinds of cellular life, from humans and other animals to plants and even bacteria. All viruses have their own genetic material, which can be either DNA or RNA, and a protein layer called capsid that protects their genes. Some viruses also have a lipid cover that surrounds them when they are not inside a cell.
These organisms, which cause diseases as common as a cold or flu and as serious as ebola or Aids, are unable to reproduce by themselves. They are transmitted through blood or saliva, through contaminated water, through the breathing of aerosol particles in which viruses travel, or through insects that act as vectors. Once inside the organism, the viruses penetrate in the host cell by fusing their cover with the cell membrane. If they don’t have a cover, the viruses enter by endocytosis, a process by which viral proteins recognise receptors at the membrane of the host cell and bind them, allowing the entry of the virus into the cell. There, the virus gets rid of the capsid and the genetic material is replicated and new viral proteins are synthesized, all using the molecular machinery of the cell. The new proteins and genetic material form new viruses, which are freed from the host cell by lysis – a process that kills the cell by bursting its membrane – or, in the case of viruses with lipid covers, by budding, taking part of the plasma membrane of the host cell as their cover. Sometimes the DNA of the virus can integrate within the genome of the host, and remain latent, without being activated, for a long time.
The fact that they are synthesized with material from the host implies that viruses are difficult to recognise as foreign and therefore to attack. Also, viruses co-evolve with the host so that, for example, the proteins in their capsid are perfectly adapted to recognise the receptors on the cell host.
Finally, there are different strains of some viruses, such as the flu virus. This explains the fact that the flu vaccine has to be modified every year according to researchers’ estimations about which strains will be more frequent.
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