Örlítið um veirur

Skrifað um January 31, 2020 · in Almennt · 595 Comments

Veirur eru ekki taldar með í flokkun lífvera, vegna þess, að þær hafa ekki frumuskipulag og eru ekki taldar til lífvera. Á hinn bóginn má með nokkrum rökum halda því fram, að þær séu á mörkum hinnar lífvana og lifandi náttúru. Veirur eru agnarsmáar eða sjaldan stærri en 200 nm að þvermáli. Einn nanómetri er einn milljarðasti úr metra.

Veirur eru ávallt úr að minnsta kosti tveimur hlutum, ytra byrði eða hjúp úr prótíni og kjarnsýru þar inni, annaðhvort erfðaefnið DNA (Deoxy-ríbósa-kjarnsýra) eða RNA (Ríbósa-kjarnsýra). Að auki kunna að vera þar nokkur enzím (lífhvatar), sem stuðla að fjölgun kjarnsýrunnar. Utan um hjúpinn er oft þunn himna, sem að hluta til er frumuhimna hýsilsins. Flokkun veirna byggist einkum á þremur atriðum: a) Gerð kjarnsýrunnar, b) lögun og stærð hjúpsins og d) hvort hjúpurinn er klæddur himnu eða ekki.

Þar eð engin starfsemi fer fram í veirum, geta þær aðeins verið virkar inni í lifandi frumum. Oft áður voru hænuegg smituð til þess að rannsaka hætti þeirra, en nú er algengara, að sýkja vefi, sem eru sérstaklega ræktaðir í þessu skyni. Veirur sýkja allar gerðir af frumum, gerlafrumur jafnt sem frumur úr mönnum. Eitt vekur sérstaka athygli við veirur. Það er hve sérhæfðar þær eru. Vissar veirur sýkja aðeins gerla, svonefndar gerilætur, tóbaksveiran leggst einungis á plöntur og aðeins spendýr smitast af hundaæðisveiru. Sumar veirur, sem hrjá menn, eru líka mjög sérhæfðar. Alnæmisveiran (HIV) ræðst einungis á sérstök blóðkorn, mænusóttarveira fjölgar sér bara í mænutaugum og veira, sem veldur lifrarbólgu, nær sér aðeins á strik í lifrarfrumum. Skýring á sérhæfingu þessari kann að vera sú, að kjarnsýra veirnanna sé upprunnin úr frumum hýslanna. Ef svo er, mynduðust veirur á eftir frumum og eru líklega enn að verða til.

Veirur geta tekið stökkbreytingum eins og frumur. Það veldur því, að lyf gegn sjúkdómum virka oft ekki nema stuttan tíma í einu. Þannig eru ávallt að koma fram nýir stofnar af veirum, sem valda inflúensu.

Veirur eru sérhæfðar að vissum frumum vegna þess, að hjúpur á þeim eða himnan, sem umlykur hann, falla eins og lykill í skrá að frumuhimnu hýsilfrumunnar. Eftir að kjarnsýra veiru hefur brotið sér leið inn í frumu, fer hún að stjórna efnaskiptum hýsilfrumunnar og tekur að framleiða nýjar veirur.

Fjölgun veirna inni í hýsilfrumu gerist með nokkuð mismunandi hætti. Í sinni einföldustu mynd gerist það þannig, þegar veira sýkir gerilfrumu, að veiran sezt utan á frumuvegg gerilsins (1). Veiran veldur því, að veggur gerilsins leysist upp (2) og DNA-þráður veirunnar smýgur inn í hýsilinn (3). Þá gerist það, að erfðaefni veirunnar tengist við DNA-þráð gerilsins (4) og hafizt getur framleiðsla á efnum veirunnar samkvæmt fyrirsögn erfðaefnisins (5), bæði nýjum DNA-þráðum og hjúpum en líka á ensímum, sem leysa upp frumuvegg gerilsins (6), svo að nýmynduðu veirurnar (7) sleppa út (8) og geta tekið að smita aðrar frumur. (Sjá mynd hér að ofan.)

Nýmyndunarferli veiru hefst ekki alltaf um leið og fruma sýkist. Smitið getur legið niðri um allnokkurn tíma, en hafizt svo skyndilega, oft fyrir tilstilli utanaðkomandi krafta.

Veirur, sem smita dýrsfrumur, hegða sér um margt líkt og gerilætur. Þó eru nokkur atriði frábrugðin. Ef veiruhjúpurinn er klæddur himnu festist hún við hýsilfrumuna og allur hjúpurinn, með erfðaefnið inni í sér, hverfur inn í frumuna. Inni í frumunni eyðist hjúpurinn og erfðaefnið, hvort heldur það er DNA eða RNA, getur nú farið að hafa áhrif á starfsemi hýsilsins. Nýju veirurnar, sem myndast, losna oft úr hýsilfrumunni við það að hluti frumunnar myndar knappskot, sem losnar frá. Þar með hafa þær fengið nýja himnu utan um hjúpinn, sem greiðir þeim leið inn í aðra frumu. Þó að hluti hýsils losni frá, leiðir það alls ekki alltaf til dauða frumunnar.

Megingerð gerilveiru. 1. prótínhjúpur höfuðs umlykur DNA-þráð, sem geymir meira en 60 gen., 2. herpanlegt halaslíður, 3. gripangar, sem festa veiruna við geril, 4. sexhyrnd grunnplata og 5. göng, sem DNA fer um.

Megingerð gerilveiru. 1. prótínhjúpur höfuðs umlykur DNA-þráð, sem geymir meira en 60 gen., 2. herpanlegt halaslíður, 3. gripangar, sem festa veiruna við geril, 4. sexhyrnd grunnplata og 5. göng, sem DNA fer um.

Til eru svo kallaðar víxlveirur, retróveirur, sem hafa kjarnsýru af RNA-gerð og sérstakt enzím, sem veldur því, að DNA-þráður afritast eftir RNA-þræðinum. Þegar víxlveiran er komin inn í dýrsfrumu, verður því til DNA-þráður, sem síðan tvöfaldast og fellur inn í litninga hýsilfrumunnar. Þar með getur hún farið að nýmynda nýjar veirur. HIV-veiran er einmitt af þessari gerð svo og ýmsar veirur aðrar, sem vitað er að geta valdið krabbameini.

 

P.s. Finna má mikinn fróðleik um veirur á netinu.


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