Ukupakishwa kwe-DNA ku-nucleus yeseli
Ngaphakathi kweseli ngalinye le-eukaryotic—njengalelo labantu, izilwane, izitshalo, kanye nesikhunta—kugcinwa imiyalelo yokuphila ngesimo se-DNA (deoxyribonucleic acid). Uma i-DNA evela esitokisini esisodwa somuntu ingasongwa, ibingaba ngamamitha amabili ubude. Inselele: umucu omude kangaka ungangena kanjani ku-nucleus yeseli, engama-micrometer ambalwa kuphela ububanzi, ngaphandle kokuxhunywa, ukuhlala uvikelekile, kodwa usatholakala lapho iseli lidinga ukukopisha noma ukuveza izakhi zofuzo? Impendulo itholakala ohlelweni lokupakisha lwe-DNA oluhlelekile kakhulu nolunamandla olwaziwa ngokuthi i-chromatin. Ukupakisha i-DNA akukhona nje “ukugoqwa” kwe-DNA; kuyinqubo yezinto eziphilayo enquma ukuthi izakhi zofuzo ziyasebenza noma zithule futhi zidlala indima enkulu ekuzinzeni kwezakhi zofuzo.
I-DNA kanye nenkinga "yesikhala" ku-nucleus
I-DNA iyi-polymer ende eyakhiwe ngama-nucleotide anezisekelo ezine eziyinhloko: i-A, i-T, i-C, kanye ne-G. Kubantu, i-DNA iyonke ku-nucleus yeseli elilodwa ifinyelela cishe ama-base pair ayizigidi eziyizinkulungwane ezintathu. Ngalobu bude obungavamile, i-DNA ayikwazi ukushiywa intante ngokukhululeka ku-nucleus. Ngaphandle kwezinkinga zesikhala, i-DNA kumele futhi ivikelwe ekulimaleni ngokomzimba nangokwekhemikhali, futhi kumele ihlelwe ukuze izingxenye ezithile zifundwe (zibhalwe) zibe yi-RNA uma kudingeka. Ngakho-ke, amaseli asebenzisa amaprotheni akhethekile "ukuhlela" i-DNA ukuze ibe ncane, icoceke, futhi isebenze.
I-Chromatin: uhlobo oluhlelekile lwe-DNA
Ku-nucleus, i-DNA ayikho yodwa, kodwa ihlotshaniswa namaprotheni—ikakhulukazi ama-histone—ukwakha i-complex ebizwa ngokuthi i-chromatin. I-Chromatin iyashintshashintsha: ngaphansi kwezimo ezithile, ingakhululeka ukuze ivumele izakhi zofuzo ukuthi zifinyelelwe, futhi ngaphansi kwezinye izimo, ingaqina ukuze yonge isikhala noma ivimbele izakhi zofuzo. I-Chromatin futhi yakha isisekelo sokwakheka kwama-chromosome, abonakala ngokucacile ngesikhathi sokuhlukaniswa kwamaseli (i-mitosis noma i-meiosis).
Ngokuvamile, i-chromatin ingahlukaniswa ngezinhlobo ezimbili:
1. I-Euchromatin: isakhiwo esikhululekile, ngokuvamile esicebile ngamajini asebenzayo, esitholakala kalula emishinini yokubhala.
2. I-Heterochromatin: isakhiwo esiminyene, izakhi zofuzo zivame ukungasebenzi, zivame ukuqukatha izindawo eziphindaphindayo futhi zidlala indima ekuzinzeni kwama-chromosome.
Lo mehluko ubalulekile ngoba ukhombisa ukuthi ukupakishwa kwe-DNA kuhlobene ngqo nokulawulwa kwezakhi zofuzo.
I-Nucleosome: iyunithi eyisisekelo yokupakisha i-DNA
Isinyathelo sokuqala ekupakishweni kwe-DNA ukwakheka kwama-nucleosome, avame ukufaniswa "nama-beads on a string." Ama-nucleosome aqukethe i-DNA wound cishe izikhathi ezingu-1,65 ejikeleza i-histone protein core. Lo mgogodla wakhiwe ngama-histone ayisishiyagalombili (ama-octamite): amakhophi amabili ama-histone ngalinye i-H2A, i-H2B, i-H3, kanye ne-H4.
Phakathi kwama-nucleosome kukhona izingxenye ze-DNA ezixhumanisayo ezibizwa ngokuthi i-linker DNA. Lapha, i-histone H1 ivame ukusebenza njenge-"clamp," esiza ekuqiniseni i-DNA coil futhi isekele ukwakheka kwesakhiwo se-chromatin esiqinile. Ukwakheka kwe-nucleosome kungafinyeza kakhulu i-DNA futhi kuyivikele ekulimaleni, njengoba ingxenye enkulu ye-DNA imbozwe ngamaprotheni.
Kusukela kuma-nucleosome kuya kuma-chromatin fibers aqinile
Uma ama-nucleosome esebunjiwe, isinyathelo esilandelayo ukuhleleka kwezinga eliphezulu. Ama-nucleosome awagcini nje ngokuhambisana kodwa angakha nemicu ye-chromatin ethe xaxa. Sekuyiminyaka eminingi, imodeli yakudala iphakamisa imicu engu-30-nm (ngephethini "ye-solenoid" noma "ye-zigzag"), yize ucwaningo lwanamuhla lukhombisile ukuthi isakhiwo se-chromatin emaseli aphilayo singaba nokuguquguquka okukhulu futhi asihlali sakha imicu engu-30-nm efanayo. Ngokusobala, ukusebenzisana kwe-inter-nucleosome, indima ye-histone H1, kanye nezimo ze-ionic ku-nucleus kunegalelo ekuminyaneni kwe-chromatin.
Ngaphezu kwalokho, ukupakishwa akugcini nje emicu. I-Chromatin iphinde yakhe ama-loop noma "imibuthano" enamathele ohlakeni lwe-nuclear protein. La ma-loop avumela ukuhlelwa kwezifunda ze-genomic: izakhi zofuzo ezidinga ukusebenza zingabekwa ezindaweni ezinobungani bokuqoshwa, kuyilapho izifunda ezidinga ukuthula zingapakishwa kakhulu.
Izizinda kanye nokwakhiwa kwe-genome kwe-3D
Ukupakishwa kwe-DNA yesimanje kuqondwa njengokuhlelwa kwe-genome okunezinhlangothi ezintathu (3D) ngaphakathi kwe-nucleus. I-genome ayihlelwanga ngokungahleliwe; yakha izindawo ezisebenzayo. Umqondo owodwa obalulekile yi-Topologically Associated Domains (TADs), izifunda ze-DNA ezivame ukusebenzisana kaningi nezifunda ezingaphakathi kwesizinda esifanayo kunezifunda ezingaphandle kwaso. Lesi sakhiwo sisiza ukuqinisekisa ukuthi izithuthukisi (izithuthukisi zokubhala) zixhumana nabathuthukisi bezakhi zofuzo abafanele, ngaleyo ndlela zilawule kangcono ukuvezwa kwezakhi zofuzo.
Ngaphezu kwama-TAD, kukhona nezingxenye ze-A ne-B: ingxenye ye-A ivame ukuba nokucebile nge-euchromatin kanye nezakhi zofuzo ezisebenzayo, kuyilapho ingxenye ye-B icebile nge-heterochromatin kanye nezakhi zofuzo ezingasebenzi. Lokhu kusikisela ukuthi ukupakishwa kwe-DNA akukhona nje ukuminyana, kodwa futhi nokuhlukanisa "izindawo" zomsebenzi ngaphakathi kwe-nucleus.
Ukuguqulwa kwe-Histone kanye ne-epigenetics
Esinye sezici ezithakazelisa kakhulu ekupakishweni kwe-DNA indima yayo ku-epigenetics, okuwukulawulwa kwezakhi zofuzo ezingashintshi ukulandelana kwesisekelo se-DNA. Imisila ye-Histone (izingxenye ze-histones eziphuma ku-nucleosome) ingaguqulwa ngamakhemikhali ahlukahlukene, isibonelo:
– I-Acetylation (ngokuvamile yenza i-chromatin ivuleke kakhulu futhi yandisa umsebenzi wezakhi zofuzo)
– I-Methylation (umthelela ungavuselela noma ucindezele izakhi zofuzo kuye ngokuthi indawo ikuphi)
- I-Phosphorylation, i-ubiquitination, nokunye
Lezi zinguquko zisebenza njenge-"code" ethinta indlela i-chromatin ehlanganiswe ngayo ngokuqinile nokuthi yimaphi amaprotheni anganamathela ku-DNA. Ngaphezu kwama-histone, i-DNA ngokwayo ingadlula ku-methylation (isibonelo, i-cytosine ezimweni ze-CpG ezilwaneni), okuvame ukuhlotshaniswa nokucindezelwa kokubonakaliswa kwezakhi zofuzo.
Ngenxa yokuthi i-epigenetics ingashintsha ngokuya ngentuthuko, indawo ezungezile, kanye nezimo zamaseli, ukupakishwa kwe-DNA nakho kuyaguquguquka. Lokhu kubalulekile ekuhlukaniseni amaseli: wonke amaseli omzimba ngokuvamile ane-DNA efanayo, kodwa amaseli ezinzwa nemisipha ayahluka ngenxa yamaphethini ahlukene okupakishwa nokubonakaliswa kwezakhi zofuzo.
Ukupakishwa kwe-DNA ngesikhathi sokuhlukaniswa kwamaseli: ama-chromosome
Uma amangqamuzana engena ku-mitosis, izinga lokuxinana kwe-DNA likhuphuka kakhulu, kwakha ama-chromosome abonakala ngaphansi kwe-microscope. Lokhu kuyadingeka ukuze kuhlukaniswe ngokunembile i-DNA ibe amangqamuzana amabili ayindodakazi. Phakathi nalesi sigaba, i-chromatin iba ncane kakhulu, futhi izakhi zofuzo eziningi azivezwa. Amaprotheni afana ne-condensin asiza ekwakheni nasekuqiniseni isakhiwo se-chromosome esihlanganisiwe. Ngemva kokuhlukana okuphelele, ama-chromosome "ayavuleka" abuyele ku-chromatin ukuze avumele ukusebenza okuvamile kweseli.
Kungani ukupakishwa kwe-DNA kubalulekile?
Ukupakishwa kwe-DNA ku-nucleus yeseli kunemisebenzi eminingana eyinhloko:
1. Ukusebenza kahle kwesikhala: I-DNA ende kakhulu ingagcinwa ku-nucleus encane.
2. Isivikelo: i-chromatin isiza ekuvikeleni i-DNA ekulimaleni nasekuqhekekeni kweketanga.
3. Ukulawulwa kwezakhi zofuzo: izinga lokuminyana kwe-chromatin linquma ukufinyelela kwezakhi zofuzo, ngaleyo ndlela kuthonye ukuvezwa kwezakhi zofuzo.
4. Ukuphindaphindwa nokulungiswa kwe-DNA: ukupakishwa kumele kukwazi ukuvulwa okwesikhashana ukuze ama-enzyme okuphindaphindwa nokulungisa akwazi ukusebenza, bese ehlanganiswa kabusha.
5. Ukuzinza kwe-Chromosome: isakhiwo se-heterochromatin kanye nokuhleleka kwe-3D kusiza ukugcina ubuqotho be-genome, ikakhulukazi ezindaweni ezifana nama-centromere nama-telomere.
Amaphutha ekupakishweni kwe-DNA angaba nemiphumela emibi kakhulu. Ukuphazamiseka kwamaprotheni e-histone, ama-enzyme e-epigenetic, noma amaprotheni alawula ukwakheka kwe-chromatin kungabangela ukuvezwa kwezakhi zofuzo okungavamile futhi kuhlotshaniswa nezifo ezahlukahlukene, kufaka phakathi umdlavuza, ukuphazamiseka kokukhula, kanye nezifo eziwohlokayo.
I-Penutup
Ukupakishwa kwe-DNA ku-nucleus yeseli kuyisibonelo esisodwa sezimangaliso zokuhleleka kwezinto eziphilayo: i-molecule yolwazi ende kakhulu icindezelwa kahle ngaphandle kokulahlekelwa ikhono layo lokufinyelelwa nokulawulwa. Ngokuklanywa kwe-nucleosome, ukuklanywa kwe-chromatin fiber, ukwakheka kwe-loop kanye ne-3D domain, kanye nokulawulwa kwe-epigenetic ngokusebenzisa ukuguqulwa kwe-histone kanye ne-DNA, amaseli akwazi ukulinganisela izidingo ezimbili ezibonakala ziphikisana: ukuhlanganisa i-DNA ngenkathi kuvumela ukuvezwa kwezakhi zofuzo okunembile. Ukuqonda lezi zinqubo akubalulekile nje kuphela kubhayoloji eyisisekelo kodwa futhi kuvula indlela yokwelashwa kwezokwelapha, njengoba izifo eziningi zibangelwa ukuphazamiseka kwe-chromatin kanye nokulawulwa kwe-epigenetic.
