Cov Txheej Txheem Sau Ntawv hauv Molecular Biology
Kev sau ntawv yog ib qho txheej txheem tseem ceeb hauv kev tshawb fawb txog cov noob caj noob ces uas txuas cov ntaub ntawv caj ces hauv DNA nrog kev tsim cov RNA molecules. Los ntawm kev sau ntawv, cov kab ke nucleotide hauv DNA yog "txhais" mus rau hauv RNA, uas tom qab ntawd tuaj yeem ua haujlwm ua tus xa xov RNA (mRNA) rau kev tsim cov protein, lossis ua lwm yam RNA ua haujlwm xws li rRNA thiab tRNA. Cov txheej txheem no yog ib feem tseem ceeb ntawm lub hauv paus ntawm kev tshawb fawb txog biology: DNA → RNA → Protein. Txawm hais tias nws yuav zoo li yooj yim, kev sau ntawv suav nrog ntau cov kauj ruam teeb tsa, ntau yam enzymes, thiab kev tswj hwm nruj kom cov hlwb tuaj yeem qhia cov noob caj noob ces raws li qhov xav tau.
Cov ntsiab lus tseem ceeb ntawm kev sau ntawv
Hauv kev sau ntawv, tsuas yog ib txoj hlua DNA xwb thiaj siv tau ua ib txoj hlua qauv. Lub enzyme RNA polymerase nyeem txoj hlua qauv ntawm 3' mus rau 5' thiab tsim cov RNA tshiab hauv qhov kev taw qhia 5' mus rau 3'. Cov kab ke RNA uas tshwm sim yog qhov sib txuas rau txoj hlua qauv DNA, nrog rau qhov sib txawv tseem ceeb: RNA siv lub hauv paus uracil (U) es tsis yog thymine (T). Yog li, yog tias A muaj nyob hauv DNA, U yog tus khub hauv paus hauv RNA, thiab yog tias C muaj nyob, G yog tus khub hauv paus (thiab rov qab).
Kev sau ntawv tuaj yeem tshwm sim hauv ntau hom noob caj noob ces. Cov noob caj noob ces uas sau cov protein tsim cov mRNA, thaum lwm cov noob caj noob ces tuaj yeem tsim cov RNA uas tsis sau cov protein uas tsis tau txhais ua cov protein tab sis muaj cov haujlwm tsim thiab tswj hwm, xws li rRNA (ribosomal building blocks), tRNA (amino acid carriers), snRNA (splicing), thiab ntau hom miRNA/lncRNA uas tswj kev qhia tawm noob caj noob ces.
Cov Cheebtsam Tseem Ceeb thiab qhov sib txawv ntawm prokaryotes thiab eukaryotes
Hauv cov prokaryotes (piv txwv li, cov kab mob), cov txheej txheem sau ntawv yooj yim heev. Feem ntau muaj ib qho RNA polymerase thawj zaug, pab los ntawm sigma factor, uas lees paub tus txhawb nqa. Ntxiv mus, vim tias cov kab mob tsis muaj lub nucleus, kev sau ntawv thiab kev txhais lus tuaj yeem tshwm sim tib lub sijhawm ntawm cov chaw sib ze.
Qhov sib txawv, hauv eukaryotes (piv txwv li, tib neeg), kev sau ntawv nyuaj dua. Muaj ntau RNA polymerases: RNA polymerase I (sau qee cov rRNAs), RNA polymerase II (sau cov mRNAs thiab qee cov snRNAs/miRNAs), thiab RNA polymerase III (sau cov tRNAs thiab 5S rRNAs). Ntxiv mus, eukaryotic DNA tau ntim rau hauv chromatin, yog li kev nkag mus rau cov noob caj noob ces xav tau kev tswj hwm ntxiv, suav nrog kev hloov kho histone, chromatin remodeling, thiab ntau yam kev sau ntawv dav dav thiab tshwj xeeb.
Theem 1: Pib sau ntawv
Kev pib yog theem pib thaum cov tshuab sau ntawv tau sib sau ua ke ntawm qhov chaw raug ntawm DNA. Cov txheej txheem no pib nrog kev lees paub tus txhawb nqa, ib qho DNA tshwj xeeb uas nyob rau sab saud ntawm ib lub noob caj noob ces uas txiav txim siab qhov pib ntawm kev sau ntawv.
Kev pib hauv prokaryotes
Hauv cov kab mob bacteria, cov yam ntxwv sigma pab RNA polymerase paub txog cov ntsiab lus tseem ceeb ntawm promoter xws li lub thawv Pribnow thiab lub thawv 35. Tom qab RNA polymerase khi rau tus promoter, ib qho "closed complex" tsim. Cov DNA nyob ib puag ncig qhov pib ces ib feem nthuav tawm, tsim ib qho "open complex," uas tso cai rau RNA polymerase pib sib sau ua ke thawj RNA nucleotides.
Kev pib hauv eukaryotes
Hauv eukaryotes, kev pib muaj ntau cov protein. Hauv cov noob caj noob ces uas tau sau los ntawm RNA polymerase II, cov neeg txhawb nqa feem ntau (txawm tias tsis yog txhua lub sijhawm) muaj cov ntsiab lus xws li lub thawv TATA. Cov protein TATA-binding protein (TBP), ib feem ntawm TFIID complex, lees paub lub thawv TATA thiab nrhiav lwm cov yam ntxwv sau ntawv dav dav (TFIIA, TFIIB, TFIIE, TFIIF, thiab TFIIH) nrog rau RNA polymerase II. Cov complex no hu ua pre-initiation complex.
TFIIH ua lub luag haujlwm tseem ceeb vim nws muaj helicase (DNA unwinding) thiab kinase kev ua ub no, uas phosphorylates CTD (C-terminal domain) ntawm RNA polymerase II. CTD phosphorylation pab RNA polymerase II "tso tawm" los ntawm tus txhawb nqa thiab nkag mus rau theem elongation, thaum tseem muab lub platform rau RNA ua cov protein.
Theem 2: Kev nthuav dav ntawm kev sau ntawv
Tom qab pib ua haujlwm, RNA polymerase txav mus raws tus qauv DNA thiab ua kom cov saw RNA ntev dua. RNA nucleotides (ATP, UTP, GTP, CTP) raug ntxiv ib qho zuj zus, thaum cov khub hauv paus tshwm sim. Thaum lub sijhawm ntev, RNA polymerase tsim ib lub npuas transcription qhov twg DNA unwinds ib ntus. Tom qab polymerase, DNA unwinds, thaum RNA tshiab tsim tawm ntawm cov enzyme complex.
Kev ntev kuj muaj lub tshuab ua pov thawj tsawg. Yog tias muaj qhov sib phim tsis raug, RNA polymerase tuaj yeem nres thiab kho qhov yuam kev los ntawm kev txiav qhov kawg tsis raug ntawm RNA, tom qab ntawd txuas ntxiv kev tsim.
Hauv cov eukaryotes, kev ntev feem ntau nrog los ntawm kev tswj hwm ntxiv, xws li kev nres RNA polymerase II ze ntawm tus promoter. Qhov kev nres no yog qhov tseem ceeb rau kev tswj hwm cov noob caj noob ces thiab kev npaj rau kev sau ntawv sai thaum lub cell tau txais cov cim tshwj xeeb.
Theem 3: Kev xaus ntawm kev sau ntawv
Kev xaus yog cov txheej txheem ntawm kev txwv tsis pub sau ntawv thiab tso RNA thiab RNA polymerase tawm ntawm DNA.
Kev xaus rau hauv prokaryotes
Muaj ob lub tshuab tseem ceeb:
1. Kev xaus ntawm cov noob caj noob ces (rho-independent): DNA muaj ib qho sequence uas tsim cov RNA uas muaj GC-rich thiab tsim ib qho hairpin structure, ua raws li ib qho U-rich sequence. Cov hairpin ua rau RNA polymerase nres thiab RNA-DNA bond tsis muaj zog ua rau cov complex sib cais.
2. Kev xaus uas nyob ntawm Rho: Cov protein rho (helicase) txuas rau RNA thiab txav mus tom qab nws kom txog thaum nws mus txog qhov RNA polymerase uas tau nres, tom qab ntawd tso RNA tawm ntawm cov txheej txheem sau ntawv.
Kev xaus rau hauv eukaryotes (RNA polymerase II)
Hauv ntau cov noob caj noob ces, RNA polymerase II dhau ib lub teeb liab polyadenylation (piv txwv li, AAUAAA hauv RNA) thiab tom qab ntawd raug txiav los ntawm ib qho kev ua haujlwm nyuaj. Tom qab txiav, kev xaus tuaj yeem tshwm sim los ntawm tus qauv "torpedo" (ib qho enzyme exonuclease rhuav tshem cov RNA seem thiab caum cov polymerase) lossis tus qauv hloov pauv conformational nyuaj. Kev xaus hauv RNA polymerases I thiab III muaj ib qho sib txawv, ntau dua qhov sib lawv liag.
Kev ua RNA hauv eukaryotes: los ntawm pre-mRNA mus rau mRNA laus
Qhov sib txawv loj ntawm prokaryotes thiab eukaryotes yog tias eukaryotic mRNA feem ntau yuav tsum tau ua tiav ua ntej nws tuaj yeem txhais lus. Qhov kev ua tiav no tshwm sim co-transcriptionally (nrog rau kev sau ntawv) thiab suav nrog:
1. Kev txuas lub hau 5': Ntxiv ib pawg 7-methylguanosine rau qhov kawg 5'. Lub hau tiv thaiv RNA ntawm kev puas tsuaj, pab xa mus rau cytoplasm, thiab yog qhov tseem ceeb rau kev pib txhais lus.
2. Kev Sib Txuas: Tshem tawm cov introns thiab kev sib txuas ntawm exons los ntawm spliceosome. Lwm txoj kev sib txuas tuaj yeem tsim ntau hom protein isoforms los ntawm ib lub gene, ua rau muaj ntau yam proteome.
3. Polyadenylation (poly-A tus tw): Ntxiv ib tus poly-A tus tw rau 3' kawg. Tus tw no ua rau mRNA ruaj khov dua, pab xa tawm, thiab cuam tshuam rau kev txhais lus zoo.
RNA uas tsis tau ua tiav (pre-mRNA) feem ntau tsis raug txhais lus. Lub kaw lus tswj xyuas zoo ntawm lub cell yuav khaws lossis ua rau RNA puas tsuaj.
Kev tswj hwm kev sau ntawv: vim li cas txhua lub noob tsis ua haujlwm txhua lub sijhawm?
Kev sau ntawv yog qhov chaw tswj hwm tseem ceeb rau kev qhia tawm cov noob caj noob ces. Cov cell tswj cov noob caj noob ces twg ua haujlwm, lawv raug sau ntawv muaj zog npaum li cas, thiab thaum twg yuav tsum nres kev sau ntawv. Txoj cai no tso cai rau cov cell teb rau kev hloov pauv ib puag ncig, cov cim tshuaj hormones, kev ntxhov siab, lossis kev xav tau kev loj hlob.
Hauv cov prokaryotes, kev tswj hwm feem ntau cuam tshuam nrog operons, xws li lac operon, uas ua kom cov lactose-splitting enzyme tsuas yog thaum lactose muaj. Cov yam ntxwv repressor thiab activator tuaj yeem tiv thaiv lossis txhim kho RNA polymerase recruitment.
Hauv eukaryotes, kev tswj hwm yog qhov nyuaj dua. Cov ntsiab lus xws li cov khoom txhawb nqa thiab cov khoom ntsiag to tuaj yeem nyob deb ntawm tus txhawb nqa tab sis cuam tshuam rau kev ua haujlwm transcriptional los ntawm DNA folding thiab protein sib cuam tshuam. Ntxiv mus, cov xwm txheej chromatin (piv txwv li, histone acetylation, uas ua rau DNA "qhib ntau dua") txiav txim siab seb lub gene puas nkag tau rau cov tshuab transcriptional.
Kev kaw
Lub tshuab ntawm kev sau ntawv yog lub hauv paus ntawm cov ntaub ntawv caj ces tau qhia rau hauv RNA, ob qho tib si ua ib qho nruab nrab rau kev tsim cov protein thiab ua ib lub molecule ua haujlwm uas ua lub luag haujlwm tswj hwm thiab cov qauv. Cov txheej txheem no tshwm sim los ntawm peb theem tseem ceeb - pib, ntev, thiab xaus - nrog rau ntau yam kev nyuaj ntawm prokaryotes thiab eukaryotes. Hauv eukaryotes, kev sau ntawv tau sib xyaw ua ke nrog RNA ua tiav xws li 5 'cap tsim, splicing, thiab polyadenylation. Vim tias kev sau ntawv kuj raug tswj hwm heev, kev nkag siab txog cov txheej txheem no yog qhov tseem ceeb hauv ntau qhov chaw, los ntawm kev tshuaj ntsuam xyuas noob caj noob ces thiab biotechnology mus rau kev kho mob molecular, suav nrog kev nkag siab txog cov kab mob uas cuam tshuam nrog kev qhia tawm noob caj noob ces tsis zoo.
