BIOL 3301 - Genetics Ch13B - Transcription in Eukaryotes Students

8/18/2019 BIOL 3301 - Genetics Ch13B - Transcription in Eukaryotes Students

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Transcription In Eukaryotes

• In addition to promoters" enhancers may be

located upstream

•#rocessing of mRNA occurs – adding of $% capand &%tail

• Initial transcripts are pre!mRNA

• 'ome ribonucleotide seuences are e(cisedfrom pre!mRNA ! splicing


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Transcription in Eukaryotes

• Eukaryotic RNA polymerases) – RNA polymerase I

• *ocated e(clusi+ely in the nucleolus

• Cataly,es synthesis of three RNAs making the ribosome -./'" 0/'and $1/'2

– RNA polymerase II• *ocated in the nucleoplasm

• 'ynthesi,e mRNAs

• 'ome snRNAs – RNA polymerase III

• *ocated in the nucleoplasm

• 'ynthesis of tRNA" $'RNA" snRNAs


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Initiation Of Transcription In

Eukaryotes• The acti+ity of RN# is regulated by)

– Cis! acting elements)

• core promoter elements -determines the binding site and

start23

• promoters

• enhancers

– Trans!acting elements – • transcription factors" facilitate binding and initiation of

transcription

• enhancers


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Structure of a Promoter

Recognized By RNA Polymerase II


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Transcription in Eukaryotes) #romoters

and Enchancers• Core promoter)

– A short seuence called initiator -Inr2 – spans the transcription start site-402

– The TATA!bo( at !&5 position" TATAAAA

– The TATA bo( binds the TATA!binding protein -T6#2 of transcriptionfactor T7II8 and determines the start site of transcription1

– In absence of elements" transcription occur on lo9 le+els

• #romoter pro(imal elements – :cat; bo( CAAT at !5 position – 6oth 9ork in either orientation

– ?utation decreases transcription initiation from promoter


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Transcription in Eukaryotes) Initiation

• Initiation of transcription reuire the assembly

of RNA polymerase II and a number of general

transcription factors -=T7s2 on the core

promoter

• 6ind in particular order to promoter elements

to produce the complete transcription initiation

comple( -#IC – pre!initiation comple(2


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Transcription In Eukaryotes)

Elongation• Once RNA polymerases ha+e been released

from initiation comple(es" they cataly,e RNA

chain elongation

• Transcription of one gene can proceed by

se+eral RNA# simultaneously


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Lampbrush Chromosomes


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Transcription In Eukaryotes

• In eukaryotes" there is further processing of

the RNA transcript in the nucleus before

protein synthesis can occur -in the cytoplasm2

• ?ost eukaryotic mRNAs are modified in the

nucleus before they are e(ported to the

cytoplasm to be translated into proteins


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RNA Chain Elongation and

Addition of $%Cap• hen gro9ing chain is about &5 nt long"

?= caps are added to the $%end

• #rotect RNA from nucleases by unusual $%to

$%bond and t9o methylated nt

• Recogni,ed by protein factors for initiation of

translation


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Termination 6y Chain Clea+age

and Addition of &%#oly-A2 Tails• &%ends are produced not by termination of transcription as in

prokaryotes but by endonucleolytic clea+age

• Termination e+ents occur at multiple sites 0555!.555 nt

do9nstream from future &%of mature transcript

• Clea+age to produce &%end occurs at a site 00 to &5 nt

do9nstream from a conser+ed region 9ith consensus

AABAAA and upstream of =B!rich region near the end of

transcription unit

• After clea+age" poly-A2polymerase adds poly-A2 tails - around

.552 ! polyadenylation

• 7or polymerases I and III termination signals e(ist


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RNA Editing In Eukaryotes

• In both prokaryotes and eukaryotes" mature"

biologically acti+e RNA has three main parts)

– A leader seuence at $% or $% untranslated region

-$% BTR2

– Coding seuence

– A trailer seuence or &% untranslated region -&%

BTR2


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Eukaryote RNA processing

• $% end) capping – addition of

– linked by three phosphates

• &% end) poly-A2 tail – addition of up to .55 adenine nucleotides

– do9nstream of AABAAA polyadenylation signal

• Intron remo+al by spliceosome – all introns ha+e $%=B and &%A= recognition seuence

-=B – A= rule2

– snRN#s of spliceosome pro+ide catalysis

– intron e(cised as lariat" destroyed


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intron e(cisionDRNA splicing

• ?ost eukaryotic genes include specific regions thatare transcribed" but that do not contain seuencemeant to encode protein" so those regions are remo+edfrom the RNADintrons -inter+ening seuence2

• The regions 9hich remain and encode proteinseuence are exons -e(pressed seuence2

• The process of remo+ing the introns is called RNA

splicing and for most genes is coordinated by proteinsthat bind to the RNA in the regions of theintronsD spliceosome


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snRNPs

• These proteins ha+e small pieces of RNA

attached to the protein subunits" so they are

called snRNPs -snurps" small nuclear

ribonucleoproteins2

• 'plicing of some genes does not reuire

snRN#s -the introns are self!splicing2" the

splicing reaction is cataly,ed by the RNAitselfDribozyme


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Eukaryote RNA processing

• Intron remo+al by spliceosome

– all introns ha+e $%=B and &%A= recognition

seuence -=B – A= rule2 – snRN#s of spliceosome pro+ide catalysis

– intron e(cised as lariat" destroyed


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ranscription In !u"aryotes

http://www.biology.com/learning/transcription/mrnaeuk.html


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Prokaryotic mRNA

Eukaryotic mRNA


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Physical definition of a gene#

Exon 1 Exon 2 Exon 3Enhancer Enhancer

intron intron

Transcription

termination

promoter

Anatomy of a gene:

What is the gene product?

Exon 1 Exon 2 Eon 3

Exon 1 Exon 2 Exon 3

Exon 1 Exon 2 Exon 3


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Re$erse %enetics

BIOL 3301 - Genetics Ch13B - Transcription in Eukaryotes Students

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