DNA REPLICATION · PPT file · Web view · 2011-12-15Transcription in...
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Transcript of DNA REPLICATION · PPT file · Web view · 2011-12-15Transcription in...
Transcription in prokaryotes
In prokaryotes, only one RNA polymerase is responsible for the synthesis of all RNA (except for the RNA primers required for replication)
Compare the mechanism of action of RNA polymerase and DNA polymerase.
[1][1]
RNA POLYMERASE
’
Core Enzyme2, ’ and
Holoenzyme2, ’ and and
Similarities• Substrate for both - nucleoside triphosphates• Pyrophosphate released for every NTP added• Always in the 5’3’ directionDifferences• Recognizes the site for initiation of
transcription• Ribose sugar and uracil• No primer required• No proofreading activity
[2][2]For genes to be transcribed by RNA
polymerase the appropriate initiation site must be recognized.
I.How does the RNA polymerase know where to start transcription?II.How does the sequence of the
Pribnow box relate to its function?
PROMOTERS
+1
TATAAT box
TTGACA
Transcription start site
~7 bases~19 bases
5’
3’
3’
5’
Pribnow box-35 sequence
Each of the two DNA strands can serve as templates for transcription
I. What determines which of the two DNA strands will be transcribed??
II. Can both the DNA strands be transcribed??
III. Which is the sense (coding) and anti- sense (noncoding) strand ??
IV. The RNA has the same sequence as which one of the two DNA strands?
[3][3]
What determines which of the two DNA strands will be transcribed??the location of the promoter
Can both the DNA strands be transcribed??Yes, but for a given gene only one strand will be the template
Which is the sense (coding) and anti- sense (noncoding) strand ??Sense strand = the same sequence as the mRNA, contains the promoterantisense = acts as the template, complementary sequence to the mRNA.
[3][3]
Transcription
[4][4]Three separate stages of transcription
INITIATION, ELONGATION, TERMINATION
TERMINATIONindependent
TERMINATION - independent
TERMINATION - independent
Action Of Antibiotics on Transcription
• Rifampin - binds to subunit of prokaryotic RNA polymerase– tuberculosis
• Dactinomycin – binds to DNA, prevents the movement of RNA polymerase– Tumor chemotherapy
TRANSCRIPTION -TRANSCRIPTION -EUKARYOTESEUKARYOTES
RNA Polymerases in Eukaryotes
RNA POLYMERASE I - rRNA
RNA POLYMERASE II - mRNA, snRNAs
RNA POLYMERASE III - tRNA and other small RNAs
Eukaryotic gene Eukaryotic gene structurestructure
Exon 1 Exon 4Exon 3Exon 2 Exon 5
PROTEIN
5’ 3’
= intron (untranslated)
= exon (translated)
Pre-mRNA
Exon 1 Exon 4Exon 3Exon 2 Exon 5mRNA
RNA Polymerase II• 10-12 subunits• The largest subunit contains a C-
terminal region or domain (CTD) with 52 repeats of the amino acid sequence PTSPSYS
T = threonineS = serineY = tyrosine
Can be phosphorylated
RNA PLOYMERSE II PROMOTERS
+1-20-30
TATA box
CAAT box
Transcription start site
25 bases45-55 bases
-70
5’
3’
3’
5’
Transcription factorsTranscription factors• Any protein that regulates transcription but is not
a part of RNA polymerase, is a transcription factor• Six general transcription factors (GTFs) required
for all promoters
– TFIIA– TFIIB– TFIID– TFIIE– TFIIF– TFIIH
+ RNA Polymerase II = Preinitiation complex
TATA box
TFIID BA
Pol IIF
E H
Complex of several proteins, TATA-Binding Protein (TBP) and ~12 TBP Associated Factors (TAFs)
TFIIDA TFIIA stabilizes the DNA and TFIID interactionB TFIIB binds to TFIID and acts as bridge between Pol II and TFIID
F TFIIF binds Pol II and targets its to the promoterPREINITIATION COMPLEX (PIC)PREINITIATION COMPLEX (PIC)E Recruits TFIIHH Promoter melting, kinase activity
TFIID BA
E HPol IIF
TFIID BA
E HPol IIF
mRNA
GTFs only produce a basal level of transcription
Enhancers/Silencers• Upstream or downstream• Close to the promoter or thousands of base
pairs away• On either of the two strands of DNA• Act through intermediary or gene specific
transcription factors proteins• Enhancers activate transcription • Silencers deactivate transcription
Enhancers/Silencers
Response Elements•Enhancers contain response elements that
are responsive to certain metabolic factors
–Cyclic AMP response element (CRE)–Glucocorticoid response element (GRE)–Heat shock element (HSE)
•REs bind transcription factors produced under certain cell conditions to activate several related genes
-amanitin and RNA Pol II
Post Transcriptional Post Transcriptional Modification – mRNAsModification – mRNAs
5’ Capping
Post Transcriptional Post Transcriptional Modification – mRNAsModification – mRNAs
Poly-A tail
Post Transcriptional Post Transcriptional Modification – mRNAsModification – mRNAs
SPLICING
Post Transcriptional Post Transcriptional Modification - rRNAsModification - rRNAs
RNA Polymerase I
Post Transcriptional Post Transcriptional Modification - tRNAsModification - tRNAs
THE END!