Gene expression DNA RNA Protein
DNA
DNA
RNA
Protein
Replication
Transcription
Translation
Degradation
Degradation
InitiationElongationProcessingExport
InitiationElongationProcessingTargeting
Chapter 6
• Transcription• Sections
• From DNA to RNA– The start of the chapter to the end of “Mature eucaryotic
mRNAs are selectively exported from the nucleus”
RNA structure
RNA is single stranded polymer of C, G, A, U
Can have secondary structure but typically not a double helix (Fig. 6-6)
RNA structure
many types of RNA made:
•mRNA: encodes proteins,
•snRNA: RNA processing
•snoRNA: rRNA processing
•tRNA: translation
•rRNA: translation
Transcription initiation
DNA acts as a template for RNA synthesis (Fig 6-9)
Transcription initiation
DNA acts as a template for RNA synthesisSynthesis by RNA polymerase -RNA Pol
RNA Pol I makes tRNA and rRNA
RNA Poly II makes mRNA in nucleus
Control of transcription
Transcription initiation by RNA Pol II requires general transcription factors (Fig 6-16)
Control of transcription(Fig. 6-16)
Transcription start site usually a TATA box (not always)
TBP (TATA-binding protein) binds, changing DNA structure (Fig 6-18).
Recruits transcription factor II proteins (TFIIA, B, …) then RNA Pol II
Collectively the transcription initiation complex
Control of transcription
Since DNA is wrapped around histones, how does RNA Pol gain access to the promoter?
How does RNA Pol know where to bind?
Control of transcription(Fig. 6-19)
Transcription initiation also requires: •activators•mediators (or co-activators), •chromatin-remodeling proteins
Activators increase the likelihood of successful transcription initiation
Mediators allow activators to communicate with RNA Pol II
Transcription factors
• DNA-binding proteins associate with specific regions on DNA (elements)
• Elements may be as small as 6 nucleotides
• Subtle differences in DNA 3 dimensional structure alter the ability of proteins to bind
RNA processing
Newly synthesized transcripts (mRNA) are processed (Fig. 6-21):
•Splice out intervening sequences (=introns) leaving expressed sequences (exons)
•“Cap” 5’ end of RNA
•Poly-adenylate 3’ end (Poly A+ tail)
RNA processing
Introns are removed in spliceosomes ( a complex of proteins and snRNA)
cut and paste RNA at specific sites (Fig 6-26, 6-29)
Requires ATP
Transcript processing
3’ end is also processedcut downstream from poly-adenylation site (AU-rich region)
Poly A polymerase adds 100’s of ATPs
Length of poly A+ tail influences half-life (degradation rate)
Transcript export
Proteins associated with mRNA mark it for export
Only mature mRNA is exported from nucleus
Exit via nuclear pore complexes
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