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