9. Transcription Chapters 26 Lehninger 5 th Edition .

9. Transcription

Chapters 26 Lehninger 5th Edition

http://www.steve.gb.com/science/transcription.html

•Transcription•DNA -> RNA

•Translation•RNA -> Protein

Genes

• 1 gene -> 1 protein?

• Transcription– Read a gene from the DNA– DNA -> RNA

• Translation– Translate the DNA code to protein code– RNA -> Protein

Transcription

• RNA Polymerase– Reads DNA

• Makes RNA copy of– One strand

– One gene

Transcription

• Open DNA at promoter• Make RNA

– 5’-> 3’

– transcription bubble • Moves along gene

– Prevent DNA knotting• DNA topo-isomerases

Transcription

• Initiation– At promoters– Control by Transcription Factors– “Gene expression”

• Termination

• Processing of the transcript (Eukaryotes)– e.g. remove introns, add stuff to ends, modify bases

• Final product – e.g. mature mRNA or mature miRNA

Bacterial Promoters• Simple• Fixed spacing of binding sites• Transcription factors

Transcription Factors Bind DNA

Bacterial Gene Structure

• Simple promoter close to start of transcript• Gene transcribed as one piece of mRNA• Transcription terminates

– Terminator signal

• Ready to translate

m

Bacterial Operons• Genes may be grouped in Operons• Co-regulation• Single transcript

Eukaryotic Gene Structure

• V. complicated promoters– e.g. Human:

• 2000 TFs

• Spread over 10s of kb

• Several kb from coding region

• Pieces of mRNA removed before translation– Introns

• Rich Roberts, Phil Sharp

Eukaryotic gene processing

• Remove introns– Splicing

• Add 5’ cap• Cleavage of 3’ end of mRNA

at polyA site• Add 3’ polyA tail

Alternative transcripts

• Alternative polyA sites

• Alternative splicing– Alternative exons

• Alternative promoters

Alternative polyA

Alternative splicing

RNA Genome?

• Central dogma:– DNA -> RNA -> protein

• RNA Virus– e.g. HIV:

• RNA -> DNA

• Reverse transcriptase

What Controls Gene Expression?

• Transcription factors• Regulatory RNAs

– miRNA– smRNA– siRNA

• Methylation• Chromatin

Transcription Factors

• Proteins which bind DNA

• Enhance or repress gene expression– Families e.g.:

• Homeodomain (Hox)

• POU domain (Oct-1)

• Helix-loop-Helix (c-Myc)

• Zinc Fingers (TFIIIA)

• Leucine Zipper (c/EBP)

• Winged Helix (Fox family)

• Helix Turn Helix

• At least 10% of genes in Human genome are TFs