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Chapter 8: Transcription and RNA processing. Fig. 8-1. Fundamental chemical distinctions of DNA and RNA. 2’ carbon. Methyl in thymine. pp. 257-8. Fig. 8-3. General types of RNA Informational : mRNA (most diverse) Functional : tRNA rRNA (most abundant) snRNA scRNA.
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Fundamental chemical distinctions of DNA and RNA 2’ carbon Methyl in thymine pp. 257-8
General types of RNA Informational: mRNA (most diverse) Functional: tRNA rRNA (most abundant) snRNA scRNA
Transcription (DNA-dependent RNA synthesis) • Requirements: • RNA polymerase • template DNA (one strand) • ribonucleoside triphosphates (ATP, GTP, CTP, UTP) • assorted cofactors (e.g., Mg++) • Directionality: • RNA always synthesized 5'3' • template DNA is read 3'5' • RNA is complementary to its template DNA
Types of RNA polymerase • Prokaryotes: single enzyme complex • Eukaryotes: three types • Pol I transcribes rRNA genes • Pol II transcribes mRNA genes • Pol III transcribes tRNA, some snRNA genes
Genes may be transcribed from either DNA strand (may be divergently transcribed) Fig. 8-4
Transcription of rDNA genes in amphibian oocyte Fig. 8-6
Polarity and complementarity of DNA and RNA strands Fig. 8-7
Prokaryote transcription initiation Fig. 8-9
Post-initiation transcription steps Fig. 8-10
Example of a prokaryotic transcription terminator (a hairpin loop structure) Fig. 8-11
Eukaryotic transcription initiation requires the ordered assembly of a preinitiation complex (general transcription factors) (includes specific transcription factors) Fig. 8-12
Pre-mRNA processing in eukaryotes 5’ cap addition (7-methylguanosine addition by guanyltransferase) Splicing to remove introns
Pre-mRNA splicing in eukaryotes Spliceosome:a ribonucleoprotein complex consisting of snRNAs + proteins + pre-mRNA Highly conserved RNA sequences at the intron/exon boundaries Recruit snRNAs and proteins that assemble into the spliceosome complex Highly ordered, sequential series of reactions cleave away intron and join together the flanking exons
Common elements of an intron that direct proper splicing Fig. 8-15
Differential splicing creates diverse mRNAs from a single gene Fig. 8-14
Pre-mRNA processing in eukaryotes • 5’ cap addition (7-methylguanosine addition by • guanyltransferase) • Splicing to remove introns • 3’ polyadenylation • - Endonuclease cleavage of pre-mRNA • ~20 nucleotides 3’ to AAUAAA • - Addition of up to several hundreds of A’s • to 3’ end by poly(A) polymerase