Post on 14-Dec-2015
Objectives
The differences between DNA and RNA
The structure and functions of RNAs
RNA synthesis (Transcription)
Post-transcriptional events (modifications)
RNA & DNA: Similarity
Both RNA & DNA:Unbranched polymers
Nucleoside monophosphate
Phosphodiester bonds
RNA & DNA: Differences
RNASingle-Strand (mostly)Cytoplasm (mainly)AGCU (exception)Modified basesRiboseProtein BiosynthesisPost-transcriptional events
DNADoubleNucleusd AGCT
DeoxyriboseStorage &transferDNA Repair
RNA: TypesMajor types:
Ribosomal RNA (rRNA) 80%
Transfer RNA (tRNA) 15%
Messenger RNA (mRNA) 5%
Others:Small nuclear RNA (snRNA)
The tRNASmallest RNA 4S (74 – 95)
At least 20 species
Unusual bases
Secondary structure Intra-chain base pairingAdaptor molecule Carries its sp. a.a. to site of protein biosynthesis
The mRNASize:Heterogeneous (500 – 6000)
Primary (precursor): hnRNA
Post-transcriptionalProcessing of
Euokaryotic mRNA
Carries genetic information from nucleus to cytoplasm(Template of protein synthesis)
RNA Synthesis (Transcription)
Transcription: the copying process which uses one of the two DNA strands (template strand) to form RNAOther DNA strand: coding strand Similar to RNA strand (with exception)
Transcription is highly selective: Some DNA regions form many, other form few or no transcripts
Transcription - 2
The selectivity is due to signals in the DNA nucleotide sequence
The signals convey a message toRNA polymerase to define: Where polymerase should start?
How often? (frequency)
Where to stop? (termination)
Transcription – 4Prokaryotic RNA
polymeraseOne species for all RNAs (Except primase for RNA primer)
Elongation: 5’– 3’ polymerizing activity
Initiation: Recognition of promoter sequence
Termination: ρ(rho)-independent
No exonuclease or proofread activity
Transcription:
Transcription – 6Prokaryotic Promoter
Promoter is DNA sequence recognized by (σ) Sigma factor of RNA polymerase holoenzyme where transcription should start
Consensus, highly conserved sequences-10 Pribnow box (TATAAT)-35 Sequence (TTGACA)
(as read 5’ –3’ on nontemplate ‘coding’ strand)
Mutant promoter affects transcription of its gene
Transcription – 8Transcription Unit
Transcription Unit: DNA region from promoter to termination
The product: primary transcript
Transcription – 9Initiation
Binding of RNA polymerase holoenzyme to promoterLocal unwindingSupercoils (Topoisomerases I & II)
Denovo synthesis: No primer required
Substrate: Ribonucleoside triphosphate
Usually begins with a purine
No proofreading
Once promoter is recognized, Sigma (σ) factor is released
Transcription – 10Elongation
Direction: 5’– 3’, antiparallel
Complementarity: as DNA, but for A there is U release of pyrophosphate
Relaxation of supercoils:
Topoisomerases I and II
Transcription – 11Termination
A-The rho (ρ)-dependent termination:The ρ-factor binds to C-rich near 3’-end of RNA
It has ATP-dependent DNA-RNA helicase activity
Hydrolyzes ATP (Energy-dependent)Unwind 3’-end of the transcript from template
Movement of (ρ) protein along RNA/DNA hybrid 5’-3’
Displacement of DNA template at termination site, releasing of RNA transcript
Transcription – 13Inhibitors
(Antibiotics)Rifampin (anti-tuberculous): binds to the β-subunit interferes with formation of first phosphodiester bond inhibit RNA synthesis and bacterial growth
Dactinomycin (Actinomycin D): binds to minor groove of DNA template interferes with movement of RNA polymerase inhibits prokaryotic transcription (antibiotic) inhibits euokaryotic replication (anticancer)