Transcription - PowerPoint PPT Presentation

1 / 44
About This Presentation
Title:

Transcription

Description:

Four stages: Initiation, Elongation, Termination, Post-transcriptional modification ... Dissociation is somehow coupled to ATP hydrolysis. Comparison with Eukaryotes ... – PowerPoint PPT presentation

Number of Views:894
Avg rating:3.0/5.0
Slides: 45
Provided by: thomas2
Category:

less

Transcript and Presenter's Notes

Title: Transcription


1
Transcription
2
Transcription
  • Transcription- the synthesis of RNA using DNA as
    a template.
  • Four stages Initiation, Elongation,
    Termination, Post-transcriptional modification

3
Similarities and Differences from Replication
  • Similarities
  • 5' ? 3' direction
  • many proteins involved
  • initiation, elongation, termination
  • transcription bubble
  • starts and stops at specific places
  • Nt not dNt
  • Differences
  • RNA polymerase instead of DNA polymerase
  • no proofreading
  • posttranscriptional modification
  • 1 strand copied not 2
  • not all copied

4
Terminology of Transcriptionsee Fig. 26-5
Lehninger POB 4th Ed.
5
Initiation
6
Transcription Start Site is Promotersee Fig.
26-4 Lehninger POB 4th Ed. (UP)
7
RNA Polymerase DNA-Dependent RNA PolymeraseFig.
26-4 Lehninger POB 3rd Ed.
  • zinc metalloenzyme
  • ??ßß? core enzyme (390 kDa)
  • with ? subunit, holoenzyme (?70 most common)
  • other subunits
  • the ? subunit causes specific binding to promoter
  • also dependent on NTPs (and Mg2 as usual)
  • no requirement for primer
  • No proofreading mistake every 104-105 bases

8
Elongation
9
Termination
  • Rho (r)-dependent vs. r-independent

10
r-Independent Termination
  • 5-(N)n CCCAGCCCGCCUAAUGAGCGGGCUUU

11
r-Dependent Termination
  • A hairpin forms
  • if protein called r is present, polymerase
    detaches
  • Dissociation is somehow coupled to ATP hydrolysis

12
Comparison with Eukaryotes
  • Prokaryotes Eukaryotes
  • promoter promoter enhancers
  • Polymerase Polymerases I, II and III
  • Rho not Rho
  • not as much processing Processing
  • 50-90 nt/sec Slower

13
Eukaryote Upstream
14
Animal RNA Polymerases
15
Primary Transcript
  • Primary Transcript- the initial molecule of RNA
    produced. AKA hnRNA
  • hnRNA-heterogenous nuclear RNA
  • In prokaryotes, DNA ? RNA ? protein in cytoplasm
    concurrently
  • In eukaryotes nuclear RNA gtgt Cp RNA

16
Post-transcriptional Processing of mRNA Occurs in
Nucleus
  • Cap
  • functions transport, recognition, prevents
    exonucleases
  • poly A tail
  • functions prevents exonucleases, recognition,
    other?
  • Less in prokaryotes
  • Introns spliced out- eukaryotes only

17
Post-transcriptional Processing of mRNA -
Cappingsee Fig. 26-12 Lehninger POB 4th Ed.
18
Post-transcriptional Processing of mRNA -
Methylating the Capsee Fig. 26-12 Lehninger POB
4th Ed.
19
Methylation of the 2-OH of First (and Second)
Base Can Also Occursee Fig. 26-12 Lehninger POB
4th Ed.
20
Post-transcriptional Processing of mRNA -
Polyadenylationsee Fig. 26-18 Lehninger POB 4th
Ed.
  • Prokaryotes n20-50
  • Eukaryotes n80-250
  • of prokaryotic mRNAs have polyA tail
  • 100 of eukaryotic mRNAs have polyA tail

21
Post-transcriptional Processing of mRNA -
Splicing out Introns
  • Complicated
  • snurps - small nuclear ribonucleoproteins
  • snurps 1-6 bind at specific times
  • RNA catalysts
  • lariat structure

22
Intron Groups and Characteristics
23
Group II Intron Consensus Sequence
intron
24
Group II Intron Splicing
  • U1 base-pairs first by 5 end of intron

25
Group II Intron Splicing
  • U1 base-pairs first by 5 end of intron

26
Group II Intron Splicing
  • U1 base-pairs first by 5 end of intron
  • U2 binds branch site and directs U1 binding there

27
Group II Intron Splicing
  • U1 base-pairs first by 5 end of intron
  • U2 binds branch site and directs U1 binding there
  • U5 and U4/U6 then bind
  • U4 released
  • U2/U6 causes cleavage?

28
Group II Intron Splicing
  • U1 base-pairs first by 5 end of intron
  • U2 binds branch site and directs U1 binding there
  • U5 and U4/U6 then bind
  • U4 released
  • U2/U6 causes cleavage?

29
Typical Structure
  • Typical exon size lt 1000 nt
  • Many 100-200 nt
  • So 30-60 amino acids long
  • typical intron highly variable
  • 50-20,000 nt

30
Processing of rRNAsee Fig. 26-21 Lehninger POB
4th Ed.
  • RNA synthesized longer than needed (polytenes)
  • structural portions methylated
  • non-methylated parts degraded
  • RNA associates with protein
  • final shape globular

31
Processing of tRNA
  • Synthesized longer than needed

32
Processing of tRNAFig. 26-23 Lehninger POB 4th
Ed.
  • Synthesized longer than neededtrimmed

33
Processing of tRNAFig. 26-23 Lehninger POB 4th
Ed.
  • Synthesized longer than neededtrimmedspliced

34
Processing of tRNAFig. 26-23 Lehninger POB 4th
Ed.
  • Synthesized longer than neededtrimmedsplicedCCA
    added

35
Processing of tRNAFig. 26-23 Lehninger POB 4th
Ed.
  • Synthesized longer than neededtrimmedsplicedCCA
    addedand bases modified.

36
Base ModificationFig. 26-24 Lehninger POB 4th Ed.
37
Final Structure Cloverleaf Fig. 27-13
Lehninger POB 4th Ed.
38
Advanced Topics
  • Regulation of mRNA half-life
  • Alternative splicing

39
mRNA Half-life
  • t½ seconds if seldom needed
  • t½ several cell generations (i.e. 48-72 h) for
    houskeeping gene
  • avg 3 h in eukaryotes
  • avg 1.5 min in bacteria
  • PEPCK
  • Insulin 30 min
  • -Insulin 3 h

40
mRNA degrades by RNase
  • Exonuclease
  • Major p/w
  • Uses 53
  • Deadenylate
  • Decap
  • Degrade
  • 3 sequence inhibits (PEPCK)

41
Alternative SplicingFig. 26-20 Lehninger POB 4th
Ed.
42
VDJC Sequence Light ChainFig. 5-1 Stites, Stobo,
and Wells Basic and Clinical Immunology 6th Ed.
43
Eukaryote Upstream
  • Promoter tells where to start
  • Boxes tell how often to start
  • TATA box has many proteins associated TBP (TATA
    binding protein), TFII_, TAFs (TBP associated
    factors)

44
Boxes and FactorsHarpers Review of Biochemistry
  • Cis- vs. Trans-acting factors

45
Enhancers Work Upstream, Downstream or in the
Middle of a Gene
  • They also work forwards or backwards
  • Possible ways of working
  • Different transcription factors
  • Order of binding (differing concentrations)
  • Affinity of transcription factors
Write a Comment
User Comments (0)
About PowerShow.com