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BSC 219
10/11/12
Transcription
13.1 RNA Consisting of a Single Strand of Ribonucleotides Participates in a Variety
of Cellular Functions
Primary structure
Secondary structure
Classes of RNA
Ribosomal RNA – rRNA-part of ribosome
Messenger RNA – mRNA-template of protein to be synthesized
Transfer RNA – tRNA-transports amino acids to ribosome for translation
Small nuclear RNAs – snRNAs
Small nuclear ribonucleoproteins – snRNPs
Small nuclear RNAs – snoRNAs
Classes of RNA
Small cytoplasmic RNAs – scRNAs
MicroRNAs – miRNAs
Small interfering RNAs – siRNAs
Piwi-interacting RNAs – PiRNAs
13.2 Transcription Is the Synthesis of an RNA Molecule from a DNA Template
The Template
The transcribed strand: template strand
Transcription will produce an RNA molecule that resembles the opposite strand or
the nontemplate strand
RNA polymerase moves along template strand in 3’-5’ direction and produces new
RNA in 5’-3’ much as in DNA replication.
The Template
The transcription unit
Promoter-initiates transcription
RNA coding sequence-contains sequence that will be reflected in RNA molecule
Terminator-halts transcription and releases RNA molecule
Initiation
The substrate for transcription:
Ribonucleoside triphosphates – rNTPs added to the 3′ end of the RNA molecule
rGTP, rCTP, rATP, and rUTP
Initiation
The transcription apparatus:
Bacterial RNA polymerase: five subunits made up of the core enzyme:
Two copies of α
Single copy of β
Single copy of β′
A stabilize enzyme: ω
The sigma  factor: binding to the promoter when transcription starts
Initiation
The substrate for transcription:
Ribonucleoside triphosphates – rNTPs added to the 3′ end of the RNA molecule
The transcription apparatus:
Eukaryotic RNA polymerases
Initiation
Bacterial promoters:
Consensus sequences: sequences that possess considerable similarity
−10 consensus: 10 bp upstream of the start site
Pribnow box:
5′ TATAAT 3′
3′ ATATTA 5′
−35 consensus sequence: TTGACA
Concept Check 2
The holoenzyme (core enzyme + sigma factor)
The sigma factor alone
The core enzyme alone
mRNA
Concept Check 2
The holoenzyme (core enzyme + sigma factor)
The sigma factor alone
The core enzyme alone
mRNA
Initiation
Initial RNA synthesis: No primer is required.
The location of the consensus sequence determines the position of the start site.
Elongation
RNA elongation is carried out by the action of RNA polymerase.
Termination
Rho-independent termination: hairpin structure formed by inverted repeats, followed by a
string of uracils
Rho-dependent termination: a hairpin slows down polymerase allowing a trailing protein
called rho to catch up and dislodge the polymerase from the template
13.4 The Process of Eukaryotic Transcription Is Similar to Bacterial Transcription
but Has Some Important Differences
Transcription and Nucleosome Structure – Chromatin modification before
transcription
Promoters:
Basal transcription apparatus
Transcriptional activator proteins
RNA polymerase II – mRNA synthesis
Core promoter TATA box TATAAAA, −25 to −30 bp, binded by transcription
factors
Transcription and Nucleosome Structure – Chromatin modification before
transcription
Promoters:
Regulatory promoter
A variety of different consensus sequences may be found in the regulatory
promoters.
Main difference between prokaryotes and eukaryotes is in assembly of
complex structures at promoter in eukaryotes
Transcription and Nucleosome Structure – Chromatin modification before
transcription
Enhancers: distant regions of DNA that increase transcription levels
Bound by initiation complex proteins and loop around to interact with
promoter region
Polymerase I and polymerase III promoters
Distinct from those of polymerase II
May sometimes be downstream of transcription start site
Initiation
RNA polymerase II + transcription factors
TATA binding protein
Elongation
Termination
RNA polymerase I-terminated by protein that binds DNA downstream of termination
sequence
RNA polymerase II-terminated by complex mechanism involving RNA cleavage and
Rat1 protein
RNA polymerase III-terminates after long poly-U transcript.
Concept Check 3
Only the core promoter has consensus sequences.
The regulatory promoter is farther upstream from the gene.
Transcription factors bind to the core promoter; transcriptional activator proteins
bind to the regulatory promoters.
Both b and c above
Concept Check 3
Only the core promoter has consensus sequences.
The regulatory promoter is farther upstream from the gene.
Transcription factors bind to the core promoter; transcriptional activator proteins
bind to the regulatory promoters.
Both b and c above
13.5 Transcription in Archaea Is More Similar to Transcription in Eukaryotes than
to Transcription in Eubacteria
This suggests a closer relationship between archaea and eukaryotes.