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Transcription
… from DNA to RNA
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The Central Dogma of Molecular Biology
DNA RNA Proteinreplication
transcription translation
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Why RNA?
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Why RNA?• Not all genes need to be turned on at
once. –We can make an RNA transcript of just
ONE GENE– Now we can make the right protein at
the right time in the right location
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Why RNA?
In EUKARYOTES…
• DNA cannot leave the nucleus• BUT proteins are built by the
ribosomes in the cytosol!• We need a messenger to transfer the
genetic code to the ribosomes
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Why RNA?
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mRNA• Messenger RNA (mRNA) is a complementary copy of a gene that CAN leave the nucleus
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Gaining Access to DNA
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4 Phases of Transcription
1.Initiation2.Elongation3.Termination4.Processing
(Eukaryotes Only)
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Initiation• RNA polymerase (RNAP) binds to the
double stranded DNA molecule at a promoter sequence (with the help of initiation factors)
• It is able to locally unzip DNA with its own built in helicase activity as it constructs an RNA transcript of the DNA
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RNA Polymerase II
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Enhancers
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Promoters• DNA sequence
upstream of the gene being transcribed
• Determines where RNAP binds and where transcription begins
• Usually rich in Thymine and Adenine (“TATA” box)
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Elongation• One strand of the unzipped DNA acts
as a template for RNA synthesis
SG
P SA
P ST
P SC
P SG
P SG
P SA
P SC
P SC
P3´ 5´
SG
PSG
PST
PSC
PSC
PSG
PSA
PST
PSC
P5´ 3´
Template Strand
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Elongation
SU
P
SG
PSG
PST
PSC
PSC
PSG
PSA
PST
PSC
P5´ 3´
Template StrandSC
PSG
P SA
P ST
P SC
P SG
P SG
P SA
P SC
P SC
P3´ 5´
SA
PSG
PSC
PSC
PSU
PSG
PSG
P
5´ 3´
Coding Strand
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More Detail: Elongation
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Elongation• mRNA is transcribed in the 5' to 3' direction
• DNA unwinds only in the region of transcription
• After transcription DNA recoils
• Several RNAPs can work on a single gene at once
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Lots of copies for lots of ribosomes
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Electron Micrograph
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Termination• A terminator sequence on the coding
strand tells RNAP when to stop transcribing the mRNA
• RNAP is released and reused and mRNA is released
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Processing• In Eukaryotic cells the RNA transcript
is called pre-mRNA (or primary RNA) because it must still be modified before it leaves the nucleus
• Why processing?– Remove introns– Protects from degradation in the
cytoplasm
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Introns and Exons
Genes contain both coding regions (exons) and non-coding regions (introns)
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Introns and Exons• To produce a final mRNA transcript,
introns must be removed
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Splicing
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Splicing• Small nuclear RNA (snRNA) in complex with
proteins are called small nuclear ribonucleic particles (snRNPs)
• These assemble with other proteins to form the Spliceosome
• snRNA binds to specific mRNA sequences at the beginning and end of an intron forming a loop
• The loop is removed and the remaining exons are linked
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Cap and Tail
S
G
PS
G
PS
U
PS
C
PS
C
PS
G
PS
A
PS
U
PS
C
P
5´ 3´
SG
PSG
PS
G
PS
G
P S
A
PS
A
PSA
PSA
P S
A
PS
A
PSA
PSA
P
cap poly A tail
mRNA transcrip
t
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Cap and Tail• To protect RNA from restriction
endonucleases in the cytosol, a poly-A tail is added to the 3‘ end of the pre-mRNA
• As an attachment site for the ribosome, a 5‘ cap of modified Gs is added
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