Download - Transcription: From DNA to RNA
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Transcription: From DNA to RNA
Chapter 8.2Chapter 8.2
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Why RNA?
• 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
• mRNA is a complementary copy of a gene that CAN leave the nucleus
Messenger RNA
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Transcription:Making mRNA from DNA
Four phases1. Initiation2. Elongation3. Termination4. Processing
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1) Initiation• RNA polymerase (RNAP) binds to the double
stranded DNA molecule at a promoter sequence• Promoters
– always upstream of the gene being transcribed
– Determines where RNAP binds and where transcription begins
– Usually rich in Thymine and Adenine (“TATA” box)
• RNAP has its own built-in helicase activity
TATATTRNAP
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2) Elongation
• One strand of the unzipped DNA acts as a template for RNA synthesis
S
G
P S
A
P S
T
P S
C
P S
G
P S
G
P S
A
P S
C
P S
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P
3´ 5´
S
G
PS
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PS
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PS
C
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G
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P5´ 3´
Template Strand
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S
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G
PS
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P5´ 3´2) Elongation
Template StrandS
C
PS
G
P S
A
P S
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P S
C
P S
G
P S
G
P S
A
P S
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P S
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3´ 5´S
A
PS
G
PS
C
PS
C
PS
U
PS
G
PS
G
P
5´ 3´
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2) 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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3) Termination
• A terminator sequence on the non-transcribed DNA strand tells RNAP when to stop transcribing the mRNA
• RNAP is released and reused
• mRNA released
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S
G
PS
G
PS
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PS
C
PS
C
PS
G
PS
A
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C
P5´ 3´3) Termination
Template Strand
S
G
P S
A
P S
T
P S
C
P S
G
P S
G
P S
A
P S
C
P S
C
P
3´ 5´
S
U
P S
C
P S
A
P S
G
P S
C
P S
C
P S
U
P S
G
P S
G
PmRNA
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4) Processing
• Occurs only in eukaryotic cells
• Why processing?– Remove introns– Protects from degradation in the cytoplasm
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4) Processing
• Splicing– mRNA still has both
introns and exons– We need to remove
introns
– Spliceosome - a complex of multiple proteins and snRNA (small nuclear)
– snRNA binds a specific mRNA sequences at the beginning and end of an intron forming a loop
– Loop is removed by spliceosome
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4) Processing
• Protecting the mRNA– From what?
• 5’ cap– Modified G added to 5’ end
of mRNA
• Poly-A tail– Many A added to 3’ end of mRNA
restriction enzymes in cytosol
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Final mRNA Molecule
S
G
PS
G
PS
U
PS
C
PS
C
PS
G
PS
A
PS
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C
P
5´ 3´
S
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PS
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PS
G
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P S
A
PS
A
PS
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P S
A
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PS
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PS
A
P
cap poly A tail
mRNA transcrip
t
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video
http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter15/animations.html
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