Transcription
… from
DNA to RNA
The Central Dogma of
Molecular Biology
transcription
replication
translation
Why RNA?
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
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
Why RNA?
mRNA
• Messenger RNA (mRNA) is a
complementary copy of a gene that
CAN leave the nucleus
Gaining Access to DNA
4 Phases of Transcription
1.Initiation
2.Elongation
3.Termination
4.Processing
(Eukaryotes Only)
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
RNA Polymerase II
Enhancers
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)
Elongation
• One strand of the unzipped DNA acts
as a template for RNA synthesis
3´
G G
T
C
C
G
A
T
C
P S P S P S P S P S P S P S P S P S
5´
G
A
T
C
G G
A
C
C
P S P S P S P S P S P S P S P S P S
3´
Template Strand
5´
G G
T
C
C
G
A
T
C
P S P S P S P S P S P S P S P S P S
U
G
A
C
P S
5´
Elongation
3´
Coding Strand
5´
3´
G
A
T
C
G G
A
C
C
P S P S P S P S P S P S P S P S P S
3´
5´
Template Strand
More Detail: Elongation
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
Lots of copies
for lots of ribosomes
Electron Micrograph
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
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
Introns and Exons
Genes contain both
coding regions
(exons) and
non-coding regions
(introns)
Introns and Exons
• To produce a final mRNA transcript,
introns must be removed
Splicing
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
A
poly A tail
A
A
A
A
A
A
A
G
G
mRNA
transcript
P S P S P S P S
U
C
C
G
A
U
C
G
cap
G
G
G
P S P S P S P S P S P S P S P S P S P S P S P S P S P S P S P S P S
3´
5´
Cap and Tail
Cap and Tail
• To protect RNA from restriction
endonucleases in the cytosol, a polyA 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