Transcription

… from DNA to RNA

The Central Dogma of Molecular Biology

DNA RNA Proteinreplication

transcription 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.Initiation2.Elongation3.Termination4.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

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

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

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

Cap and Tail

S

G

PS

G

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U

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P

5´ 3´

SG

PSG

PS

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P S

A

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P S

A

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A

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PSA

P

cap poly A tail

mRNA transcrip

t

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