Transcription: From DNA to RNA

Chapter 8.2Chapter 8.2

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

Transcription:Making mRNA from DNA

Four phases1. Initiation2. Elongation3. Termination4. Processing

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

2) Elongation

• One strand of the unzipped DNA acts as a template for RNA synthesis

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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

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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PmRNA

4) Processing

• Occurs only in eukaryotic cells

• Why processing?– Remove introns– Protects from degradation in the cytoplasm

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

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

Final mRNA Molecule

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cap poly A tail

mRNA transcrip

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