Transcription … from DNA to RNA

The Central Dogma of Molecular Biology DNA RNA Protein transcription translation replication

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

mRNA Messenger RNA (mRNA) is a complementary copy of a gene that CAN leave the nucleus

Gaining Access to DNA

4 Phases of Transcription Initiation Elongation Termination 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 S G P T C A 5´ 3´ S G P A T C 3´ 5´ Template Strand

Elongation S C P S G P S C P S U P S G P S G P S A P S U P S C P 3´ 5´ Coding Strand 5´ 3´ S G P A T C 3´ 5´ Template Strand

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

One more look at Elongation

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 The Spliceosome is a complex of multiple proteins (snRNPs) and small nuclear RNA (snRNA) snRNA binds a specific mRNA sequences at the beginning and end of an intron forming a loop The loop is removed and exons are linked

Cap and Tail cap mRNA transcript poly A tail 5´ 3´ S G P S G P U C A S

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