Transcription from DNA to RNA
The Central Dogma of Molecular Biology replication DNA RNA Protein transcription translation
Why call it transcription and translation? transcription is such a direct copy really, of letter for letter translation is between two different languages Watson: all information flows from nucleotides to proteins
Why RNA? Early Earth developed conditions in which complex molecules covered portions of the planet in a soup this is called the primordial soup part of that soup were free floating RNA nucleotides. these formed chains this was the precursor to life
Why does DNA need to be transcribed? all the machinery for making proteins evolved during RNA World DNA was invented only later! the machinery still prefers to work with RNA instead of DNA
Why does DNA need to be transcribed? DNA is a better information storing molecule, it is more stable, it protects the nucleotides better but RNA is bendier, performs better in reactions, better at doing stuff. and as mentioned previously, the machinery involved in the back half of the Central Dogma evolved when there was just RNA, so they prefer working with it.
DNA can t leave nucleus, Why must RNA be able to leave? this question must be more than just a matter of who can leave the nucleus. something has to because the ribosomes that synthesize proteins are out in the cytosol. for reasons mentioned in previous slides, life chose to store info in reliable and safe DNA, but sacrifices some fidelity for reactivity, workability, and familiarity when it is time to make proteins
Why RNA?
mrna Messenger RNA (mrna) is a complementary copy of a gene that DOES leave the nucleus it is the messenger for transferring the genetic code to the ribosome.
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
http://www.chemgapedia.de/vsengine/ supplement/vlu/vsc/de/ch/5/bc/vlus/ gen_protein.vlu/page/vsc/de/ch/5/bc/ gen_protein/initiation_eu.vscml/ Fragment/ 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 5 3 P S P S P S P S P S P S P S P S P S C T A G 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 C C T G G
P S P S P S P S P S P S P S P S P S G U A C P S C T A Elongation 5 3 G C C Coding Strand 5 3 T G G 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
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 premrna (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 5 3 A A A A A A A A G G U C C G A U C G 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 P S P S P S P S cap mrna transcript poly A tail
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