Ch3 Transcription

• Gene


  The functional unit of heredity, present on chromosomes, a DNA sequence that encodes a gene product that can be either a protein or non-coding RNAs
• Central Dogma of Molecular Biology


  The information stored in DNA is transferred to RNA molecules during transcription then to proteins during translation
• RNA


  • Chemically different from DNA in 3 ways: usually single stranded, contains Uracil instead of Thymin, contains ribose instead of deoxyribose as the pentose sugar
• mRNA


  • Composed of several regions: 5'UTR (not translated, plays role in regulation and translation initiation), Coding region (codes for protein), 3'UTR (not translated, plays role in regulation of gene expression)
• Monocistronic


  (Eukaryotic mRNA) Encoding a single protein
• Polycistronic


  (Prokaryotic mRNA) Encoding for multiple proteins
• Transcription in prokaryotes


  Initiated by RNA polymerase holoenzyme (core enzyme + sigma factor), occurs in 3 stages: Initiation, Elongation, Termination
• Sigma factor


  • Ensures specific binding at the promoter of the specific gene and allows for efficient initiation of transcription, different sigma factors regulate different genes
• Transcription initiation in prokaryotes

  Scanning for the promoter, binding to the promoter to form the closed complex, unwinding a portion of the double helix to form the open complex
• Promoter


  A region in the gene where RNA polymerase will bind, contains 2 primary sequences: 5'-TTGACA-3' at position -35, 5'-TATAAT-3' at position -10 (Pribnow box)
• Transcription elongation in prokaryotes

  RNA polymerase moves in the 5' to 3' direction synthesizing RNA in a continuous manner
• Transcription termination in prokaryotes

  Rho-dependent, Rho-independent (hairpin structure in RNA causes separation of DNA and RNA)
• Transcription in eukaryotes


  • Initiated by an enzyme called RNA polymerase (3 types: I, II, III), occurs in 3 stages: Initiation, Elongation, Termination
• Pre-initiation complex


  Composed of an RNA polymerase bound to the promoter region along with many proteins called transcription factors
• Promoter class II


  • Two types: Promoters containing a TATA box, TATA-box less promoters
• Transcription initiation in eukaryotes

  Many transcription factors play a role
• Transcription elongation and termination in eukaryotes

  Enzyme moves synthesizing mRNA in a continuous manner, coupled to RNA processing, termination occurs in a reaction coupled to RNA-3'end processing
• RNA processing


  Involves 3 steps: 5' capping, Poly A tail addition, Splicing
• 5' Capping


  Removal of one phosphate from the 5'end, addition of a GTP molecule, addition of a methyl group to the GMP molecule
• 5' Cap


  Protects the 5' end from degradation, helps to position the mRNA correctly on ribosomes during translation
• Poly A tail addition


  Polyadenylation: pre-mRNA cleavage, addition of the Poly A tail at the cleavage site, requires CPSF, CstF, CF I and CF II, PAP enzyme, PAB II
• RNA splicing


  Removal of non-coding introns, joining of exons, accomplished by 5 principle snRNPs (small nuclear Ribonucleoproteins)
• Splice site consensus sequences

  5'GU (start of intron), 3'AG (end of intron), internal A nucleotide inside the intron sequence
• Spliceosome assembly


  U2 binds to branchpoint, U4-U5-U6 complex binds to 5' splice site, U1 is displaced, U6 binds to U2 at branchpoint, U5 binds exons sequences, intron is removed