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IB HL Biology 2.7 and 7.2 Transcription (Transcription: process by which…

- - - - Promoter: non-coding sequence responsible for transcription initiation; core promoter located immediately upstream of coding sequence
        
        Binding site for RNA polymerase; mediated by array of transcription factors in eukaryotes, which bind to proximal control elements or distal control elements
        
        Transcription factors form complex w/ polymerase at promoter (polymerase cannot initiate transcription w/out these factors
        
        Regulatory proteins bind to DNA sequences outside of promoter, generally distal control elements, interact w/ transcription factors
        
        Chemical signals w/in cell can trigger changes in levels, allowing gene expression to change in response to alterations in external/internal environment
        
        Control elements: some located close to promoter, others more distance
        
        Types
        
        Activator proteins bind to enhancer sites, increase rate of transcription
        
        Repressor proteins bind to silencer sequences, decrease rate of transcription
      - Coding sequence: causes DNA strands to unwind/separate after polymerase bound to promoter
        
        Coding sequence: region of DNA transcribed
      - Terminator: polymerase will continue to transcribe until it reaches terminator sequence (differs b/t prokaryotes and eukaryotes)
    - - Anti sense/template strand: transcribed, complementary to RNA sequence; "DNA version" of tRNA anticodon sequence
      - Sense/coding strand: not transcribed, identical to RNA sequence (T instead of U), "DNA version" of RNA sequence except for T/U
- - - - 2) Elongation occurs as RNA polymerase moves along coding sequence, synthesizing RNA in 5' to 3' direction
        
        Termination: polymerase reaches terminator, both enzyme and RNA strand detach, DNA rewinds
    - - 1) Capping: addition of methyl group to 5' end, provides protection against degradation by exonucleases, allows transcript to be recognized by translational machinery (ribosomes)
        
        2) Polyadenlyation: addition of long chain of adenine nucleotides to 3' end, improving RNA transcript stability, facilitates export from nucleus
        
        3) Splicing: introns (non-coding sequences in eukaryotic genes) must be removed; coding regions called exons, fused w/ introns when latter removed to form continuous sequence
        
        Introns are intruding sequences; exons are expressing sequences
        
        Can result in exon removal (alternative splicing); selective removal will result in formation of different polypeptides from same gene sequence
- - - - Direct methylation of DNA (vs. to histone tails): increasing will decrease gene expression, preventing binding of transcription factors (non-transcribed genes exhibit more DNA methylation)
        
        Factors
        
        Fertilisation (parent of origin)
        
        Pregnancy (maternal diet)
        
        Infancy (exposure to microbes)
        
        Young Adult (Environment, diet, lifestyle)
        
        Senior (age-related)
        
        Epigenetics: study of changes in phenotype as result of variations in gene expression levels, shows patterns may change over time
        
        Influenced by heritability, not always pre-determined