Molecular Genetics: Transcription

Transcription 

initiation →elongation→termination→premRNA→RNA processing→mRNA→transcription

making an mRNA (sometimes rRNA or tRNA as well) copy of a message on a DNA template strand (main enzyme is RNA polymerase)

  • RNA polymerase: pries strands of RNA apart and joins together RNA nucleotides complementary to DNA template strand → elongating RNA polynucleotide 

    • only assembled in 5’→  3’ direction
    • can only start chain from scratch

  • Initiation

    • Bacteria: 
    • promoter: DNA sequence where RNA polymerase attaches and initiates transcription
    • transcription unit: stretch of DNA downstream from promoter that is transcribed into RNA
      • direction of transcription: “downstream”
    • terminator sequence: sequence in bacteria that signals end of transcription
    • Eukaryotes:
    • primary transcript:  rNA transcript immediately after transcription in the nucleus, before rNA splicing or polyadenylation to form the mature mRNA.
    • uses Promoter 
    • transcription factors: mediate binding of RNA polymerase and initiation of transcription in eukaryotes
      • interaction of promoter, proteins, and RNA polymerase tells whether transcription will occur
    • TATA box: DNA sequence in eukaryotic promoter crucial in forming complex
    • Transcription initiation complex: complex of transcription factors and RNA polymerase II bound to promoter

  • Elongation

    • single gene can be transcribed simultaneously by several RNA polymerase

  • Termination 

    • in bacteria
    • transcription goes until terminator sequence
    • in eukaryotes
    • RNA polymerase II transcribes DNA sequence called the polyadenylation signal sequence (specifying signal in mRNA)
      • about 10-35 nucleotides downstream from the signal, a protein cuts RNA transcript free from polymerase → releasing pre-mRNA

  • RNA processing: modification of RNA primary transcripts, including splicing out of introns, joining together exons, and alternation of 5’ and 3’ ends

    • 5’ cap: modified form of guanine added to 5’ end after 20-40 nucleotides have been transcribed 
    • poly-A tail: 50-250 adenine nucleotides added to the 3’ end by an enzyme 
    • RNA splicing: portions of RNA molecule are removed and the remaining are reconnected -- editing of eukaryotic transcripts before leaving the nucleus -- only in eukaryotes 
    • introns: noncoding segments of nucleic acids 
    • exons: coding regions 
    • spliceosomes: large complex made up of proteins and RNA molecules
      • removes introns from a transcribed pre-mRNA, a type of primary transcript.
    • alternative RNA splicing: mRNA molecules are produced from same primary transcript, depending on which RNA segments are treated as exons or introns 
      • accounts for the fact that so many different proteins can be made by a smaller number of genes

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