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Transcription. Chapter 17. Objectives. Understand the process of transcription Recognize the role of RNA Polymerase Recognize the significance of promoter and terminator regions of DNA

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Chapter 17


  • Understand the process of transcription

  • Recognize the role of RNA Polymerase

  • Recognize the significance of promoter and terminator regions of DNA

  • Explain how transcribed RNA is modified prior to exiting the nucleus. Understand the significance of this process


  • Consists of three stages

    • Initiation: attachment of RNA Polymerase to the promotor region on DNA

    • Elongation: building of the mRNA from the 3’ end of the nucleotide polymer

    • Termination: release of RNA polymerase and mRNA following transcription of the terminator region of the DNA


  • Genes on the DNA begin with a promoter region consisting of a sequence of A & T (TATA box) and the first nucleotide involved in the peptide sequence

  • Transcription factors (proteins that assist the binding of RNA polymerase to the promoter) are found in association with the promoter region

  • Transcription initiation complex: transcription factors & RNA polymerase bound to the promoter region of the DNA

Elongation termination

Once initiation is complete the 2 strands of the DNA unwind

RNA polymerase builds a mRNA strand complimentary to the DNA transcription unit (60 bases/sec)

Once the RNA Polymerase passes the DNA strands reform their double helix

When the RNA Polymerase transcribes the terminator region of the DNA, the Polymerase releases the mRNA

The transcribed termination sequence on the mRNA is AAUAAA

Elongation Termination

Modification of mrna
Modification of mRNA

  • Transcribed mRNA (pre-mRNA) must be modified before leaving the nucleus

  • modifications include:

    • addition of 5’cap

      • Prevents “unraveling”

    • addition of poly A tail

      • Prevents “unraveling”

      • Helps ribosome attach

      • Assists in the export of mRNA from nucleus

Further modifications
Further Modifications

  • Transcribed RNA is too long and is shortened in the nucleus

  • Exons are segments of the pre-mRNA that contain information that will be reflected in the polypeptide

  • Introns are segments of the pre-mRNA that separate (intervene) exons

How is this done
How is this done?

  • Small nuclear ribonucleicproteins (snRNP) recognize intron ends and together with proteins form a structure called a spliceosome

  • Spliceosomes remove introns while connecting exons together

  • Ribozymes may also catylyze this process in some organisms (introns may act as ribozymes)

Why bother with introns
Why bother with introns?

  • Introns may regulate gene activity and the passage of mRNA into the cytoplasm

  • Genes may play roles in multiple proteins, introns may enable a gene to be diverse in function

  • May increase recombination of genetic material (easier to cut and paste)