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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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Transcription

Chapter 17


Objectives

Objectives

  • 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


Transcription1

Transcription

  • 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


Initiation

Initiation

  • 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

Elongation

  • Once initiation is complete the 2 strands of the DNA unwind due to the zipper region of the enzyme

  • 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


Termination in prokaryotes

Termination In prokaryotes

  • Different mechanisms

    • Rho-independent mechanism

      • GC rich area followed by Uracils

    • Rho-dependent mechanism

      • a specific protein called a rho factor binds to the end of the RNA chain

      • slide along the strand towards the polymerase digesting the strand


Termination in eukaryotes

Termination In Eukaryotes

  • When the RNA Polymerase transcribes the terminator region of the DNA, the polymerase releases the mRNA due to the action of proteins

  • The transcribed termination sequence, also known as the polyadenylation signal in the pre-mRNA, is AAUAAA.

  • Polymerase continues to synthesize RNA until an enzyme catches up to it and causes it too fall off


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”

      • Helps ribosome attach

    • addition of poly A tail

      • Prevents “unraveling”

      • Assists in the export of mRNA from nucleus


Further transcript modifications

Further Transcript Modifications

  • Transcribed RNA is “too long” and is shortened in the nucleus through RNA splicing

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

  • Introns are segments of the 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)


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