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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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  1. Transcription Chapter 17

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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)

  11. 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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