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Transcription in Eukaryotes


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Detailed Description of Transcription in eukaryotes.

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Transcription in Eukaryotes

  1. 1. TRANSCRIPTION IN EUKARYOTES Presented By:- Ruchi Bio & Nano Technology Dept, GJUST, HISAR
  3. 3. TRANSCRIPTION ➢ It is the synthesis of an RNA molecule from DNA template. ➢ The template strand is the strand from which the RNA is actually transcribed. It is also termed as Antisense Strand. ➢ The coding strand is also called as Sense strand.
  4. 4. RNA POLYMERASE ✓ An enzyme that catalyses RNA synthesis. ✓ It does not need a Primer for initiation of transcription. ✓ Eukaryotes have 3 RNA polymerases but bacteria have only a single RNA polymerase. ✓ Shape: RNA polymerase resembles a crab claw.
  5. 5. Steps in Eukaryotic Transcription • Initiation • Elongation • Termination
  7. 7. • Transcription in eukaryotes is undertaken by different RNA polymerases. • Several Initiation factors are required for efficient & promoter- specific initiation in eukaryotes, and are called as General Transcription Factors(GTFs). • Sometimes additional factors are required such as Mediator Complex, DNA binding regulatory proteins and chromatin modifying enzymes.
  8. 8. Pol II core Promoter • The Eukaryotic core promoter refers to the minimal set of sequence elements required for accurate transcription initiation by the Pol II machinery.
  9. 9. PRE INITIATION COMPLEX FORMATION • RNA Polymerase II Forms a Preinitiation Complex with General Transcription Factors at the Promoter. • Once assembled at the Promoter, Pol II leaves the preinitiation complex upon addition of the nucleotide precursors required for RNA synthesis and after phosphorylation of serine residues within the enzyme’s Tail.
  10. 10. ELONGATION ➢ Elongation requires another set of factors, such as TFIIS & hSPT5, Known as Elongation factors. ➢ Various Proteins are thought to stimulate elongation by Pol II. ➢ The protein P- TEFB stimulates elongation in 3 separate steps- 1). Phosphorylation at ser-2 position of CTD repeats; 2). It also activates another protein ,called hSPT5 which is an elongation factor; 3). It activates another elongation factor called TAT-SF1.
  11. 11. POLYADENYLATION & TERMINATION ➢ Once the elongation is completed, it proceeds through the RNA processing events i.e., polyadenylation & termination. ➢ The CTD tail of polymerase carries 2 protein complexes- CPSf and CstF. ➢ The sequences which one transcribed into RNA, trigger the transfer of these factors to the RNA, are called as poly- A signals. ➢ Once CPSF & CstF bound to the RNA, it results in RNA clevage and then polyadenylation.
  13. 13. RNA POLYMERASE I • This enzyme is related to Pol. II, but they initiate transcription from distinct promoters and transcribed different genes. • Pol. I is required for the expression of only one gene that encoding the rRNA precursor. • The promoter of rRNA genes comprises 2 parts: core elements &UCE. • In addition to Pol. I, initiation requires 2 other factors, called SL1 & UBF. • SL1 comprises TBP & three TAF’s specific for transcription. • These complex bound to the UCE in the presence of UBF and stimulates transcription for core promoter by recruiting Pol I.
  14. 14. RNA POLYMERASE III • This enzyme is also related to Pol.II and they initiate transcription from distinct promoters and transcribed distinct genes. • Pol. III promoters come in various forms and being located downstream of the transcription start site. • Some Pol III promoters consist of 2 regions called Box A & Box B. • Same as Pol I & II, transcription by Pol III also requires transcription factors alongwith polymerase. • The factors are TFIIIB &TFIIIC. • Firstly TFIIIC binds to the promoter region. • After that the factor TFIIIB containing TBP is also binds to the template strand. • This complex is forced which stimulates the initiation of transcription.
  16. 16. Differences Between Prokaryotic vs Eukaryotic Transcription
  17. 17. Prokaryotic Transcription Eukaryotic Transcription • Coupled Transcription- Translation is the rule. • Occurs in the cytoplasm. • A single RNA Polymerase synthesises all three types of RNA . • RNAs are released & processed in the cytoplasm. • Polycistronic • σ factor present for initiation. • mRNA is devoid of 5’guanosine cap and Poly A tailing. • Coupled Transcription- Translation is not possible. • Occurs in the Nucleus. • The RNA polymerases I, II,III synthesises rRNA,mRNA, tRNA respectively. • RNAs are released & processed in the nucleus. • Monocistronic • σ absent and initiation of transcription require proteins called Transcription factors. • Capping occurs at 5’ end and Poly A tail at 3’ position of mRNA.
  18. 18. Bibliography 1). Watson (2009), Molecular Biology of gene, 5th Edition, Pearson Education, New Delhi. 2). Lewin B. (2004), Gene VIII, Pearson Prentice and Hall, New Delhi. 3). Lodish et al (2013), Molecular Cell Biology, 7th Edition, W.H. Freeman Publisher