mRNA PostTranscriptional Modification in Eukaryotes

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mRNA Post-Transcriptional Modification in
Eukaryotes
mature mRNA 5-cap- exons -3 Poly A tail
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Electron Micrograph of a DNA-RNA Hybrid
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tRNA Transcription Post Modification
E. coli genes are clustered
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rRNA Transcription Post Modification
E. coli genes are clustered
small subunit 16S rRNA 21 proteins
Large subunit 23S rRNA, 5S rRNA 34 proteins
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Translation

• requires mRNA, ribosomes, tRNA, aminoacyl tRNA
  synthetases, and initiation, elongation and
  termination factors

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Initiation of Translation

• Aminoacyl tRNA synthetases enzymes which attach
  a.a.s to tRNAs to form charged tRNAs
• Initiation factors help assemble molecules

aa

• Initiation complex mRNA tRNA-met
• small ribosomal subunit
• attaches to large subunit
• tRNA-met occupies P (peptidyl) site

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Elongation of Translation

• A second charged tRNA occupies the A (aminoacyl)
  site

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Elongation of Translation

• met is transferred from its tRNA
• to a.a. at A site
• Peptide bond links amino acids

Peptidyl transferase A ribozyme that catalyzes
peptide bond formation
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Elongation of Translation

• Unchqrged tRNA enters E site, then exits
• Translocation of ribosome, requires EF-G
• Elongation cycles require EF-Tu-GTP

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Direction of Synthesis
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Termination of Translation

• No tRNA can bind to stop codon
• Releasing factor binds to A site
• Dissociation of molecules occurs

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Antibiotics as Translation Terminators
Puromycin
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Polysomes

• Several ribosomes can move in tandem

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Polycistronic mRNA

• Prokaryotes mRNA may contain multiple
  translation initiation sites
• Polycistronic mRNAs allow coordinate regulation
  of synthesis of more than one protein