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Ch 17 Gene Expression I: Transcription

Ch 17 Gene Expression I: Transcription. What is this?. Basic Principles of Gene Expression. DNA encodes hereditary information (genotype) -> decoded into RNA -> protein (phenotype). DNA. Transcription. RNA. Translation. Protein. LE 17-3-1. DNA. TRANSCRIPTION. Prokaryotic cell.

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Ch 17 Gene Expression I: Transcription

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  1. Ch 17 Gene Expression I: Transcription What is this?

  2. Basic Principles of Gene Expression DNA encodes hereditary information (genotype) -> decoded into RNA -> protein (phenotype) DNA Transcription RNA Translation Protein

  3. LE 17-3-1 DNA TRANSCRIPTION Prokaryotic cell

  4. LE 17-3-1 DNA TRANSCRIPTION Prokaryotic cell

  5. LE 17-3-2 DNA TRANSCRIPTION mRNA Ribosome Prokaryotic cell Polypeptide Prokaryotic cell

  6. DNA TRANSCRIPTION LE 17-3-3 mRNA Ribosome TRANSLATION Polypeptide Prokaryotic cell Nuclear envelope DNA TRANSCRIPTION Eukaryotic cell

  7. DNA TRANSCRIPTION LE 17-3-4 mRNA Ribosome TRANSLATION Polypeptide Prokaryotic cell Nuclear envelope DNA TRANSCRIPTION Pre-mRNA RNA PROCESSING mRNA Eukaryotic cell

  8. DNA TRANSCRIPTION LE 17-3-5 mRNA Ribosome TRANSLATION Polypeptide Prokaryotic cell Nuclear envelope DNA TRANSCRIPTION Pre-mRNA RNA PROCESSING mRNA Ribosome TRANSLATION Polypeptide Eukaryotic cell

  9. Transcription: DNA->RNA Structure of a gene • Promoter: DNA sequence where RNA polymerase binds to transcribe the gene • Transcription start site: the nucleotide where RNA pol initiates transcription • Transcription unit: the transcribed DNA

  10. Basic components for transcription dsDNA with a promoter RNA polymerase rNTPs (ribonucleotides triphosphates) ATP, CTP, GTP, UTP

  11. LE 17-7 Transcription unit Promoter 5¢ 3¢ 3¢ 5¢ DNA Start point RNA polymerase

  12. Elongation Non-template strand of DNA RNA nucleotides LE 17-7 RNA polymerase 3¢ 3¢ end 5¢ Direction of transcription (“downstream”) 5¢ Template strand of DNA Newly made RNA

  13. Synthesis of an RNA Transcript • The three stages of transcription: • Initiation • Elongation • Termination

  14. Promoter Transcription unit 5 3 3¢ 5¢ DNA Start point RNA polymerase LE 17-7 Initiation 5¢ 3¢ 5¢ 3¢ Template strand of DNA RNA tran- script Unwound DNA Elongation Rewound DNA 5¢ 3¢ 3¢ 3¢ 5¢ 5¢ RNA transcript Termination 5¢ 3¢ 3¢ 5¢ 5¢ 3¢ Completed RNA transcript

  15. Termination of Transcription Different in prokaryotes and eukaryotes • In prokaryotes • RNA pol stops transcription at the end of the terminator (DNA sequence) • In eukaryotes • pre-mRNA is cleaved from the growing RNA chain • RNA pol eventually falls off the DNA

  16. RNA processing in eukaryotes, not prokaryotes Draw 1. Addition of methylated cap to 5’ end of messenger RNA (mRNA)-> increases stability and translation of mRNA 2. Addition of poly(A) tail to 3’ end (polyadenylation) -> increases stability and translation of mRNA • Splicing removal of introns and joining together of exons All processing events occur in nucleus before transport to cytoplasm

  17. LE 17-10 5¢ Exon Intron Exon Intron Exon 3¢ Pre-mRNA 5¢Cap Poly-A tail 1 30 31 104 105 146 Introns cut out and exons spliced together Coding segment (mature) mRNA Poly-A tail 5¢Cap 1 146 UTR 5¢ 3¢ UTR

  18. RNA splicing: carried out by spliceosomes • Spliceosomes complex of proteins and several small nuclear ribonucleoproteins (snRNPs) Recognize splice sites (specific RNA sequences) cleave out introns and splice together exons (coding region)

  19. RNA transcript (pre-mRNA) 5¢ Exon 1 Intron Exon 2 LE 17-11 Protein Other proteins snRNA snRNPs Spliceosome 5¢ Spliceosome components Cut-out intron mRNA 5¢ Exon 1 Exon 2

  20. Ribozymes • Catalytic RNAs molecules that function as enzymes; involved in splicing • Non-protein biological catalyst Can you think of a ribozyme with a different function? Telomerase

  21. Functional and Evolutionary Importance of Introns • Some genes can encode more than one kind of polypeptide -different combinations of exons can be spliced together • Called alternative RNA splicing • Increases the potential number of different proteins (and thus functions) in an organism • Increased adaptive potential Draw Splice Variants

  22. Exons and protein domains • In many cases, different exons code for the different domains in a protein • Protein domains • Distinct conformational regions often with discrete functions

  23. Gene DNA Exon 1 Intron Exon 2 Intron Exon 3 LE 17-12 Transcription RNA processing Translation Domain 3 Domain 2 Domain 1 Polypeptide

  24. LE 17-9 Architecture of eukaryotic mRNA Protein-coding segment Polyadenylation signal 5¢ Start codon Stop codon 3¢ UTR Cap 5¢ UTR 5¢ Poly-A tail UTR: untranslated regions

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