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Ch 17 Gene Expression I: Transcription What is this?

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Presentation on theme: "Ch 17 Gene Expression I: Transcription What is this?"— Presentation transcript:

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 RNA Protein Transcription Translation

3 LE TRANSCRIPTION DNA Prokaryotic cell

4 LE TRANSCRIPTION DNA Prokaryotic cell

5 LE TRANSCRIPTION DNA Prokaryotic cell Ribosome Polypeptide mRNA Prokaryotic cell

6 LE TRANSCRIPTION TRANSLATION DNA mRNA Ribosome Polypeptide DNA Prokaryotic cell Nuclear envelope TRANSCRIPTION Eukaryotic cell

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

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

9 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 Transcription: DNA->RNA Structure of a gene

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

11 LE 17-7 Promoter Transcription unit RNA polymerase Start point DNA

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

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

14 LE 17-7 Promoter 3 5 Transcription unit DNA Initiation RNA polymerase Start point Template strand of DNA RNA tran- script Unwound DNA Elongation Rewound DNA RNA transcript Termination 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 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 3.Splicing removal of introns and joining together of exons All processing events occur in nucleus before transport to cytoplasm Draw

17 LE ExonIntronExonIntronExon 3 Pre-mRNA Coding segment Introns cut out and exons spliced together Cap Poly-A tail 5 3 UTR (mature) mRNA

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 LE Exon 1 5 IntronExon 2 Other proteins Protein snRNA snRNPs RNA transcript (pre-mRNA) Spliceosome 5 components Cut-out intron mRNA Exon 1Exon 2 5

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

23 LE Gene Transcription RNA processing Translation Domain 2 Domain 3 Domain 1 Polypeptide Exon 1IntronExon 2IntronExon 3 DNA

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


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