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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 TRANSCRIPTION Prokaryotic cell
LE DNA TRANSCRIPTION Prokaryotic cell

4 TRANSCRIPTION Prokaryotic cell
LE DNA TRANSCRIPTION Prokaryotic cell

5 DNA TRANSCRIPTION mRNA Ribosome Prokaryotic cell Polypeptide
LE DNA TRANSCRIPTION mRNA Ribosome Prokaryotic cell Polypeptide Prokaryotic cell

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

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

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

9 Transcription unit: the transcribed DNA
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 Promoter Transcription unit 5¢ 3¢ 3¢ 5¢ DNA Start point RNA polymerase
LE 17-7 Promoter Transcription unit 5¢ 3¢ 3¢ 5¢ DNA Start point RNA polymerase

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

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

14 LE 17-7 Promoter Transcription unit 5 3 3¢ 5¢ DNA Start point RNA polymerase Initiation 5¢ 3¢ 3¢ 5¢ RNA tran- script Template strand of DNA 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 (mature) mRNA 5¢ Exon Intron Exon Intron Exon 3¢ 1 30 31 104 105 146 1
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 5¢Cap Poly-A tail 1 146 5¢ UTR 3¢ UTR

18 carried out by spliceosomes Spliceosomes
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)
LE 17-11 RNA transcript (pre-mRNA) 5¢ Exon 1 Intron Exon 2 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 Transcription
LE 17-12 Gene DNA Exon 1 Intron Exon 2 Intron Exon 3 Transcription RNA processing Translation Domain 3 Domain 2 Domain 1 Polypeptide

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

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