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
TRANSCRIPTION Prokaryotic cell LE 17-3-1 DNA TRANSCRIPTION Prokaryotic cell
TRANSCRIPTION Prokaryotic cell LE 17-3-1 DNA TRANSCRIPTION Prokaryotic cell
DNA TRANSCRIPTION mRNA Ribosome Prokaryotic cell Polypeptide LE 17-3-2 DNA TRANSCRIPTION mRNA Ribosome Prokaryotic cell Polypeptide Prokaryotic cell
LE 17-3-3 DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Polypeptide Prokaryotic cell Nuclear envelope TRANSCRIPTION DNA Eukaryotic cell
LE 17-3-4 DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Polypeptide Prokaryotic cell Nuclear envelope TRANSCRIPTION DNA Pre-mRNA RNA PROCESSING mRNA Eukaryotic cell
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
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
Basic components for transcription dsDNA with a promoter RNA polymerase rNTPs (ribonucleotides triphosphates) ATP, CTP, GTP, UTP
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
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
Synthesis of an RNA Transcript The three stages of transcription: Initiation Elongation Termination
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
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
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
(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
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)
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
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
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
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
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
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