ch 17 gene expression i: transcription
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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
RNA
Protein
Transcription
Translation
LE 17-3-1
TRANSCRIPTIONDNA
Prokaryotic cell
LE 17-3-1
TRANSCRIPTIONDNA
Prokaryotic cell
LE 17-3-2
TRANSCRIPTIONDNA
Prokaryotic cellRibosome
Polypeptide
mRNA
Prokaryotic cell
LE 17-3-3
TRANSCRIPTION
TRANSLATION
DNA
mRNA
Ribosome
Polypeptide
DNA
Prokaryotic cell
Nuclearenvelope
TRANSCRIPTION
Eukaryotic cell
LE 17-3-4
TRANSCRIPTION
TRANSLATION
DNA
mRNA
Ribosome
Polypeptide
DNA
Pre-mRNA
Prokaryotic cell
Nuclearenvelope
mRNA
TRANSCRIPTION
RNA PROCESSING
Eukaryotic cell
LE 17-3-5
TRANSCRIPTION
TRANSLATION
DNA
mRNA
Ribosome
Polypeptide
DNA
Pre-mRNA
Prokaryotic cell
Nuclearenvelope
mRNA
TRANSLATION
TRANSCRIPTION
RNA PROCESSING
Ribosome
Polypeptide
Eukaryotic cell
• 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
Basic components for transcription
dsDNA with a promoter
RNA polymerase
rNTPs (ribonucleotides triphosphates)ATP, CTP, GTP, UTP
LE 17-7
Promoter Transcription unit
RNA polymeraseStart point
DNA
53
35
LE 17-7
ElongationNon-templatestrand of DNA
RNApolymerase
RNA nucleotides
3 end3
5
5
Newly madeRNA
Templatestrand of DNA
Direction of transcription(“downstream”)
Synthesis of an RNA Transcript
• The three stages of transcription:– Initiation– Elongation– Termination
LE 17-7Promoter
35
Transcription unit
DNA
InitiationRNA polymerase
Start point
Template strandof DNA
RNAtran-script
UnwoundDNA
Elongation
3
3
53
5
5
3 5
RewoundDNA
5 3
35 35
RNAtranscript Termination
35
5 3Completed 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
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. Splicingremoval of introns and joining together of exons
All processing events occur in nucleus
before transport to cytoplasm
Draw
LE 17-10
5 Exon Intron Exon Intron Exon 3Pre-mRNA
1 30 31 104 105 146
Codingsegment
Introns cut out andexons spliced together
1 146
5Cap
5Cap
Poly-A tail
Poly-A tail
5 3UTR UTR
(mature) mRNA
• 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)
LE 17-11
Exon 15
Intron Exon 2
Other proteinsProtein
snRNA
snRNPs
RNA transcript (pre-mRNA)
Spliceosome
5
Spliceosomecomponents
Cut-outintron
mRNA
Exon 1 Exon 25
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 potentialDraw Splice Variants
• 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
LE 17-12
Gene
Transcription
RNA processing
Translation
Domain 2
Domain 3
Domain 1
Polypeptide
Exon 1 Intron Exon 2 Intron Exon 3
DNA
LE 17-9
5Protein-coding segment
5 Start codon Stop codon Poly-A tail
Polyadenylation signal
5 3Cap UTR UTR
Architecture of eukaryotic mRNA
UTR: untranslated regions