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
TRANSCRIPTION
Prokaryotic cell
DNA
LE 17-3-1
TRANSCRIPTION
Prokaryotic cell
DNA
LE 17-3-2
TRANSCRIPTION
DNA
mRNA
Ribosome
Prokaryotic cell
Polypeptide
Prokaryotic cell
LE 17-3-3
DNA
TRANSCRIPTION
mRNA
Ribosome
TRANSLATION
Polypeptide
Prokaryotic cell
Nuclear
envelope
TRANSCRIPTION
Eukaryotic cell
DNA
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
DNA
TRANSCRIPTION
Pre-mRNA
RNA PROCESSING
mRNA
Ribosome
TRANSLATION
Polypeptide
Eukaryotic cell
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
LE 17-7
Promoter
Transcription unit
5
3
Start point
RNA polymerase
DNA
3
5
LE 17-7
Elongation
Non-template
strand of DNA
RNA nucleotides
RNA
polymerase
3
3 end
5
Direction of transcription
(“downstream”)
5
Newly made
RNA
Template
strand of DNA
Synthesis of an RNA
Transcript
• The three stages of transcription:
– Initiation
– Elongation
– Termination
LE 17-7
Promoter
Transcription unit
5
3
Start point
RNA polymerase
3
5
DNA
Initiation
5
3
3
5
RNA Template strand
Unwound tran- of DNA
DNA
script
Elongation
Rewound
DNA
5
3
3
5
3
5
RNA
transcript
Termination
5
3
3
5
5
Completed RNA transcript
3
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
3. Splicing
removal of introns and joining together of exons
All processing events occur in nucleus
before transport to cytoplasm
LE 17-10
5 Exon Intron
Pre-mRNA
Exon
Intron
Exon
3
5 Cap
Poly-A tail
1
30
31
Coding
segment
(mature) mRNA
104
105
146
Introns cut out and
exons spliced together
Poly-A tail
5 Cap
5 UTR
1
146
3 UTR
• 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
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
LE 17-12
Gene
DNA
Exon 1
Intron
Exon 2
Intron
Exon 3
Transcription
RNA processing
Translation
Domain 3
Domain 2
Domain 1
Polypeptide
LE 17-9
Architecture of eukaryotic mRNA
Protein-coding segment
Polyadenylation signal
5
5 Cap
5 UTR
Start codon
Stop codon
3 UTR
UTR: untranslated regions
Poly-A tail