gene expression from gene to protein dna rna protein transcription and splicing translation
TRANSCRIPT
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Gene expression
From Gene to Protein
DNA
RNA
ProteinTranscriptionand
Splicing
Translation
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• The genetic information of all organisms is stored
in long strains of DNA (desoxyribonucleic-acid).
• Genes are the functional subunits of the genome.
• They are arranged in a succession on the DNA.
• Usually one gene encodes one protein.
• The DNA sequence determines the sequence of
amino acids of the resulting protein.
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Transcription• Transcription is the first step of
genexpression.
• The template for transcription is DNA.
• The product of this process is messenger RNA (mRNA).
• RNA polymerase is the enzyme performing transcription.
• Transcription proceeds in the nucleus in eucaryotes; in the cytoplasm in procaryotes.
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The Three Steps of Transcription
•Initation
•Elongation
•Termination
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Transcription InitiationProcaryotes
• RNA polymerase binds to the DNA and is
associated with the so called sigma factor.
• The sigma factor aids in finding the starting
point of transcription: the region -10 and -
35 basepairs downstream of the promoter.
• The initation complex opens and the first
phosphodiester bond is formed.
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Transcription InitiationEucaryotes
• Transcription factors mediate binding of
the RNA polymerase.
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Transcription Elongation
• One DNA strand is used as the template
for transcription (the 3‘–5‘ strand).
• The RNA polymerase traverses the
template strand. It produces an RNA copy
that is complementary to the template (T
are replaced with U).
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Transcription TerminationProcaryotes
• Two different termination strategies:
– Rho dependent: protein factor Rho destabilizes the
interaction between DNA and RNA, releasing the
RNA.
– Rho independet: termination occurs when the
transcript forms a G-C rich hairpin loop, followed by a
run of Us, which leads to relase of the mRNA from
the DNA template.
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Transcription TerminationEukaryotes
• The termination process is less well
understood than in procaryotes.
– It involves cleavage of the new transcript.
– template independent addition of As at the 3‘
end (poly-adenylation).
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Translation
The Way From RNA to Protein
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? How does the information in mRNA codons get translated into an amino acid sequence
and v therefore in polypeptides ?
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Through adapter molecules called
transfer RNAs tRNAs.
The tRNA anticodon base pairs with the codon in the
mRNA and carries an amino acid corresponding to that
codon.
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Transfer RNAs (tRNAs)• About 80 nucleotides long RNA with a
complex secondary and tertiary
structure.
• Contain non-standard base pairs,
stems and loops, and modified bases.
• Each cell contains different types of
tRNAs that can incorporate one of
the 20 different amino acids into
protein.
• Some tRNAs can recognize more
than one codon.
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? What is the correspondence
between the mRNAVnucleotides and the amino acids of the
protein??
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Codons of one nucleotide:AGCU
Codons of two nucleotides:AA GA CA UAAG GG CG UGAC GC CC UCAU GU CU UU
Proteins are formed from 20 amino acids in humans.
Can only encode 4 amino acids Can only encode 16 amino acids
One codon consists of THREE nucleotides
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The 3rd Base Position is Variable
The genetic code is nearly universal
Exceptions:Yeast
Mitochondria
Tetrahymena
Mycoplasma
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The Three Steps of Translation
• Initiation• Elongation• Termination
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Translation Initiation
• Translation begins at a START codon: AUG
(methionine)
• The small ribosomal subunit binds to the
mRNA.
• Initiator tRNA (fMet-tRNA) binds and builds
H-bonds with its anticodon to the AUG codon
on the mRNA
(codon-anticodon interaction).
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Translation I nitiation
Leadersequence
mRNA
5’ 3’
mRNA
A U GU U C G U C G G A C G AU G U A A G A
Small ribosomal subunit
Assembling to begin translation
Met
U A C
I nitiator tRNA
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Translation Elongation
• The large ribosomal subunit binds to the
initiation complex.
• The ribosome has three tRNA binding sites:
A-site, P-site, E-site.
• The incoming tRNA, carrying the amino acid
corresponding to the next codon, binds to the
A-site.
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Translation Elongation
• A peptide bond is formed between the amino
acids of the P-site and A-site tRNAs.
• After transfer of the amino acid to the
growing peptide chain tRNAs leave the
ribosome via the E-site (E: exit).
• These steps are repeated until the ribosome
reaches a STOP codon on the mRNA.
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Translation Elongation
CU A
Met
mRNA5’ 3’
C C U
Gly
U U U CG G G G GGA A A A A
AAC
Cys
Small ribosomal subunit
Large ribosomal subunit
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Translation Elongation
mRNA5’ 3’
Met
C C U
Gly
C
UA
U U U CG G G G GGA A A A A
A AC
Cys
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Translation Elongation
mRNA5’ 3’
Met
A AC
Cys
C
UU
Lys
C C U
Gly
U U U CG G G G GGA A A A A
CU
A
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Translation Elongation
mRNA5’ 3’
CC
U
MetGly
CU U
Lys
Lengtheningpolypeptide(amino acid chain)
A AC
Cys
U U U CG G G G GGA A A A A
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Translation Elongation
mRNA5’ 3’
MetGly
C UG
Arg
CU U
Lys
A AC
Cys
U U U CG G G G GGA A A A AC
CU
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Translation Elongation
mRNA5’
U U U CG G G G GGA A A A A U A A
Stop codon
C UG
Arg
CU U
Lys
MetGly
Cys
Releasefactor
A
AC
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Translation Elongation
mRNA5’
CU U
Met Gly CysLys
Stop codonRibosome reaches stop codon
C UG
Arg
U U U CG G G G GGA A A A A U A A
Releasefactor
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Translation Termination
• A stop codon on the mRNA leads to
binding of a release factor.
• The ribosomal subunits disassemble
and are released separately.
• The completed peptide chain is
released.
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Translation Termination
UU U
CG G G G G
GAA A A A U A A
C UG
MetGly
CysLys
Arg
Releasefactor
Once a stop codon is reached, the elements disassemble.