Transcript Genexpression_070808

Gene expression
From Gene to Protein
Translation
RNA
DNA
Transcription
and
Splicing
Protein
• 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.
Transcription
The way from DNA to RNA
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.
The Three Steps of Transcription
•Initation
•Elongation
•Termination
Transcription Initiation
Procaryotes
• 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.
Transcription Initiation
Eucaryotes
• Transcription factors mediate binding of the
RNA polymerase.
Transcription Initiation
RNAP = RNA polymerase
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).
Transcription Elongation
Transcription Termination
Procaryotes
• 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.
Transcription Termination
Eukaryotes
• 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).
Transcription: Termination
Translation
The Way From RNA to Protein
?
How does the information in mRNA codons get
translated into an amino acid sequence and
therefore in polypeptides
?
v
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.
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.
?
What is the correspondence between
Vnucleotides and the amino
the mRNA
acids of the protein??
Proteins are formed from 20 amino acids
in humans.
Codons of one nucleotide:
A
G
C
U
Can only encode 4 amino acids
Codons of two nucleotides:
AA GA CA UA
AG GG CG UG
AC GC CC UC
AU GU CU UU
Can only encode 16 amino acids
One codon consists of THREE nucleotides
The 3rd Base Position is Variable
The genetic code
is nearly universal
Exceptions:
Yeast
Mitochondria
Tetrahymena
Mycoplasma
The Three Steps of Translation
• Initiation
• Elongation
• Termination
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).
Translation Initiation
Leader
sequence
Small ribosomal subunit
5’
3’
mRNA
mRNA
U U C G U C A U G G G A U G U A A G C G A A
U A C
Assembling to
begin translation
Initiator tRNA
Met
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.
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.
Translation Elongation
Small ribosomal subunit
mRNA
5’
3’
A U G G G A U G U A A G C G A
U A C C C U
A C
A
Met
Large ribosomal subunit
Gly
Cys
Translation Elongation
mRNA
5’
3’
A U G G G A U G U A A G C G A
C C U A C A
U
A
C
Gly
Me
t
Cys
Translation Elongation
mRNA
5’
3’
A U G G G A U G U A A G C G A
C C U A C A
U
A
U
C
Gly
t
e
M
Cys
Lys
U
C
Translation Elongation
mRNA
3’
5’
A U G G G A U G U A A G C G A
C
C
U
A C A U U C
Gly
Lengthening
polypeptide
(amino acid chain)
Me
t
Cys
Lys
Translation Elongation
3’
A U G G G A U G U A A G C G A
U
A C A U U C
C
G C
U
C
mRNA
5’
Gly
t
Me
Cys
Lys
Arg
Translation Elongation
Stop codon
mRNA
5’
A U G G G A U G U A A G C G A U A A
C
U U C G C U
A
A
Cys
Gly
M
et
Lys
Arg
Release
factor
Translation Elongation
Stop codon
Ribosome reaches stop codon
5’
mRNA
A U G G G A U G U A A G C G A U A A
G C U
U
U
C
Release
factor
Arg
Lys
Met
Gly
Cys
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.
Translation Termination
Once a stop codon is reached,
the elements disassemble.
A U GG
GA UG U
AA G C
G A U
A A
G C
Release
factor
U
Arg
s
Ly
s
Cy
Met
Gly