Transcript Gene expression

Gene expression
Gene expression
•
The information encoded in a gene is converted into a
protein
 The genetic information is made available to the
cell
•
Phases of gene expression
1. Transcription
2. Translation
3. Protein folding
 Functional protein
1.
DNA’s information is copied into
messanger RNA (mRNA) molecule
in transcription
2. mRNA directs synthesis of a protein with
amino acid sequence determined by the base
sequence of the codons in mRNA
Translation
Folded
Unfolded
3. Correct folding of a protein is needed to achieve
functional activity
Transcription:
• a mRNA copy of a DNA sequence
is produced
• RNA polymerases make RNAs
• Other strand is used as a template
• mRNA copy has one strand
• Beside the coding area also other
information is added to mRNA
molecule
• Sequence is complementary for DNA
• Ts are replaced with uracils, U
•mRNA is produced and
processed in the nucleus:
Nucleus
1.Introns are cutted off
2.Methyl cap is added to 5’ end
3.Poly A tail is added to 3’ end
•The ready mRNA molecule is
transported to the cytoplasm
Cytoplasm
From a mRNA to a protein…
• Decoding mRNAs codon sequence to protein is
dependent on transfer RNAs (tRNA)
• All tRNAs have similar structure
•amino acid part
•anticodon part
Amino acid
• Anticodon part base pairs with it’s
anticodon structure in mRNA
• Amino acid part carries correct amino
acid to the place of protein synthesis
tRNAs are needed for recognition
and transport
mRNA
Protein synthesis
• Protein synthesis takes place in the ribosomes
• Ribosomes are located to the
cytoplasm
• Ribomes recognize the
initiation codon from mRNA
• Elongation of a protein chain
includes three steps main steps
Step 0.
mRNA arrives to the ribosome and the ribosome
starts to ”read” mRNAs code
Step 1.
tRNA forms a pair with the corresponding
codon in mRNA
Step 2.
A bond is formed by ribosome between the
adjacent amino acids
Step 3.
The ribosome translocates to the next mRNA codon
and the ”used” tRNA is discharged from the ribosome
• Previous steps are repeated until the ribosome
arrives to the stop codon
Step 4.
Termination is carried out with the help
of termination factors
• After termination the nascent protein is released from
the ribosome, the ribosome dissociates and the mRNA
is released
Step 5.
Following the translation proteins are folded and
sometimes also chemically modificated
Protein folding
• Protein´s folding is dictated by it’s amino acid sequence
• Correct folding is needed for the protein to achieve
proper functional properties
•Proteins assisting in the folding process are known
•3D structure can be predicted from the aa-sequence
•The function of a protein can be predicted from it’s
structure
Protein folding…
Expression control
•
The action of a cell is dependent on it’s proteins
•
Amount of the proteins are determined by:
1. Concentration of the RNA
2. Frequency at which the RNA in translated to the
protein
3. Stability of the protein
•
Only a small portion of the genes in a cell are expressed
 Depends on the cell type, developmental
stage, environmental factors…
• Regulation can happen at any stage of gene expression
• Control of the transcription initiation is the most important
• Different kind of control elements are found
• In eukaryotes, the control elements of transcription can be
found from the inside and outside the gene area
• Most important control element is the promoter
 Initiation place
 Directs binding of the enzymes
needed to produce RNA
Control of the initiation of transcription
•
5’ regulatory sequences  control the site of
transcription initiation  The promoter
•
RNA polymerase can`t recognise transcription start sites
•
Start sites are positioned 25 bp to 3’ direction from a
nucleotide sequence motif called the TATA BOX
•
General transcription factors guide RNA polymerase
to the start site
 TFIID-protein binds to TATA BOX
 Directs the binding of the RNA polymerase
• Other transcription factors are also needed
 TFIIA, TFIIB, TFIIE and TFIIH bind close
to the start site
• Some transcription factors bind to the RNA polymerase
• Critical properties are brought by transcription factor
 needed for example to unwind the DNA
• Also enhancer are needed for activation of transcription
 Are found from the genome
 Binding sites for activators
Thank you all for
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