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

Unfolded

Folded

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:

1.Introns are cutted off2.Methyl cap is added to 5’ end3.Poly A tail is added to 3’ end

•The ready mRNA molecule is transported to the cytoplasm

Nucleus 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

• 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

Amino acid

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´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

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 protein3. 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 your attention!