Gene expression

From Gene to Protein

DNA

RNA

ProteinTranscriptionand

Splicing

Translation

• 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 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.

Transcription InitiationEucaryotes

• 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 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.

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).

Transcription: Termination

Translation

The Way From RNA to Protein

? How does the information in mRNA codons get translated into an amino acid sequence

and v therefore in polypeptides ?

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 the mRNAVnucleotides and the amino acids of the

protein??

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

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 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

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

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

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

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

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

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

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

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

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

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.