gene expression from gene to protein
DESCRIPTION
Gene expression From Gene to Protein. Translation. RNA. DNA. Protein. Transcription and Splicing. The genetic information of all organisms is stored in long strains of DNA (desoxyribonucleic-acid). Genes are the functional subunits of the genome. - PowerPoint PPT PresentationTRANSCRIPT
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• 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 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 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).
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.