Gene expression From Gene to Protein

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Gene expressionFrom Gene to Protein
Translation
RNA
DNA
Protein
Transcription and Splicing
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• The genetic information of all organisms is
  stored in long strains of DNA (desoxyribonucleic-a
  cid).
• 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.

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Transcription
The way from DNA to RNA
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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.

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The Three Steps of Transcription

• Initation

• Elongation

• Termination

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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.

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Transcription InitiationEucaryotes

• Transcription factors mediate binding of the RNA
  polymerase.

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Transcription Initiation
RNAP RNA polymerase
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Transcription Elongation

• One DNA strand is used as the template for
  transcription (the 35 strand).
• The RNA polymerase traverses the template strand.
  It produces an RNA copy that is complementary to
  the template (T are replaced with U).

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Transcription Elongation
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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.

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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).

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Transcription Termination
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TranslationThe Way From RNA to Protein
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? How does the information in mRNA codons get
translated into an amino acid sequence and v
therefore in polypeptides ?
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• 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.

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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.

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? What is the correspondence between the
mRNAVnucleotides and the amino acids of the
protein??
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Proteins are formed from 20 amino acids in
humans.

• Codons of one nucleotide
• A
• G
• C
• U

• Codons of two nucleotides
• AA GA CA UA
• AG GG CG UG
• AC GC CC UC
• AU GU CU UU

Can only encode 4 amino acids
Can only encode 16 amino acids
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The 3rd Base Position is Variable

• The genetic code is nearly universal
• Exceptions
• Yeast
• Mitochondria
• Tetrahymena
• Mycoplasma

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The Three Steps of Translation

• Initiation
• Elongation
• Termination

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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).

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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.

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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.

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Small ribosomal subunit
Large ribosomal subunit
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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.

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