Protein Synthesis

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Protein Synthesis

• The Three Ts
• 1. Transcription
• 2. Translation
• 3. Termination

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Transcription in Eukaryotesie Animal cells
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Protein Synthesis in Prokaryotesie single
celled Bacteria
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Where does All this Happen
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Protein Synthesis

• Nucleic Acids (DNA RNA) carry the hereditary
  information.
• This information is contained in codons.
• What are Codons?

A codon is a set of three base pairs (A, T, C, G)
that directs or codes for amino acids. (ex. GCU
Alanine)
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How Many Amino acids?

• There are 20 essential amino acids, however they
  can be combined in any order, just like the four
  nucleotides. This permits the production of the
  many different proteins which let organisms grow
  and function.

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Transcription

• This process occurs in the nucleus.
• A section of DNA called a gene is unwound and
  unzipped.

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• A RNA copy of one of the DNA strands is made.
  This strand is made complimentary to the
  nucleotides on the the DNA Strand.
• DNA Bases Adenine, Thymine, Cytosine, Guanine
• RNA Bases Adenine, Uracil, Cytosine, Guanine

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• DNA serves as a template for the synthesis of
  RNA.
• A regulatory protein binds to the promoter
  sequence.
• An enzyme (RNA Polymerase) binds to the promoter.
• Together they open the DNA double helix.
• RNA Polymerase proceeds down one strand moving in
  the 3 to 5 direction, as it does it assembles a
  complementary strand of RNA.
• Each ribonucleotide is inserted into the growing
  RNA strand following the rules of base pairing.
• Transcription stops when the termination sequence
  is reached.
• The completed RNA copy is now called messenger
  RNA or mRNA and carries the coded message to the
  ribosomes in the cytoplasm.

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DNA Strand 1 AGCTATCGAGCAT
DNA Strand 2 TCGATAGCTCGTA
RNA copy AGCUAUCGAGCAU
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Transcription
RNA polymerase (enzyme) attaches to the promoter
region of the DNA
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Translation

• The small subunit of the ribosome binds to a site
  on the 5 end of the start of the message strand
• The ribosome moves downstream (5?3) until it
  encounters the start codon AUG
• At this time, the large subunit joins and the
  initiator t-RNA enters the scene
• After this, the tRNA binds to the p site on the
  ribosome
• The first amino acid that starts this sequence of
  peptide bonds is Methionine (one of the 20
  a.a.s)
• Eucaryotes methionine Bacteria f Met
  (modified)

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T RNA
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1. Ribosome attaches to mRNA and reads first
codon. 2. A transfer RNA molecule (tRNA for
short) brings the correct amino acid to the
ribosome drops it off. ex AUG (codes for
methionine) tRNA brings methionine to ribosome.
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3. The ribosome moves on to the next codon and
another tRNA brings the next amino acid. 4. The
amino acids in the growing chain are linked
together by a peptide bond. The growing chain is
called a polypeptide or protein molecule. 5.
When the ribosome reaches a STOP codon the
polypeptide is released and the mRNA falls off.
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Elongation more specifically!

• As another tRNA arrives with its associated a.a.
  and binds to the next codon site, it becomes
  covalently linked to the incoming a.a. with a
  peptide bond
• The initiator tRNA is released from the P site
• The ribosome moves downstream (5?3)
• The more recently-arrived tRNA w/ its peptide
  moves to P site and opens the A site for the
  arrival of a NEW tRNA and its corresponding a.a.

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Termination

• End of the message is a STOP codon
• STOP codons UAA, UAG, UGA
• Protein release factor sees these codons at the A
  site and tells the polypeptide chain to release
  from the ribosome site
• Ribosome splits into its subunits until it is
  time to make more protein!

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Termination
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Check out this link for an animation on Protein
Synthesis

• http//www.lewport.wnyric.org/jwanamaker/animation
  s/Protein20Synthesis20-20long.html