TRANSLATION

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TRANSLATION

• The reading of the mRNA transcript and the
  creation of the polypeptide chain.
• RIbosomes
• Composed of a small subunit (40S) and a large
  subunit (60S)
• INITIATION
• Ribosome recognizes the 5' cap on the mRNA
  transcript and binds to the RNA
• The subunits act as a clamp (80S) around the mRNA

http//www.mtmi.vu.lt/pfk/funkc_dariniai/nanostruc
tures/images/0901whitesides_ribo.jpg
http//www.modares.ac.ir/elearning/Dalimi/Proto/Le
ctures/week2/ribosome_15B15D.gif
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GENETIC CODE

• There are 20 amino acids found in proteins, only
  4 bases in mRNA (U C A G)
• Codons sequences of three bases used to code for
  an a.a.
• 4364 possible codons (some amino acids have more
  than one codon)
• Ex. UUU UUC, UCU, UCC all code for phenylalanine
  (a.a.)
• This redundancy helps to reduce errors
• AUG start codon (Methionine) used 99 of the
  time
• UAA, UAG, UGA stop codons
• The mRNA transcript is read in sets of 3
  nucleotides (one codon) to determine which a.a.
  is next.

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http//anthro.palomar.edu/biobasis/images/protein_
synthesis_at_ribosomes.gif
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ELONGATION of the POLYPEPTIDE CHAIN

• AUG is the first codon read for every protein
  (start codon)
• Ensures the correct reading frame (phase in which
  the mRNA is read) is used by the ribosome
• The large subunit of the ribosome has two sites
  for tRNA
• A (acceptor) site
• P (peptide) site
• tRNA carrying the methionine enters the P site.
• The next tRNA carrying the required a.a. enters
  the A site.
• At the catalytic site (b/w A and P) a peptide
  bond forms between the two amino acids.
• The ribosome moves over one codon. The second
  a.acid moves to the P site, a tRNA brings the
  next a.acid to the A site and it is added to the
  chain.
• The first tRNA is released to be recycled for use
  with another amino acid.
• The process repeats with the polypeptide chain
  trailing in the cytoplasm.

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http//sun.menloschool.org/birchler/cells/animals
/ribosome/synthesis.GIF
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tRNA

• tRNA delivers amino acids to the ribosome
• Looks like a cloverleaf
• Has an anticodon sequence of three bases that
  recognize the mRNA codon (complementary to)
• ex. The GCC codon specifies the a.a. alanine
• The anticodon on the tRNA carrying alanine would
  be CGG
• Opposite arm carries the amino acid binding site
• Aminoacyl t-RNA synthases add the amino acids to
  tRNA at the binding site, the tRNA is now a
  aminoacyl-tRNA

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TERMINATION

• Ribosome will reach a stop codon (UGA, UAG, UAA)
• No tRNA exists for a stop codon
• A release-factor protein helps the two ribosome
  subunits to fall off the mRNA and the polypeptide
  chain is released.
• MODIFICATIONS
• Some amino acids may be glycosylated (sugar
  added) or phosphorylated (phosphate added) or
  altered in another way.
• Enzymes may cleave (cut) the chain at specific
  places.

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Wobble Hypothesis

• In codons the third base may differ between 2
  codons that code for the same amino acid (UAU and
  UAC both code for tyrosine).
• If the tRNA's anticodon is AUA it can still bind
  to UAC.
• This flexibility allows for the correct amino
  acid to be added to the polypeptide chain despite
  errors in the gene sequence.
• The proposal that tRNA can recognize more than
  one codon by unusual base pairing is known as the
  wobble hypothesis.