translation n.
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TRANSLATION. The reading of the mRNA transcript and the creation of the polypeptide chain. Occurs on the Ribosomes : Composed of a small subunit (40S) and a large subunit (60S) Made up of ribosomal RNA and protein

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  • The reading of the mRNA transcript and the creation of the polypeptide chain.
  • Occurs on the Ribosomes:
    • Composed of a small subunit (40S) and a large subunit (60S)
    • Made up of ribosomal RNA and protein
  • The mRNA transcript is read by the ribosome in sets of 3 nucleotides to determine the amino acid sequence.
  • tRNA deliver the appropriate amino acid to the ribosome where the polypeptide chain is built.
genetic code
Genetic Code
  • There are 20 amino acids found in proteins, but only 4 bases in the code (U C A G of mRNA)
  • Codons: sequences of three nucleotide bases used to code for an amino acid. Ex. UAC codes for Tyrosine
  • 43=64 possible codons
    • therefore some amino acids can have more than one codon
    • Ex. UUU UUC, UCU, UCC all code for phenylalanine
    • This redundancy helps toreduce errors
the genetic code
The Genetic Code
  • AUG: start codon used 99% of the time (codes for Methionine)
  • UAA, UAG, UGA: stop codons
  • tRNA delivers amino acids to the ribosome
  • Looks like a cloverleaf
  • Has an anticodon – sequence of three bases that recognize (are complementary to) the mRNA codon
    • 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
  • Amino acids are added by Aminoacyl t-RNA synthases to make aminoacyl-tRNA
1 initiation
1) Initiation
  • The small subunit of the ribosome recognizes the 5' cap on the mRNA transcript and binds to the RNA.
  • The ribosome will position itself at AUG (the first codon read for every protein)
    • Ensures the correct reading frame (phase in which the mRNA is read) is used by the ribosome
  • The large subunit binds and the subunits act as a clamp (80S) around the mRNA.
sites on the ribosome
Sites on the Ribosome
  • The large subunit of the ribosome has two sites that the mRNA moves through:
    • A (acceptor) site: where new amino acids enter the ribosome
    • P (peptide) site: where the polypeptide chain grows as the amino acids are combined.
  • At a third site, the catalytic site, the peptide bonds are made between the amino acid in the A site and the P site.
2 elongation of the polypeptide chain
2) Elongation of the Polypeptide Chain
  • The AUG codon will be in the P site, tRNA carrying the corresponding amino acid (methionine) enters this 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 tRNA moves to the P site, a new tRNA brings the next a.a. to the A site and the a.a. 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.
3 termination
3) Termination
  • Ribosome will reach a stop codon (UGA, UAG, UAA)
  • No tRNA or a.a. 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.
4 post translational modifications
4) Post-Translational Modifications
  • Polypeptide chains travel through the RER and are modified to become functional proteins.
  • 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.
wobble hypothesis
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”.