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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”.

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