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Translation Translation. – The Next Step The next step is for the mRNA to leave the nucleus and move into the cytoplasm to be translated. More on Ribosomes. Ribosomes are made up of two subunits – 60s – large and 40s Small

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Translation translation
Translation Translation

  • – The Next Step

  • The next step is for the mRNA to leave the nucleus and move into the cytoplasm to be translated

More on ribosomes
More on Ribosomes

  • Ribosomes are made up of two subunits – 60s – large and 40s Small

  • The two subunits bind to the mRNA much like a clamp – mRNA is between them.

  • The ribosome moves along the mRNA in the 5’ to 3’ direction adding a new amino acid each time it reads a codon.

  • The large ribosome unit contains two sites the A site (acceptor) and the P site (peptide)

Trna bob the builder
tRNA – Bob the Builder

  • This unique clover shaped structure brings the amino acids to the ribosome to the P site where the polypeptide chain is being built.

Mrna trna working together
mRNA – tRNA – Working together

  • The recognition of mRNA codon by tRNA anticodon

  • Anticodon – group of three complementary bases that recognizes and pairs with a codon on mRNA

How the trna move the a a
How the tRNA move the a.a.

  • tRNA can exist in two states

  • – charged – carrying the amino acid

    - uncharged – lacks amino acid

  • An enzyme is responsible for adding the appropriate a.a. called aminoacyl – tRNA synthases.

  • 20 different enzymes for the 20 different amino acids.

Trna versatility
tRNA - versatility

  • They think that because there is more than one codon that tRNA can still bind if 2 of the 3 bases match up - can also be UAC for tyrosine

  • They call this the wobble hypothesis.

Building the protein
Building the Protein

  • Three step process

  • Initiation

  • Elongation

  • Termination


  • Ribosome moves along the mRNA, reading the code in triplets – codons.

  • Translation does not occur until it reads the triplet…..AUG start codon – corresponds to the aa methionine – this means that the initial protein always starts with methionine.

  • When the start codon is in the P site it delivers the amino acid methionine. tRNA recognizes the codon because of the complimentary anti-codon


  • The second codon is now in the A site – the appropriate tRNA delivers the next amino acid in the sequence.

  • A peptide bond is formed and the ribosome moves to the next codon.

  • The first tRNA is released to pick up another a.a.

  • Again another tRNA moves into the A site to continue building the peptide.


  • This continues until the stop codon is read in the A site UAG, UGA, UAA causes the ribosome to stall.

  • A protein known as a release factor recognizes the stall and causes the ribosomal unit to disassemble releasing the mRNA and the new protein.

Put it together

Put it together

Additional changes
Additional changes

  • The protein is folded and modified and then targeted to the area of the cell that it is required.

  • In some cases parts of the polypeptide are cleaved, including the methionine that started the process.

  • Sugars or phosphates can be added in a process known as glycosylation and phosphorylation.

  • These modifications occur in the Endoplasmic Reticulum