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Lets say that the cell wants to make a particular protein. Transcription of the appropriate gene is done and the mRNA is made. mRNA attaches to the ribosome and the translation is initiated in a "normal" way.

What exactly would happen if an amino acid required in the protein is absent ? I know that the protein would not be manufactured properly but I want to exactly know what would happen to the ribosome and the mRNA. Are they destroyed? Are they simply separated? Who regulates this?

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2 Answers 2

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It is not true that the anticodon of an uncharged tRNA can't bind to the mRNA.

Bacteria have a mechanism called stringent response. This response is complicated, here is a shorter and simplified explanation. Further informations can be found at the wikipedia pages linked in the text.

If during translation a certain amino acid is absent, an uncharged tRNA binds in the ribosome, causing the synthesis of the alarmone (p)ppGpp by activation of the RelA enzyme (in E. coli), associated with the 50S subunity of the ribosome. The translation blocks and (p)ppGpp interacts with β subunit of RNApol causing:

  1. the reduction of its affinity for the σ70 transcription factor and therefore reducing the transcription of rRNA genes;
  2. the increase of its affinity for σS and σN, transcription factors needed, rispectively, for the adaptation at the stationary phase and the lack of N. Also biosynthesis of amino acids is enhanced.

Once the situation is restored, the enzyme SpoT (coli) hydrolizes (p)ppGpp.

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Point to know : aminoacyl-tRNA binds to mRNA its not just t-RNA..

So if there is no Amino-acid there is no aminoacyl-tRNA of that aminoacid.. so if there is no aminoacyl-tRNA, the anticodon of tRNA doesn't form a bond with mRNA, so the protein production halts (until the Amino acid produced).

If the protein is not formed within a certain long time, the premature protein folds itself and breaks. Then a molecule (example: ubiquitin for certain type of bacteria) attaches to this premature protein and marks it for degradation.

But there are still chances of protein production with slight mutations.

Mutation: tRNA with the same anticodon and different aminoacid can easily bind to the A site of the ribosome in the absence of the correct aminoacyl-tRNA, so the protein production doesn't stop in this case but the protein has a mutation with different aminoacid.

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Could you give a source for your answer? –  biogirl Sep 2 '13 at 15:32
    
How can a tRNA with the same anticodon bind to another amino acid ? –  biogirl Sep 2 '13 at 15:32

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