Translation

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Translation involves taking the message that's in the messenger RNA and in a sense decoding the message from the language of nucleic acids to the language of proteins or polypeptides. For translation to happen, the messenger RNA goes to the cytoplasm where it is attached to a cellular structure called a ribosome. Ribosomes are two part molecular assemblies consisting of various proteins plus a special kind of RNA called ribosomal RNA. Ribosomal RNA is involved in catalyzing some of the chemical reactions of translation.

In addition to the ribosome, another kind of RNA called tRNA carries amino acids to the mRNA when it is attached to a particular part of the ribosome's small subunit, called a binding site. A critical feature of mRNA and how it is translated is the fact that each three nucleotides in the mRNA is called a codon and it is the codon that is translated. Thus the sequence of codons corresponds to the sequence of amino acids in the polypeptide. You will see that the tRNA molecules have a set of three nucleotide bases at one end that are complementary to a corresponding codon. The bases on the tRNA are called the anti codon. This is critical because the anti codons make the connection between the codons and the correct amino acids that go with each codon.

Scientists have cracked the code involved in translation and given a stretch of mRNA can tell what the corresponding sequence of amino acids is. Learning how to do this was one of the two or three big advances in the 20th century and has laid the foundation for many advances in biotechnology as well as more basic biology such as the study of evolution. This genetic code is virtually universal in that a particular codon will usually translate to the same amino acid regardless of of the organism doing the translation.

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Sample mRNA transcript showing codon's nucleotide triplets. Read from left to right. This mRNA has 5 codons. Press on the image to go to the next step!
mRNA

Attachment to small ribosome unit
Initiation Step 1.

The mRNA joins to the small ribosomal unit at the 5' untranslated region. This binds to a special binding site on the small ribosomal subunit.

The large ribosomal subunit has 3 binding sites, E, P, and A.

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Attachment of large ribosomal subunit
Initiation Step 2.

The large ribosomal subunit attaches to the small subunit such that the first codon is aligned at the P binding site.

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attachment of met tRNA at start codon
Initiation step 3.

A tRNA carrrying the amino acid methionine attaches to the start codon (AUG) on the messaenger RNA. This inititates elongation.

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Elongation Step 1   Elongation step 1.

Attachment of first amino acid carrying tRNA to A binding site.

A  tRNA and its amino acid attaches to the A binding site.

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Joining of met to amino acid on the A binding site tRNA
Elongation Step 2.

Peptide bond formation between the met and the amino acid carried at the A bindiung site. Our polypeptide chain is now:

Met -Thr

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Ribosome moves down the reading frame by 3 bases
Elongation Step 3.  

Ribosome moves in the 3' direction down the messenger RNA  by three bases or one codon shifting the tRNA and polypeptide chain to the P Binding site. The A binding site is open and a  vacant tRNA is in the E binding site.

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met tRNA leaves the E binding site
Elongation Step 4.

 tRNA ejected from the E binding site.

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Next tRNA and amino acid attaches to A binding site
Repeat Elongation Step 1 - 4 

 until stop codon encountered.

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Ribosome moves to stop codon, old tRNA leaves E site
Elongation ends with:

  • Old tRNA ejected from the E Binding site

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STOP codon reached. End of elongation/ start of termination.
Termination Step 1.

The polypeptide chain is at the P site. The stop codon at the A site.

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Release factor protein attaches to A binding site
Termination Step 2.

A Release factor protein binds to the stop codon at the A binding site.

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Polypeptide released
Termination Step 3.

Release factor protein initiates separation of polypeptide chain:

Met-Thr-His-Asp-Gly

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Ribosomal subunits, mRNA separate
Termination Step 4.

Separation of translation machinary. Polypeptide chain may go to cytoplasm for further processing.

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created 03/07/00 pgd