Translation

Translation is the final step of protein synthesis, in which the mRNA leaves the nucleus to make proteins.

The Players

Messenger RNA (mRNA)

• Is single-stranded.
• Contains the genetic message transcribed from the DNA.
• In prokaryotes the mRNA is simple.
• In eukaryotes the mRNA has gone through extensive processing, as shown below.

Modified figure 17.8, page 302, Campbell's Biology, 5th Edition

Transfer RNA (tRNA)

• Is single-stranded.
• Folds into a 3D clover or “L” shape (due to hydrogen bonding between the bases).
• Associates an amino acid with its code on the mRNA.
  • It links two things: an amino acid and an anticodon.
  • An amino acid is attached to the 3' end.
  • An anticodon (complement to the mRNA codon) is located at another place on the molecule.
• Only 45 different tRNA molecules recognize 61 different codons (remember the three stop codons don't code for anything).
  • Some tRNA molecules have anticodons that can recognize two or more different codons.
  • This is due to a phenomenon called wobble, which allows the last base in an anticodon to violate the base pairing rules.
    • Normally, the only combinations are A—U and C—G.
    • With wobble, the base U of a tRNA anticodon can pair with either A or G in the third position of a mRNA codon!

Figure 17.12, page 305, Campbell's Biology, 5th Edition

Aminoacyl-tRNA Synthetase

• Attaches amino acids to tRNA molecules.

Ribosome

• A mixture of protein and RNA.
  • This RNA is called ribosomal RNA (or rRNA).
• This is where protein synthesis happens: amino acids are added one-by-one to create a protein.
• The ribosome is a very complicated organelle made of two separate pieces that come together to get the job done.
  • The small subunit
  • The large subunit, which has three sites for the tRNAs, named the E site, P site, and A site (remember “EPA” or "APE")
    • A: Arrival
    • P: Polypeptide (is created)
    • E: Exit
      
      

Figure 17.14, page 307, Campbell's Biology, 5th Edition

The Process

• Translation, like transcription, is a 3 step process:
  1. Initiation
  2. Elongation
  3. Termination
• Goal: To translate the mRNA code into a protein.
• Each each triplet codon must be read from the mRNA and used to add the appropriate amino acid to the polypeptide.

1. Initiation

• The small ribosomal subunit binds to the mRNA at the 5' end of the mRNA molecule.
• The initiator tRNA
  • Has methionine as an amino acid
  • Has an anticodon that is complementary to the start codon on mRNA (normally AUG).
  • This anticodon (normally UAC) binds to the mRNA.
• The large ribosomal subunit binds to the tRNA at the P site.
  • This creates the the initiation complex.

Figure 17.15, page 307, Campbell's Biology, 5th Edition

2. Elongation

• Codon recognition: the codon in the A site of the ribosome pairs with the appropriate tRNA molecule.
• Peptide bond formation: the two amino acids are linked.
• Translocation:
  • The tRNAs in the P and A site move left.
    • A moves to P.
    • P moves to E.
  • The tRNA in the E site exits the ribosome.

Figure 17.16, page 308, Campbell's Biology, 5th Edition

3. Termination

• Occurs when one of the three stop codons (UAA, UAG, UGA) arrives in the P site.
• A release factor binds to the A site, causing the whole thing to fall apart.
  • The large and small ribosomal subunits separate from each other and from the mRNA.
  • The new protein is released and can now fold into its 3D shape and get to work in the cell.

Figure 17.17, page 309, Campbell's Biology, 5th Edition

The End of Protein Synthesis!

• We can finally say that the gene has now been expressed!
• A protein has been synthesized and is ready to serve its function in the organism.
• However, remember that a protein cannot function until it takes on its unique 3D shape, which gives it its unique function in the organism.

Links

• Protein folding simulation
• The genetic code at the DNA Learning Center
• Go to “Copying the Code” and then “Putting It Together”
• Go to “Reading the Code” and then “Putting It Together”