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Honors Biology
Translation Summary
Translation is the process used by the cell to read the information encoded in an mRNA molecule and
use it to build a protein. Recall that a protein is a long polymer (or chain) of amino acids. The amino acids are
linked together by peptide bonds, made through the process of dehydration synthesis
Two structures are needed to read the mRNA and construct the protein. The larger of the two is the
ribosome. A ribosome is composed of both proteins and ribosomal RNA (rRNA). The proteins serve as a
framework that holds the rRNA in place. The rRNA serves as a ribozyme—an RNA that works as a catalyst. The
rRNA speeds the formation of peptide bonds between the amino acids, linking them together. The ribosome
surface contains three grooves, called the E slot (Exit), P slot (Peptide or Protein) and A slot (amino acid slot).
These three groves are vital to linking the amino acids together in the proper order.
The second molecule is tRNA (transfer RNA). This is a RNA strand that loops and coils around itself,
forming an “L” shape. In old diagrams, tRNA was often drawn as a cloverleaf-like shape. There are many
different tRNAs, at least one for each amino acid. All but three of the tRNAs have a specific amino acid attached
at one end (the three for the stop codons do not). The other end of the tRNA has three exposed RNA bases
called the anticodon. The anticodon is complimentary to the codon in mRNA. For example, the mRNA codon
‘AUG’ is read by a tRNA with the anticodon “UAC”. Attached to this tRNA is the amino acid methionine. The
mRNA codon ‘UUU’ is read by a tRNA molecule with the anticodon “AAA” and carrying the amino acid
phenylalanine.
After the mRNA gets its methyl-G cap, polyA tail, and is spliced together, it leaves the nucleus through
a nuclear pore and finds a ribosome. A ribosome binds to the mRNA, and aligns it so that the start codon, AUG,
is in the middle P slot. The tRNA that reads this codon, and carries the amino acid methionine, slides into the P
slot and the anticodon and codon bases hydrogen bond. The second codon is under the A slot. The second
tRNA that reads the second codon slides in. This puts the two amino acids, methionine on the first tRNA and
what ever is on the second tRNA, next to each other. The ribosome then catalyzes the formation of a peptide
bond between the two amino acids. It takes the energy equivalent of three ATP to power this process and
make one peptide bond. The first amino acid is released from the tRNA, and is now bound to the second amino
acid.
After the peptide bond forms, the ribosome slides along the mRNA by one codon. The first tRNA,
which is now empty, is in the E slot where it exits the ribosome and floats away. The two-amino acid peptide
chain is in the P slot. The A slot is empty, and ready for the next tRNA to enter carrying the third amino acid in
the chain. When it enters, the ribosome peptide bonds it to the earlier amino acids. Then the ribosome moves
along the mRNA, reading the next codon. This continues until a stop codon is reached. This stop codon is read
by a tRNA that does not have an amino acid attached. The ribosome tries to make a peptide bond, but it
cannot. The ribosome then aborts translation and releases the new protein so that it can fold into its final shape.
The ribosome.
tRNA