The Protein Synthesis Process

Mitochondrial DNA
Mitochondria (and chloroplasts in plants) have their own small loops of DNA and reproduce on their own, independent of what goes on in the nucleus. This DNA codes for some mitochondrial proteins, but others are provided from the DNA stored in the nucleus. Mitochondria resemble an early form of bacteria, which is thought to have been captured into eukaryotic cells early in the history of life on Earth. The bacteria coexisted with the cell (endosymbiosis) and evolved into mitochondria. Another unique aspect of mitochondrial DNA is that you inherit it only from your mother (the mitochondria that exist in the egg cell). Although the sperm cell that fertilizes the egg contains a mitochondrion from the father, it does not get released and passed on.

Now let's look at the order of events in the synthesis of our protein from our sample mRNA:

  1. A ribosome binds to mRNA with the AUG codon in the P-site and the UUU codon in the A-site.
  2. An amino acyl-tRNA (anti-codon = UAC) with an attached methionine comes into the P-site of the ribosome
  3. An amino acyl-tRNA (anti-codon = AAA) with an attached phenylalanine comes into the A-site of the ribosome
  4. A chemical bond forms between the methionine and phenylalanine (in a protein, this covalent bond is called a peptide bond).
  5. The methionine-specific tRNA leaves the P-site and goes off to gather another methionine
  6. The ribosome shifts so that the P-site now contains the UUU codon with the attached phenyl-alanine tRNA and the next codon (ACA) now occupies the A-site.
  7. An amino acyl-tRNA (anti-codon) with an attached threonine comes into the A-site of the ribosome.
  8. A peptide bond forms between the phenylalanine and the threonine.
  9. The phenylalanine-specific tRNA leaves the P-site and goes off to find another phenylalanine.
  10. The ribosome shifts down one codon so that the stop sequence is now in the A-site. Upon encountering the stop sequence:
    • The ribosome detaches from the mRNA and splits into its two parts
    • The threonine-specific tRNA releases its threonine and leaves
    • The new protein floats away

Several ribosomes can attach to a molecule of mRNA one after another and begin making proteins. So several proteins can be made from one mRNA. In fact, in E. coli bacteria, translation of the mRNA begins even before transcription is finished.

In the next section we'll look at DNA mutation.