The Synthesis of Proteins The sequence of bases in DNA ultimately determines the sequence of amino acids in proteins. At a time of protein synthesis some of hydrogen bonds break and the DNA molecule partly unravels to expose a sequence of bases.

The essential molecular structures concerned with protein synthesis are: DNA - The molecule that contains all the information needed to transfer hereditary traits. The information is comprised in the sequence of bases in the polymer chains. Messenger RNA (m-RNA) - A single-stranded RNA molecule that carries the information from the DNA molecule to structures (ribosomes) outside the cell nucleus where protein synthesis takes place. m-RNA determines the particular protein to be synthesised. Transfer RNA (t-RNA) - A much smaller single-stranded RNA molecule consisting of three bases that selects specific amino acids and “escorts” them to the growing protein chain so that they join at just the proper position. Ribosome - A structure outside the cell nucleus where protein synthesis occurs. ATP (adenosine triphosphate) - A molecule that “stores” energy and can release its energy under specific conditions. Amino acids ‑ Of general formula H 2 NCRHCOOH. Activated amino acids - Amino acid is complexed with ATP.

How can the base language of DNA be translated to the amino acid language of proteins? There are four kinds of bases (A, C, G, T) in DNA and 20 amino acids. If each base coded for one particular amino acid then only 4 of the 20 amino acids could be used in protein synthesis. If a pair of bases coded for an amino acid then 16 different amino acids could be used. This is still not enough. In fact, a group of 3 bases codes for an amino acid. 64 (4 3 ) different amino acids could be accounted for in this way. AA ACCA AGGA ATTA CC CGGC CTTC GG GTTG TT These are the possible base pairs.

The steps in protein synthesis may be represented as follows: m-RNA is synthesised in the cell nucleus by pairing with an untwisted portion of a DNA chain DNA molecule partly unravels. m-RNA molecule is synthesised by base pairing. The m-RNA then migrates outside of the nucleus and becomes complexed with a ribosome. An amino acid (a 1, a 2, a 3, etc.) interacts with an ATP molecule and becomes activated so that it can complex with a molecule of t-RNA. a 1 + ATP  a 1 *

The activated amino acid complexes with a molecule of t ‑ RNA that has the proper structure to accept this particular amino acid. The t ‑ RNA amino acid complex then undergoes base pairing with an appropriate section of the m ‑ RNA. Such a section comprises three nucleotides, and the set of three bases included in this section of m-RNA is called a codon. The complementary codon of the t-RNA is called an anti-codon. One codon corresponds to a particular amino acid.

Another t-RNA amino acid complex undergoes a similar process. The particular amino acid that is selected is determined by the next codon in the m-RNA sequence. This amino acid then forms a peptide bond with its neighbour. This process continues and the polypeptide chain grows to form a protein. As the protein chain breaks loose, the t-RNA molecules are released and recycled.