The triplet code.

• Genes:  are sections of DNA containing the coded information for making polypeptides.

• The information is in the form of a specific sequence of bases along the DNA molecule.

• Polypeptides combine to make proteins, so genes determine the proteins of the an organism

• Genes determine the nature and development of all organisms.

• A gene is a sequence of amino acids. 

Scientists suggested that there must be a minimum of 3 bases that coded for each amino acid because:

• Only 20 amino acids regularly occur in proteins.
• Each amino acid must have its own code of bases on the DNA
• Only four different bases (adenine, guanine, cytosine, thymine) are present in DNA
• If each base coded for a different amino acid, only four differnet amino acids could be coded for.
• Using a pair of bases 16 (4^2) different codes are possible, which is still inadequate.
• Three bases produce 64 (4^3) different codes, more than enough to satisfy the requirements of 20 amino acids.

As the code has three bases it is called the triplet code. As there are 64 possible codes and only 20 amino acids, it follows that some amino acids have more than one code. In eukaryotes much of the nuclear DNA does not code for amino acids. These sections are called introns and can occur within genes and as repeats between genes.

What is a gene?

A selection of DNA containing coded information for making polypeptides.

How many bases are required to code for a chain of six consecutive amino acids?

18

Expain how a change in one base along a DNA molecule may result in an enzyme becoming non-functional.

A different base might code for a different ammino acid. The sequence of amino acids in the polypeptide produced will be different. This change to the primary structure of the protein might result in a different shaped tertiary structure. The enzyme shape will be different and may not fit the substrate. The enzyme-substrate complex cannot be formed and so the enzyme is non-functional.