AQA AS Biology Chapter 8.2 - TRIPLET CODE

Revision notes on chapter 8.2 of the AQA AS Biology textbook concerning the triplet code.

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Chapter 8.2 ­ The Triplet Code
What is a gene?
Genes are sections of DNA that contain the coded information for making polypeptides.
The coded information is in the form of a specific sequence of bases along the molecule.
Polypeptides combine to form proteins in an organism. Enzymes are a specialised form
of protein, and as they control chemical reactions they are responsible for an organism's
development and activities. So, basically, genes determine the nature and development
of all organisms.
The triplet code:
Scientists suggested that there must be a minimum of three bases that coded for each
amino acid, because:
Only 20 amino acids regularly occur in proteins.
Each AA must have its own code of bases on the DNA.
There are only four different bases in DNA ­ A, C, T and G.
If each base coded for a different AA, only four different AAs could be coded for.
Using a pair of bases, 16 different codes are possible, which is still inadequate.
Three bases produce 64 different codes, which is more than enough to satisfy the
requirements of 20 AAs.
As there are 64 possible codes, and only 20 AAs, it follows that some AAs have more
than one code. In eukaryotes, much of the nuclear DNA does not code for AAs. These
sections are called introns and can occur within genes and as muliple repeats between


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