The Genetic Code

Biology Unit 4

The Genetic Code

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Biology Unit 4
Revision Notes
Topic 6: Infection, Immunity and Forensics
2. Explain the nature of the genetic code (triplet code,
nonoverlapping and degenerate).
DNA is the molecule that carries all our inherited information. It has a double helix structure, which
contains chains of nucleotide bases ­ adenine, thymine, cytosine and guanine ­ joined together in
pairs. Each different type of living organism must have a distinctive genetic message that produces
particular cells.
Also, within each species are many individuals, each of which is unique and therefore must have its
own unique genetic message. The information needed to give this enormous variety is found within
the DNA. The sequence of the base pairs in the molecule is used as the genetic code. The code
determines which amino acids are joined together to form proteins.
Property of the Why it is important
genetic code
Triplet code With 20 amino acids and start and stop signals to code for and only four bases
to do it, one base per amino acid will not do and neither will two. Using 3 bases
gives 64 codons which is more than enough.
Non-overlapping Each set of 3 bases forms one triplet. The triplets do not overlap, so no base
from one triplet is part of another triplet, avoiding confusion about which
amino acid is being coded for.
degenerate Some amino acids have more than one codon. E.g. there are four different
codons for the amino acid proline. As long as the codon starts with CC the
amino acid proline will be put into the polypeptide. The code is said to be
degenerate. This offers some protection against mutation.
Text Book: p. 72 - 74


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