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The triplet code
Starter
A DNA molecule is 23% guanine.
What percentage of each of the
other bases would it contain?
G = 23%
C + G = 46%
C = 23%
A = 27%
So A + T = 54%
T = 27%
(to make a total of 100%)
Aims:
•What is a gene?
•How do genes code for polypeptides?
A gene is a section of DNA nucleotides that
codes for the sequence of amino acids
that make up a particular polypeptide.
Depending on the specific groups in each
amino acid the polypeptide chain then
forms secondary & tertiary structures
The triplet code
• Only 20 amino acids
• Each amino acid has its own code of bases on
the DNA
• 3 bases code for an amino acid (triplet code)
• 64 codes so some amino acid have more than
one code.
• In eukaryotes much of the DNA doesn’t code for
amino acids – introns
• Sections of DNA that code for a.a - exons
• Introns found between and in genes
Features of the triplet code
• Few amino acids have only a single triplet
code
• Most have between 2-6 triplet codes each
• Degenerate code
• Always read in one direction
• Start of a sequence is always the same triplet
code – methionine
• 3 triplet codes are stop codons
• Code is non-overlapping and universal
Understanding checkpoint
An enzyme is a protein that is responsible
for speeding up chemical reactions. An
enzyme’s shape is crucial to it’s function.
How would a change in one base result in
an enzyme becoming non-functional?
Aims: Explain to your partner
•What is a gene?
•How do genes code for polypeptides?