4.2.3Additional lessons
The Genetic Code — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 32 of 101
Part 1 of 5Properties of the Code
The genetic code links base sequences to amino acid sequences.
Triplet code
- Each codon is three bases long, because two bases would give only 16 combinations, which is too few for 20 amino acids.
Degenerate
- Most amino acids are coded by more than one codon, so some base changes do not alter the protein made.
Universal
- Nearly all organisms use the same code, which supports a common ancestry and allows genes to be moved between species.
Non-overlapping
- Bases are read in separate groups of three, so each base belongs to one codon only.
| Codons | Number | Meaning |
|---|---|---|
| Total possible | 64 (4³) | All triplets of four bases |
| Amino acid codons | 61 | Specify 20 amino acids |
| Stop codons | 3 | UAA, UAG, UGA |
| Start codon | 1 | AUG, also codes for methionine |
Part 2 of 5Using a Codon Table
1
Write down the mRNA sequence in the 5' to 3' direction and mark the first AUG as the start of translation.
2
Split the sequence into groups of three bases from the start codon without leaving gaps or overlaps.
3
Look up each codon using the first base, then the second, then the third, to find the amino acid.
4
Continue until a stop codon appears, then list the amino acids in order as the primary structure.
Part 3 of 5Consequences of the Code
Consequences of the Code
Genetic engineering
- Because the code is universal, a human gene can be expressed in bacteria which then make a human protein.
Mutation effects
- Degeneracy means a change in the third base of a codon often has no effect on the amino acid, called a silent change.
Efficiency
- Having 61 sense codons for 20 amino acids gives a buffer against the damage caused by point mutations.
Exceptions
- A few organelles and organisms use slightly different codon meanings, so universality is almost but not entirely complete.
Part 4 of 5Worked Example
Question
- A polypeptide of 150 amino acids is made from a gene.
- Calculate the minimum number of bases in the mRNA coding region including the stop codon.
- Each amino acid needs one codon of three bases: 150 × 3 = 450 bases.
- Add one stop codon of three bases: 450 + 3 = 453.
Answer 453 bases (a longer mRNA is expected because of untranslated regions).
Part 5 of 5Exam Checklist & Tips
4.2.3 – Key Points for the Exam
- Codon Three consecutive bases of mRNA that code for an amino acid or stop.
- Degenerate Describes a code in which several codons can specify the same amino acid.
- Universal Used with the same meanings by almost all living things.
- Stop codon A codon that ends translation and does not code for an amino acid.
- Reading frame The way the base sequence is divided into triplets.
Common Mistakes
- Stating that the code is universal without the word nearly or almost, because exceptions exist.
- Mixing up degenerate with ambiguous; each codon gives one amino acid only.
- Counting 64 amino acids; there are 20 amino acids but 64 codons.
- Reading the DNA template instead of the mRNA when using a codon table.
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