9. Appendix 4: DNA codes for amino acid
This table is only for reference. You do not need to learn any of these codes.
It is included to show two things.
- That most amino acids can be made from more than one combination of bases.
- That some mutations are easy and some are difficult. Easy mutations only need one letter to change. The most complex mutations need to change all three bases.
| T | C | A | G | |
| T | TTT phenylalanine | TCC serine | TAT tyrosine | TGT cysteine |
| TTC phenylalanine | TCC serine | TAC tyrosine | TGC cysteine | |
| TTA leucine | TCA serine | TAA stop | TGA stop | |
| TTC leucine | TCG serine | TAG stop | TGC tryptophan | |
| C | CTT leucine | CCT proline | CAT histidine | CGT arginine |
| CTC leucine | CCC proline | CAC histidine | CGC arginine | |
| CTA leucine | CCA proline | CAG glutamine | CGA arginine | |
| CTG leucine | CCG proline | CAA glutamine | CGG arginine | |
| A | ATT isoleucine | ACT threonine | AAT asparagine | AGT serine |
| ATC isoleucine | ACC threonine | AAC asparagine | AGC serine | |
| ATA isoleucine | ACA threonine | AAA lysine | AGA arginine | |
| ATG methionine | ACG threonine | AAG lysine | AGG arginine | |
| G | GTT valine | GCT alanine | GAT aspartic acid | GGT glycine |
| GTC valine | GCC alanine | GAC aspartic acid | GGC glycine | |
| GTA valine | GCA alanine | GAA glutamic acid | GGA glycine | |
| GTG valine | GCG alanine | GAG glutamic acid | GGG glycine |
Valine is coded by four different combinations: GTT, GTC, GTA and GTG.
This shows that some changes will not change the amino acid.
If one mutation changes the third base from a T to C – as in GTT to GTC – the amino acid at that junction will still be valine.
Some amino acid changes require one base change, some require two base changes and some require all three bases to change.
The number of base changes needed to change one amino acid to another is related to how quickly a mutation develops.
This chart shows that the M184V is an easy mutation – like getting one bell on a fruit machine.
To change from methionine (M) to valine (V) only requires a change at one base: ATG to GTG.
The mutation M184V that stops 3TC from working, is commonly the first mutation to be detected in any 3TC-containing combination if that treatment fails. As an easy mutation, it can occur quickly. With 3TC monotherapy (ie when 3TC is the only drug) resistance would be likely to occur within two weeks.
By comparison, the T215Y mutation is more complex – perhaps like getting three cherries.
Threonine (T) can be made from four different base combinations: ACT, ACC, ACA and ACG.
Tyrosine (Y) can be made from only two: TAT and TAC.
Importantly, a change from threonine to tyrosine needs at least the first two bases to change (from ACT to TAT or ACC to TAC). In some cases, it might need all three bases to change (from ACA or ACG to TAT or TAC).
The T215Y mutation is one of the main mutations associated with resistance to AZT. However, as a more complex mutation, it is less likely to occur by chance. In practice, this mutation might take six months to develop in someone taking AZT monotherapy.
These mutations are like fruit machines: getting one bell is easier than getting a row of three cherries to show. You need to play more often and for longer to get the difficult combinations.
Last updated: 1 January 2023.