The standard codon chart
| First base | Second base | |||
|---|---|---|---|---|
| U | C | A | G | |
| U | UUU PheUUC PheUUA LeuUUG Leu | UCU SerUCC SerUCA SerUCG Ser | UAU TyrUAC TyrUAA StopUAG Stop | UGU CysUGC CysUGA StopUGG Trp |
| C | CUU LeuCUC LeuCUA LeuCUG Leu | CCU ProCCC ProCCA ProCCG Pro | CAU HisCAC HisCAA GlnCAG Gln | CGU ArgCGC ArgCGA ArgCGG Arg |
| A | AUU IleAUC IleAUA IleAUG Met | ACU ThrACC ThrACA ThrACG Thr | AAU AsnAAC AsnAAA LysAAG Lys | AGU SerAGC SerAGA ArgAGG Arg |
| G | GUU ValGUC ValGUA ValGUG Val | GCU AlaGCC AlaGCA AlaGCG Ala | GAU AspGAC AspGAA GluGAG Glu | GGU GlyGGC GlyGGA GlyGGG Gly |
The standard codon chartstringmash.com/codon-chart
Reading the codon chart
Find the first base of the codon down the side, the second along the top, and the third inside the box. AUG, in the A row under U, codes for methionine and is also where translation starts. UAA, UAG and UGA are the stop codons, and they end the protein.
There are 64 codons and only 20 amino acids, so most amino acids have more than one codon. Leucine, serine and arginine have six each. Tryptophan and methionine have one.
The chart uses mRNA letters, so U where DNA has T. The translator does that swap for you.
Using the translator
Paste a sequence and set Sequence is. A coding strand reads straight across, with each T turned into U. A template strand is complemented base by base, A to U, T to A, C to G and G to C. It's read 3′ to 5′ from left to right, the way most worksheets print it. The page opens on a template strand.
Reading starts at the first AUG and stops at the first stop codon, as a ribosome does. Set Reading to From the first base and it translates every codon, stops included, which is useful for checking a worksheet reading frame.
Show picks three-letter amino acids, one-letter codes, or the mRNA alone, so students can check the transcription step before the translation. Spaces, numbers and a FASTA header line are ignored.
UUU, the first word
In May 1961, at the National Institutes of Health, Marshall Nirenberg and his postdoctoral fellow J. Heinrich Matthaei added an artificial RNA made of nothing but uracil to an extract of E. coli. The extract built a protein made of nothing but phenylalanine. UUU codes for phenylalanine, and the first codon was read.
Poly-C gave proline next. Two other labs had tried poly-A and seen nothing, because the protein it makes, polylysine, dissolves in the acid they used to collect proteins.
Nirenberg presented the result in Moscow that August, first to a small group and then, at Francis Crick's urging, to about a thousand people. He shared the 1968 Nobel Prize with Robert Holley and Har Gobind Khorana for reading the rest of the code.
Questions
How do I convert DNA to amino acids?
Transcribe the DNA to mRNA, then read the mRNA three bases at a time on the codon chart. Paste the DNA here, choose coding or template strand, and the translator does both steps.
What's the difference between the coding and template strand?
The coding strand has the same sequence as the mRNA, with T instead of U. The template strand is its complement, the one RNA polymerase actually reads. Worksheets give either, so check which one yours is.
Which codons are start and stop codons?
AUG is the start codon and codes for methionine. UAA, UAG and UGA are stop codons and code for no amino acid.
Does this work for mitochondrial DNA?
No. It uses the standard genetic code, NCBI's table 1. Mitochondria and some microbes read a few codons differently.