Question 1

How do you read a codon chart?

Accepted Answer

Read the three bases of the codon in order, from the 5-prime end. The first base picks the row block, the second base picks the column, and the third base picks the line within that cell. For example, for AUG you find the A row block, move to the U column, then look at the G line, which gives methionine. Always read an mRNA codon 5-prime to 3-prime, because reading it backwards gives a completely different amino acid.

Question 2

What are the start and stop codons?

Accepted Answer

There is one start codon, AUG, which codes for methionine and sets the reading frame for translation. There are three stop codons, UAA, UAG, and UGA, which code for no amino acid at all. Instead of being read by a tRNA, a stop codon is recognized by a release factor that ends translation and frees the finished polypeptide. Because AUG doubles as the start signal, nearly every newly made protein begins with methionine, though it is often removed afterwards.

Question 3

What does it mean that the genetic code is degenerate?

Accepted Answer

Degenerate means most amino acids are coded by more than one codon. Leucine, serine, and arginine each have six codons, while methionine and tryptophan have only one each. The redundancy sits mostly in the third base, which is why it is called the wobble position: changing the third base often gives the same amino acid. This buffers the cell against mutations, since many third-base changes are silent and do not alter the protein at all.

Question 4

What is the wobble position?

Accepted Answer

The wobble position is the third base of a codon, where base pairing with the tRNA anticodon is less strict. Because of this looser pairing, a single tRNA can often recognize several codons that differ only at that third base. This is why the genetic code clusters so neatly in the chart: in many cells all four third-base options give the same amino acid, and a mutation at that position is frequently silent.

Question 5

What is the difference between a DNA codon chart and an RNA one?

Accepted Answer

They carry the same information but use different letters. RNA uses uracil (U) where DNA uses thymine (T), so the RNA codon AUG appears as ATG on the coding strand of DNA. Codon charts are normally written in RNA because translation reads mRNA, but a DNA version is handy when you are working from a gene sequence. This chart switches between the two so the codons match whichever sequence you have in front of you.

Fact	Value	Why it matters
Total codons	64 (4³)	Three bases, four options each
Coding for amino acids	61	64 minus the three stops
Start codon	AUG (Met)	Sets the reading frame; also codes Met mid-sequence
Stop codons	UAA, UAG, UGA	Read by release factors, not tRNA
Most codons	Leu, Ser, Arg (6 each)	Heavily buffered against mutation
Fewest codons	Met, Trp (1 each)	Any change alters the protein

Codon Chart — The Genetic Code

How to read the chart

The numbers worth memorizing

FAQ