The genetic code is a triplet, degenerate, near-universal code that maps codons to amino acids.

What this dot point is asking

You need to describe the key properties of the genetic code and use a codon table to translate an mRNA sequence into a sequence of amino acids.

What the genetic code is

The genetic code is the relationship between the sequence of bases in mRNA and the sequence of amino acids in a polypeptide. Because there are only 4 bases but 20 standard amino acids, the cell reads bases in groups rather than singly.

Key properties of the code

It is a triplet code

Bases are read three at a time. Each group of three mRNA bases is a codon. Three bases give 4 × 4 × 4 = 64 possible codons, more than enough to specify 20 amino acids plus start and stop signals.

It is degenerate

Because there are 64 codons but only 20 amino acids, most amino acids are coded for by more than one codon. This redundancy is called degeneracy. For example, leucine has six codons. Degeneracy means some base changes (especially in the third position) do not change the amino acid - these are silent.

It is non-overlapping and has no gaps

Codons are read consecutively from a fixed starting point, base by base, without skipping or sharing bases. This is why the reading frame set by the start codon is critical.

It is (nearly) universal

The same codons specify the same amino acids in almost all organisms, from bacteria to humans. This universality is strong evidence for a common evolutionary origin of life and is what makes genetic engineering between species possible. A few minor exceptions exist (for example, slightly different codon assignments in mitochondria), which is why the code is described as "near-universal" rather than perfectly universal.

It is unambiguous

Each codon specifies one and only one amino acid. Degeneracy works one way only: an amino acid may have several codons, but a given codon never codes for two different amino acids. This is essential for accuracy, because it means a ribosome reading a particular codon always inserts the same amino acid.

Why 64 codons?

The arithmetic explains the structure of the code. With 4 bases, a singlet code would specify only $4$ amino acids and a doublet code only $4 \times 4 = 16$, both too few for 20 amino acids. A triplet code gives $4^3 = 64$ combinations, comfortably enough to encode 20 amino acids plus start and stop signals. Of the 64 codons, 61 are sense codons (specifying amino acids) and 3 are stop codons.

Reading a codon table

A codon table lists the amino acid specified by each of the 61 sense codons. To use it, read the mRNA from the start codon in groups of three, look up each codon, and list the amino acids in order until you reach a stop codon.

Why degeneracy matters

Degeneracy buffers the organism against some mutations. A point mutation in the third base of a codon often produces a silent mutation because the new codon still codes for the same amino acid. This links the genetic code directly to the study of mutations and their effects.

Codons, anticodons and the template strand

Exam questions often ask you to move between three sequences, so keep them straight. The DNA template strand is read by RNA polymerase; the mRNA codon is its complementary copy (with U for T); the tRNA anticodon is complementary to the codon. For example, a template triplet $3'\text{-TAC-}5'$ gives the mRNA codon $5'\text{-AUG-}3'$, which is read by a tRNA with anticodon $3'\text{-UAC-}5'$ carrying methionine. Always note that codons are written $5'$ to $3'$ unless told otherwise, because that is the direction the ribosome reads.

Connecting to the bigger picture

The properties of the genetic code explain why some mutations are harmless and others are catastrophic, and why genes can be transferred between species in biotechnology. Understanding codons is essential for predicting the effect of mutations and for interpreting transcription and translation problems. A frameshift caused by an insertion or deletion shifts the reading frame so every downstream codon is misread, which is why the non-overlapping, fixed-frame nature of the code matters so much for predicting mutation effects.