How does a sequence of bases specify a sequence of amino acids?
The genetic code is a triplet, degenerate, near-universal code that maps codons to amino acids.
The properties of the genetic code: triplet codons, degeneracy, the start and stop codons, and how a codon table is used to translate mRNA into amino acids.
Reviewed by: AI editorial process; not yet individually human-reviewed
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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.
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.
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.