A sequence of three DNA bases, called a triplet, codes for a specific…

The idea

Each sequence of three DNA bases, a triplet, codes for one amino acid. Three properties of this code matter. It is universal: the same triplet codes for the same amino acid in almost every organism, which is why a human gene inserted into a bacterium can yield the human protein, provided the gene is in a form the bacterium can express (for example cDNA without introns, or with suitable processing). It is non-overlapping: each base belongs to one triplet only and the bases are read in a fixed reading frame from a fixed starting point, which is why adding or deleting a base causes a frame shift that alters every triplet downstream, whereas substituting one base alters only that triplet. It is degenerate: most amino acids are coded for by more than one triplet, so a substitution may not change the amino acid sequence.

What earns the marks

• The genetic code being universal, that is the same triplets code for the same amino acids in almost all organisms.
• Non-overlapping meaning that each base is part of only one triplet, read in one fixed frame.
• Degenerate meaning that most amino acids are coded for by more than one triplet.
• Using degeneracy to explain why a substitution may not change the amino acid sequence.
• Explaining that a frame shift alters every triplet downstream, whereas a substitution alters only one triplet.
• Stating that a human gene inserted into a bacterium can produce the human protein because the code is universal, with the caveat that introns may need to be removed or the gene supplied as cDNA.

Common mistakes and how to put them right

• Answering that the code is degenerate when asked why a bacterium can make a human protein, which is rejected.

• Writing that DNA is universal without qualifying it, which gains nothing.

• Saying degenerate means one triplet can code for several amino acids, which is the reverse of the truth.

• Confusing the triplet on DNA with the codon on messenger RNA and the anticodon on transfer RNA.

• Explaining a frame shift as affecting only the triplet in which the base was added or lost.

• Claiming that transcription and translation are identical in all species, ignoring differences such as splicing and coupled transcription-translation in prokaryotes.

Exam tips

• Write universality out in full as the same triplets coding for the same amino acids in almost all organisms; the bare word is not enough.
• Keep triplet, codon and anticodon straight by naming the molecule each one sits on.
• When using the human gene in bacterium example, mention that introns may need to be removed or the gene supplied as cDNA.