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4.1.6Genetics, Variation and Interdependence

The Triplet Code — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 9 of 46 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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4.1.6 The Triplet Code — AQA A Level Biology 7402 notes: Describe the properties of the genetic code and use a table of mRNA codons to work out amino acid sequences.DNA contains four bases (A, T, C and G), but proteins are built from twenty different amino acids.A code using one …

  • Non-overlapping
  • Degenerate
  • Start and stop signals
  • frameshift
  • Exam-style question 1
  • Exam-style question 2

Section 1 of 9A code in groups of three

DNA contains four bases (A, T, C and G), but proteins are built from twenty different amino acids.

A code in groups of three
A code using one base per amino acid could specify only four, and one using two bases could specify only 16 (42), which is not enough.
Triplet code
A triplet code of three bases gives 64 (43) combinations, enough to specify twenty amino acids plus signals for starting and stopping.
Codon
In mRNA, each group of three bases is a codon.
Anticodon
The corresponding group of three bases on tRNA is the anticodon.

Section 2 of 9Properties of the genetic code

Triplet
Triplet three bases code for one amino acid.
Non-overlapping
Non-overlapping each base is part of only one triplet, and the code is read in order from a fixed starting point,
  • so a sequence is read as separate groups, such as AUG GCU CAA, rather than overlapping groups.
Degenerate
Degenerate most amino acids are coded by more than one triplet.
  • For example, leucine has six codons.
  • This means that some changes in the base sequence do not alter the amino acid, which reduces the effect of some mutations.
Universal
Universal the same triplets code for the same amino acids in almost all organisms, from bacteria to humans.
  • This allows genes to be transferred between species in genetic engineering and shows common ancestry.
  • A few exceptions exist, for example in mitochondria.
Start and stop signals
AUG is the start codon, which also codes for methionine.
  • UAA, UAG and UGA are stop codons, which code for no amino acid and signal the end of translation.

Section 3 of 9Reading the code

The base sequence on the template (antisense) strand of the DNA is transcribed to give the complementary mRNA, in which thymine is replaced by uracil. For example, a template strand reading TAC CGA TTG gives the mRNA codons AUG GCU AAC. Using the table of codons, these code for methionine, alanine and asparagine.

In the exam you will be given a codon table, so you do not need to memorise the codons, but you should be able to follow the logic: DNA template to mRNA codons to amino acids (using the mRNA codon table), or the other way round when working out which DNA sequence could code for a given peptide. Because the code is degenerate, working backwards from the amino acids to the bases gives more than one possible answer.

Section 4 of 9Effects of changes in the code

A substitution that changes one base may have no effect (silent, because of degeneracy), may change one amino acid (missense), or may create a stop codon (nonsense) which shortens the polypeptide.

Frameshift
An addition or deletion of one base shifts the reading of every triplet after it (a frameshift), usually altering the whole sequence after that point.
These mutations are discussed in the later topic on genetic mutations.

Section 5 of 9Links

The triplet code connects the structure of nucleic acids to protein structure: it allows the base sequence of a gene to set the primary structure of a polypeptide. It also underpins transcription and translation, and the use of DNA sequence comparisons for classification.

Section 6 of 9Worked reasoning about mutations and the code

Consider the effect of altering a codon in a short mRNA reading AUG GAA CCU.

1
A substitution of the third base of GAA by G gives GAG,
  • which also codes for glutamic acid,
  • so the polypeptide is unchanged: a silent mutation made possible by degeneracy.
2
Changing the first base so that GAA becomes AAA gives lysine instead of glutamic acid: a missense mutation,
  • which may or may not affect the protein, depending on where the change occurs and how different the new amino acid is.
3
Changing a codon like UAC (tyrosine) to UAA creates a stop codon, which is a nonsense mutation leaving a shorter polypeptide.
4
Deleting the first base of GAA changes the reading frame of all the codons after that, giving a completely different amino acid sequence.

When answering, always say which codons are involved and how the number and order of amino acids change. If asked to compare two sequences, mark the differences, and say whether the polypeptide is shorter, longer or the same length. Remember that tRNA anticodons are complementary to the mRNA codons, so the anticodon for GAA is CUU.

Section 7 of 9Exam-style questions

  1. Explain why the genetic code must be at least a triplet code. [2]2 marks

  2. A section of the DNA template strand has the sequence TAC GGT CTT. Give the mRNA sequence. [2] Explain why you cannot be certain of the DNA sequence if you are given only the amino acid sequence. [2]2 marks

  3. State what is meant by a universal genetic code and why it is important in genetic engineering. [2]2 marks

Section 8 of 9Key terms

TermMeaning
TripletA group of three bases that codes for one amino acid.
CodonA triplet of bases on mRNA.
AnticodonA triplet of bases on tRNA complementary to a codon.
Degenerate codeThe property that most amino acids are coded by more than one triplet.
Non-overlappingEach base is part of only one triplet.
Universal codeThe same codons code for the same amino acids in almost all organisms.
Stop codonA codon that signals the end of translation.
Start codonThe codon AUG, which signals the start of translation.

Section 9 of 9Exam technique

1
Calling the DNA triplet a codon. The codon is on mRNA; the DNA triplet is the template triplet.
2
Writing thymine in mRNA. mRNA contains uracil.
3
Saying degenerate means 'imperfect' or 'faulty' rather than that several codons code for the same amino acid.
4
Confusing overlapping with non-overlapping; each base is read only once.
5
Forgetting that the mRNA is complementary to the template strand, not identical to it.

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