What you need to know
- 2.11Triplet, non-overlapping and degenerateDescribes the genetic code: most amino acids are coded for by more than one codon. genetic codeThe rules by which triplets of bases code for amino acids. It is a triplet, non-overlapping, degenerate and universal code..
- 2.12A geneA sequence of DNA bases that codes for a polypeptide or a functional RNA. as a DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. base sequence coding for a polypeptideA chain of many amino acids joined by peptide bonds.’s amino-acid sequence.
By the end, you should be able to…
- Define gene and codonA sequence of three bases on mRNA that codes for one amino acid or a stop signal. precisely.
- Explain triplet, non-overlapping and degenerate.
- Use a supplied codon key without confusing DNA and RNARibonucleic acid: a single-stranded polynucleotide with ribose and uracil. mRNA, tRNA and rRNA take part in protein synthesis..
Build your understanding
From a base sequence to an amino-acid sequence
For this specification, a geneA sequence of DNA bases that codes for a polypeptide or a functional RNA. is a sequence of DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. bases that codes for the amino-acid sequence of a polypeptideA chain of many amino acids joined by peptide bonds.. Its order of bases matters because that order determines the sequence used during protein synthesis. Proteins may contain more than one polypeptide chain.
During expression, information is copied into messenger RNAA single-stranded RNA copy of a gene that carries the code from the nucleus to a ribosome.. A codonA sequence of three bases on mRNA that codes for one amino acid or a stop signal. is a group of three bases on mRNA. Codon tables conventionally show RNARibonucleic acid: a single-stranded polynucleotide with ribose and uracil. mRNA, tRNA and rRNA take part in protein synthesis. bases, so they contain U rather than T. Check the labels before using any sequence or table.
Three bases provide enough combinations
Four different bases provide 4¹ = 4 one-base combinations, 4² = 16 two-base combinations and 4³ = 64 three-base combinations. A triplet code provides enough different combinations to specify the 20 amino acidThe monomer of proteins: a central carbon with an amine group, a carboxyl group, a hydrogen and an R group. used in ordinary protein synthesis as well as stop signals.
The standard code is degenerateDescribes the genetic code: most amino acids are coded for by more than one codon.: several different codons can specify the same amino acid. This does not mean that a single codon normally specifies several different amino acids. For example, UUU and UUC both specify phenylalanine.
Read a continuous sequence in the correct frame
The code is non-overlapping: in a given reading frame, each base belongs to one codon, rather than being reused in the next codon. Read AUGGCUUUCUAA as AUG | GCU | UUC | UAA in the frame shown. It specifies methionine–alanine–phenylalanine followed by a stop signal using the supplied key.
The start position sets the reading frame. A stop codon ends translationMaking a polypeptide on a ribosome, using the sequence of codons on mRNA. tRNAs bring the amino acids. and does not code for an amino acid. Do not count it as one more amino acid when calculating polypeptide length. Start and stop roles are developed in 2B6.
Use the code to make careful predictions
Degeneracy explains why some substitutions leave the amino-acid sequence unchanged. Other substitutions change an amino acid or create a stop codon. You must inspect the actual codons; degeneracy does not protect every position from every change.
A simplified coding region for n amino acids contains 3n bases for those amino acids, plus three more for a stop codon if included. This is not the total length of an entire chromosomeA long DNA molecule wound around histone proteins. After replication it is two sister chromatids joined at a centromere. or of a eukaryoticDescribes cells with a nucleus and membrane-bound organelles, such as animal and plant cells. gene with additional non-coding regions. State what your count includes.
NOT: degenerate does not mean one codon codes for several amino acids.
- Non-overlapping: each base is in one codon in that frame
- Example frame: AUG GCU UUC UAA
- Changing the start changes every following codon
- A stop codon ends the chain and is not counted as an amino acid
Two traps this diagram blocks
- Do not translate a DNA template directly with an RNA codon table without first making the mRNA.
- Do not say a one-base insertion changes only one amino acid. It shifts the reading frame.
From the diagram alone
Question
Explain what is meant by a triplet, non-overlapping and degenerate code. [3]
Show a model answer
A codon is three mRNA bases. In a reading frame those triplets do not share bases with the next codon. Degenerate means more than one codon can specify the same amino acid. It does not mean one codon specifies several amino acids.
| Codon (5′ → 3′) | Meaning |
|---|---|
| AUG | Methionine / start in the context used |
| GCU | Alanine |
| UUU / UUC | Phenylalanine |
| UUA | Leucine |
| UAA | Stop |
Make your reasoning visible
Define degeneracy the right way round
Several codonA sequence of three bases on mRNA that codes for one amino acid or a stop signal. can specify one amino acid. Avoid “one codon makes several amino acidThe monomer of proteins: a central carbon with an amine group, a carboxyl group, a hydrogen and an R group.”.
Check the sequence type
Use messenger RNAA single-stranded RNA copy of a gene that carries the code from the nucleus to a ribosome. codons with an RNARibonucleic acid: a single-stranded polynucleotide with ribose and uracil. mRNA, tRNA and rRNA take part in protein synthesis. codon key. A DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. template sequence must first be converted by complementary pairing, including U opposite A.
State your counting assumptions
Separate amino-acid codons from stop codons and separate a coding sequence from a whole geneA sequence of DNA bases that codes for a polypeptide or a functional RNA..
Key explanations
- AUG GCU UUC UAA is read AUG | GCU | UUC | UAA, not by reusing bases in the next codon.
Avoid this misconception
The claim: A three-base code means there are only three different bases.
The correction: Each codon has three positions, each chosen from four possible bases.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Why is a two-base code insufficient to specify 20 different amino acids if each codeword has one meaning?
Show suggested answer
Four bases in two positions give 4² = 16 combinations, fewer than 20. Three positions give 64.
Practice question
Retrieval and explanation · suggested answer
Using the key below, translate AUG | GCU | UUC | UAA and state the number of amino acids.
Show suggested answer
Methionine–alanine–phenylalanine: three amino acids. UAA is a stop codon, not an amino acid.
Practice question
Retrieval and explanation · suggested answer
A simplified sequence contains 90 amino-acid codons and one stop codon. How many RNA bases does it contain?
Show suggested answer
(90 + 1) × 3 = 273 bases. Of these, 270 specify amino acids and three form the stop codon.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
Two coding-sequence substitutions change UUU to UUC and UUU to UUA. Using the supplied key, which leaves the amino-acid sequence unchanged? Does this prove that the other substitution changes the phenotype?
Show a hint
Separate codon meaning from the final effect on protein function.
Explore the reasoning
UUU → UUC still specifies phenylalanine. UUU → UUA changes phenylalanine to leucine, but the effect on protein function depends on the position and chemical consequences. A changed amino acid does not guarantee an observable phenotype.
Recall this lesson
Lesson recall questions
Question 1 of 5: The genetic code is a ________ code that is ________ and ________.
The genetic code
Practice question
Retrieval and explanation · suggested answer
The genetic code is a ________ code that is ________ and ________.
Lesson: 2B 5: The Genetic Code
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Cracking the code
2B · The Genetic CodeThe 1968 Nobel Prize in Physiology or Medicine recognised Holley, Khorana and Nirenberg for work interpreting the genetic code and its role in protein synthesis. Artificial RNA sequences…Specification 2.11Read & use the scienceClose article
Why this matters
The 1968 Nobel Prize in Physiology or Medicine recognised Holley, Khorana and Nirenberg for work interpreting the genetic code and its role in protein synthesis. Artificial RNA sequences allowed researchers to connect controlled inputs with the polypeptides produced. A repeating U sequence is particularly informative because every triplet is UUU; mixed repeating sequences need more careful interpretation of possible reading frames. This reveals the code as an experimentally tested relationship rather than a table scientists simply assumed. The detailed historical methods are enrichment. The specification connection is the logic of triplet coding and the need for enough experiments to distinguish competing assignments.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.11 understand the nature of the genetic code (triplet code, non-overlapping and degenerate)
Use the science
If an artificial RNA made only of U produced a chain of phenylalanine, what codon assignment would that support?
Explore a possible response
Every complete triplet is UUU, so it supports assigning UUU to phenylalanine. Further experiments are needed for other codons.
Explore the source
- The genetic code: historyNobel Prize
Could a cell use fewer genetic code words?
2B · The Genetic CodeThe genetic code is degenerate: several codons can specify the same amino acid. In 2019, researchers used that redundancy to construct an E. coli genome in which selected…Specification 2.11 · 2.12Read & use the scienceClose article
Why this matters
The genetic code is degenerate: several codons can specify the same amino acid. In 2019, researchers used that redundancy to construct an E. coli genome in which selected codons were replaced with alternatives, producing an organism with a reduced set of codons. This was not a change from triplets to pairs, nor did it mean the bacterium stopped making the usual twenty amino acids. The work also showed why apparently synonymous changes need testing: translation is a biological process, and codon choices can matter to how successfully it runs. Synthetic biology turns a property of the code into a design question.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.11 understand the nature of the genetic code (triplet code, non-overlapping and degenerate)
- 2.12 know that a gene is a sequence of bases on a DNA molecule that codes for a sequence of amino acids in a polypeptide chain
Use the science
How can replacing a codon change the DNA sequence without changing the amino-acid sequence, and why is that not enough to prove the cell is unaffected?
Explore a possible response
A synonymous codon specifies the same amino acid, but translation efficiency or other sequence-dependent processes can still change. Organism-level function needs testing.
Explore the source
- Total synthesis of Escherichia coli with a recoded genomeFredens et al., Nature (2019) · Abstract