OCR Specification focus:
‘Define the triplet, non-overlapping, degenerate, and universal nature of the genetic code.’
The genetic code is the cellular language linking nucleotide sequences in DNA and RNA to amino acid sequences in proteins, ensuring precise control of gene expression and protein synthesis.
The Concept of the Genetic Code
The genetic code is a set of rules that determines how the sequence of nucleotides in messenger RNA (mRNA) specifies the sequence of amino acids in a polypeptide. It acts as the bridge between the genotype (genetic information) and the phenotype (observable traits) by dictating how genetic instructions are translated into proteins.
Each group of three nucleotide bases on mRNA, known as a codon, codes for one specific amino acid or a control signal such as a start or stop codon. Because proteins are central to virtually every biological process, the genetic code is fundamental to all living systems.
The Triplet Nature of the Code
Each amino acid in a protein is specified by a triplet of bases, called a codon, on the mRNA strand. With four different RNA bases — adenine (A), uracil (U), guanine (G), and cytosine (C) — there are 43=644^3 = 6443=64 possible codons, more than enough to code for the 20 naturally occurring amino acids.
The standard genetic code table maps each three-base codon to its amino acid (or to start/stop signals). The multiple codons per amino acid illustrate the degenerate nature of the code, while its widespread conservation across life reflects near universality. The layout emphasizes that codons are read as non-overlapping triplets. Source.
Triplet code: A system where three nucleotide bases in sequence (a codon) specify one amino acid in a polypeptide chain.
The triplet nature ensures that sufficient combinations exist to encode all amino acids, as well as start and stop signals. The discovery of this triplet code was key to understanding how DNA directs protein synthesis.
Non-Overlapping Code
The genetic code is non-overlapping, meaning that each base in an mRNA sequence is read only once, as part of a single codon. When the ribosome reads a sequence, it begins at a start codon and reads continuously in triplets until it encounters a stop codon.
Reading-frame diagram showing the same sequence partitioned into three different triplet frames. In actual translation, the ribosome selects one frame at the start codon and proceeds codon by codon without overlap. This highlights why insertions or deletions can cause frameshift effects. Source.
For example, if the sequence AUGCCUAAA is read as AUG, CCU, AAA, each codon contributes one amino acid.
In a non-overlapping code, bases are not reused in different codons.
This ensures accurate translation and prevents confusion in amino acid placement.
Non-overlapping code: A system in which each nucleotide base contributes to only one codon and is read once during translation.
A mutation that adds or removes one base can shift the entire reading frame, creating a frameshift mutation, which usually produces a completely different and non-functional protein.
Degenerate Code
The genetic code is degenerate because most amino acids are represented by more than one codon. There are 64 codons in total, but only 20 amino acids; therefore, multiple codons can specify the same amino acid.
Degenerate code: The redundancy of the genetic code, where more than one codon specifies the same amino acid.
This degeneracy provides fault tolerance in the genetic system:
Silent mutations may occur when a change in one base does not alter the amino acid produced, due to codon redundancy.
It reduces the likelihood that random mutations will produce harmful effects.
For example:
GAA and GAG both code for glutamic acid.
This flexibility helps maintain protein integrity despite genetic variation.
Universal Code
The genetic code is almost universal across all known organisms, from bacteria to humans. The same codon specifies the same amino acid in nearly all species.
Universal code: The principle that the same codons specify the same amino acids in almost all living organisms.
This universality is strong evidence for a common evolutionary origin of life. The few known exceptions, such as in some mitochondrial or protozoan genomes, involve minor deviations but the overall pattern remains conserved.
The universality of the code allows genes from one organism to function in another, forming the basis of genetic engineering and biotechnology applications.
The Start and Stop Codons
Certain codons perform special regulatory functions in translation:
Start codon: The codon AUG codes for methionine and signals where translation begins on the mRNA strand.
Stop codons: UAA, UAG, and UGA do not code for any amino acid but instead signal the end of translation.
These codons ensure that polypeptides are synthesized correctly from a defined starting point to a precise termination point.
Codon–Anticodon Interaction
During translation, the sequence of codons on the mRNA interacts with anticodons on transfer RNA (tRNA) molecules.
Translation schematic showing a ribosome reading mRNA in non-overlapping triplets. A tRNA’s anticodon pairs with the mRNA codon, delivering the specified amino acid to the growing polypeptide. The diagram also indicates termination when a stop codon is reached; this extra detail complements the notes’ start/stop section. Source.
Anticodon: A sequence of three bases on a tRNA molecule complementary to a codon on mRNA, enabling correct amino acid addition.
Each tRNA molecule carries a specific amino acid, which is joined to the growing polypeptide chain by the ribosome as codon–anticodon pairing ensures accurate translation of the genetic message.
Redundancy and Reading Frame Maintenance
The degeneracy and non-overlapping nature of the genetic code combine to maintain reliability in translation. Once the ribosome establishes the reading frame from the start codon, it proceeds codon by codon without overlap, ensuring that each amino acid is correctly sequenced.
Insertion or deletion of a base alters the reading frame and changes all subsequent codons.
The triplet, non-overlapping system is therefore crucial for protein fidelity.
Mutation resilience and universality make the genetic code a robust system, capable of preserving function despite occasional base changes.
Summary of the Four Defining Features
The genetic code can be described using four essential characteristics specified in the OCR syllabus:
Triplet – Three bases code for one amino acid.
Non-overlapping – Each base is read once in one codon.
Degenerate – Most amino acids have multiple codons.
Universal – The same codons specify the same amino acids in nearly all organisms.
Together, these properties enable accurate and efficient translation of genetic information, supporting life’s continuity and adaptability across all biological systems.
FAQ
Stop codons (UAA, UAG, and UGA) do not code for any amino acid. Instead, they signal the end of translation when reached by the ribosome.
A release factor binds to the stop codon in the A site of the ribosome, triggering the release of the newly synthesized polypeptide chain. The ribosome then detaches from the mRNA, and its subunits separate.
This mechanism ensures proteins are produced with the correct length and amino acid sequence.
The start codon AUG codes for methionine, marking the beginning of translation. Ribosomes recognize this codon as the universal start signal.
Even when methionine is not part of the final protein, it is initially added and may later be removed during post-translational modification.
This process ensures that all protein synthesis begins at a consistent, defined point along the mRNA strand.
A mutation that converts a codon into a stop codon is known as a nonsense mutation.
Consequences include:
Premature termination of translation.
Production of a truncated polypeptide, usually non-functional.
Possible degradation of the mRNA through nonsense-mediated decay, preventing wasteful protein synthesis.
The overall effect depends on where the mutation occurs within the gene — early stop codons are usually more damaging.
The wobble hypothesis states that the third base of a codon can pair flexibly with more than one complementary base in the anticodon of tRNA.
This explains the degeneracy of the genetic code — fewer tRNA molecules are needed than there are codons.
For example:
The codons GAA and GAG both code for glutamic acid because one tRNA can recognize both through wobble pairing.
This mechanism maintains translation efficiency while preserving accuracy in protein synthesis.
Although the genetic code is highly conserved, a few organisms and organelles exhibit minor variations.
Examples include:
Mitochondria, where UGA, normally a stop codon, can code for tryptophan.
Some protozoa use different stop codons for specific amino acids.
These exceptions arose through evolutionary divergence but remain rare. The overall near-universality still supports the concept of a shared evolutionary ancestry among all forms of life.
Practice Questions
Question 1 (2 marks)
Explain what is meant by the term degenerate genetic code and state one advantage of this feature for living organisms.
Mark scheme:
The genetic code is degenerate because more than one codon codes for the same amino acid (1 mark).
This protects against mutations, as a change in one base may not alter the amino acid sequence (1 mark).
Question 2 (5 marks)
The genetic code is described as triplet, non-overlapping, degenerate, and universal.
Explain what each of these terms means and describe how these features contribute to accurate and reliable protein synthesis.
Question 2 (5 marks)
Triplet: Each amino acid is coded for by three bases (a codon) on the mRNA (1 mark).
Non-overlapping: Each base is read once and is part of only one codon (1 mark).
Degenerate: Most amino acids have more than one codon (1 mark).
Universal: The same codon codes for the same amino acid in almost all organisms (1 mark).
Explanation of contribution to protein synthesis (1 mark total, any one of the following):
The triplet and non-overlapping nature ensures correct reading of the sequence.
Degeneracy reduces the impact of mutations on the resulting protein.
Universality allows genes to be transferred between species and expressed correctly (e.g. in genetic engineering).
