DNA, Genes, and the Genetic Code Revision Notes | A Level AQA Biology

1. From DNA to Protein: Why the Code Matters

Your entire body — every enzyme, every structural protein, every hormone — is built from instructions written in DNA. The question is: how does a molecule made of just four different bases produce thousands of different proteins? The answer lies in the genetic code, and once you understand its logic, everything else in molecular biology clicks into place.

Think of DNA as a master recipe book locked away in the nucleus. Each recipe is a gene, and each gene specifies exactly which amino acids to string together, and in which order, to build one particular polypeptide. Get the sequence right, and you get a functional protein. Change even one base in the wrong place, and the whole recipe can fail.

2. What Is a Gene?

A gene is a base sequence of DNA that codes for the amino acid sequence of a polypeptide or a functional RNA molecule (such as tRNA or rRNA). That definition is precise and examiners expect you to reproduce it accurately. Notice two things: it is a base sequence (not a protein, not an RNA molecule), and it codes for either an amino acid sequence (the primary structure of a polypeptide) or, in some cases, a functional RNA.

The phrase 'amino acid sequence of a polypeptide' is doing important work here. A polypeptide is a chain of amino acids joined by peptide bonds. The order of those amino acids — determined entirely by the gene — dictates how the polypeptide folds, and therefore what function it performs. Change the sequence, change the protein, change the function.

3. The Locus: Every Gene Has a Fixed Address

Now that you know what a gene is, here is a crucial detail about where it lives. Every gene occupies a fixed position on a particular DNA molecule — this position is called its locus (plural: loci). Picture a chromosome as a very long street, and each gene as a house with a specific address that never changes.

Because the locus is fixed, every member of the same species carries the gene for, say, haemoglobin at exactly the same position on chromosome 11. This consistency is what makes genetics predictable and heritable. Different versions of the same gene (alleles) still occupy the same locus — they are just different 'editions' of the same recipe at the same address.

The concept of the locus also underpins how we track inheritance. When geneticists say a gene is 'linked' to another gene, they mean the two loci are close together on the same chromosome, making them less likely to be separated during crossing over. The fixed nature of the locus is therefore fundamental to understanding both gene expression and inheritance patterns.

4. The Triplet Code: Three Bases, One Amino Acid

Here is where the logic of the genetic code becomes elegant. There are 20 different amino acids used to build proteins, but DNA only has four bases: adenine (A), thymine (T), guanine (G), and cytosine (C). How can four bases code for 20 amino acids?

The answer is that bases are read in groups of three, called triplets. With four bases and groups of three, the number of possible combinations is $4^3 = 64$. That gives 64 possible triplets — more than enough to code for all 20 amino acids, with combinations left over. Each triplet codes for one specific amino acid.

To make this concrete: the DNA triplet TAC codes for the amino acid methionine (via its mRNA codon AUG). The triplet ATA codes for tyrosine (via mRNA codon UAU). Change one base in the triplet, and you may change the amino acid — or you may not, because of a property called degeneracy, which we will examine next.

5. Properties of the Genetic Code: Universal, Non-Overlapping, and Degenerate

The genetic code has three key properties that AQA expects you to know precisely. These are not arbitrary facts — each property has a biological consequence worth understanding.

The universal nature of the code is particularly powerful in biotechnology. Because the triplet for insulin is the same in humans and in Escherichia coli, scientists can insert the human insulin gene into bacterial DNA and the bacteria will produce human insulin. The code is the same language, spoken by every living organism.

Non-overlapping means the reading frame is critical. If you read the sequence ATGCTT as a non-overlapping code, you get two triplets: ATG and CTT. If it were overlapping, you would also read TGC, GCT, and CTT — producing far more amino acids from the same stretch of DNA. The non-overlapping system keeps the message clean and unambiguous.

Degeneracy is the most conceptually rich of the three properties. Because 64 triplets code for only 20 amino acids, most amino acids have multiple triplets assigned to them. The variation is usually in the third base of the triplet — sometimes called the 'wobble position'. A mutation that changes only the third base often produces a synonymous triplet that codes for the same amino acid, so the polypeptide sequence is unchanged. This is why degeneracy is described as offering partial protection against mutation.

6. Putting It Together: From Gene to Polypeptide

Now that all four spec points are in place, it is worth seeing how they connect. A gene is a sequence of DNA bases at a fixed locus. Within that sequence, every three bases form a triplet that codes for one amino acid. The full sequence of triplets in the gene therefore specifies the full sequence of amino acids in the polypeptide — and that sequence determines the protein's shape and function.

Worked example: Reading a DNA Triplet Sequence

A section of a DNA template strand reads: 3'-TAC-GGT-AAA-5'. Identify the number of amino acids coded for and explain what property of the code makes this reading unambiguous.

1. TAC | GGT | AAA — three groups of three bases — The non-overlapping property means each base belongs to exactly one triplet. We read along the template strand in groups of three without sharing bases between triplets.
2. Three triplets → three amino acids — Each triplet codes for one specific amino acid. This section of the gene therefore specifies a sequence of three amino acids in the polypeptide.
3. Non-overlapping: reading frame is TAC / GGT / AAA, not TAC / ACG / CGG / GGT... — If the code were overlapping, the same bases would appear in multiple triplets, producing ambiguity. The non-overlapping property ensures each triplet is read once and only once.
4. If the final base of AAA mutated to AAG, the amino acid coded might remain the same — Because the code is degenerate, a change in the third base of a triplet (the wobble position) often produces a synonymous triplet coding for the same amino acid — so the polypeptide sequence is unaffected.

Answer: Three triplets code for three amino acids. The non-overlapping property ensures unambiguous reading. Degeneracy means a mutation in the third base position may not alter the amino acid sequence.

7. Exam Technique and Common Mistakes

AQA questions on this topic tend to target three areas: the precise definition of a gene, the properties of the genetic code (especially degeneracy), and the mathematical logic of the triplet code. The marks are available for precise language, so vague answers cost you.

For extended-response questions that ask you to evaluate or compare properties of the genetic code, structure your answer by stating the property, explaining what it means at the molecular level, and then linking it to a biological consequence. Examiners award marks for the chain of reasoning, not for listing facts in isolation.

Across all four spec points — the definition of a gene, the locus, the triplet code, and the three properties — the thread connecting them is this: the genetic code is a precise, consistent, and robust system for translating a linear sequence of DNA bases into a specific sequence of amino acids, and every property of that code has a functional reason for existing.