DNA, the Genome and Protein Synthesis

The genetic material in the nucleus of a cell is composed of a chemical called DNA (deoxyribonucleic acid).

DNA is a polymer — a long molecule built from many small repeating units joined together. It is made up of two strands that twist around each other to form a shape called a double helix. Picture a ladder that has been twisted along its length.

DNA is contained in structures called chromosomes. Body cells in humans carry chromosomes in 23 pairs (46 in total), and each chromosome is one very long molecule of DNA coiled up tightly so it fits inside the nucleus.

DNA is the chemical that stores all the genetic instructions an organism needs to grow, develop and function. It is a double-stranded polymer found in the chromosomes inside the nucleus.

These instructions matter because they control which proteins a cell makes — and proteins do almost all the work in a living organism, acting as enzymes, hormones and structural materials.

These four terms are easy to mix up. Each one describes a different scale of the same genetic material — from a single instruction up to the whole set.

Term	Meaning
DNA	The chemical that genetic information is stored in; a double-stranded polymer.
Gene	A small section of DNA on a chromosome that codes for a particular sequence of amino acids to make a specific protein.
Chromosome	A long, coiled molecule of DNA found in the nucleus; carries many genes.
Genome	The entire genetic material of an organism — every gene on every chromosome.

A useful way to keep the scale straight:

• A gene is one instruction (e.g. the instruction to make one protein).
• A chromosome is a long structure carrying thousands of genes.
• The genome is the complete set of all the genetic material an organism has.

A gene is a small section of DNA on a chromosome. Each gene codes for a particular sequence of amino acids, which join together to make one specific protein.

Because different genes code for different proteins, the genes an organism inherits help determine its characteristics.

The genome of an organism is its entire genetic material. The whole human genome has now been studied, and this is expected to have great importance for medicine in the future.

AQA limits what you need to discuss to three specific uses:

Use of understanding the human genome	What it allows
Searching for genes linked to disease	Identify which genes are associated with different types of disease.
Inherited disorders	Improve the understanding and treatment of disorders passed from parents to offspring.
Tracing human migration	Use genetic patterns to trace how human populations migrated in the past.

Worked discussion point — a question might ask: "Give two ways that understanding the human genome could be useful in medicine."

A full-mark answer draws from the list above, for example:

1. It helps scientists find genes linked to particular diseases, so people at higher risk can be identified.
2. It improves the understanding and treatment of inherited disorders.

Stick to AQA's three uses: genes linked to disease, inherited disorders, and tracing migration patterns. Tracing migration is the one students most often forget.

(Biology only — this nucleotide detail appears only on the AQA GCSE Biology paper, not Combined Science. It is assessed at both Foundation and Higher tier.)

The DNA polymer is built from repeating units called nucleotides. DNA is made from four different nucleotides.

Each nucleotide is made of three parts:

• a common sugar,
• a phosphate group, and
• one of four different bases attached to the sugar.

The four bases are A, C, G and T (adenine, cytosine, guanine, thymine).

The two long strands of DNA each consist of alternating sugar and phosphate sections, forming the "backbone." Attached to each sugar is one of the four bases. A sketch of one strand looks like this:

Because the backbone is the same alternating sugar–phosphate pattern all the way along, it is the order of the bases that carries the genetic information.

A nucleotide = one sugar + one phosphate group + one base. DNA is a polymer of repeating nucleotide units. You must be able to interpret a diagram of DNA structure, but you will not be asked to draw it from memory.

(Biology only — the triplet code is assessed at both Foundation and Higher tier on the Biology paper. The complementary base-pairing detail later in this section is Higher Tier only.)

A sequence of three bases is the code for a particular amino acid. Because the code is read in groups of three, it is called a triplet code. The order of bases controls the order in which amino acids are assembled to produce a particular protein.

Worked example 1 — counting amino acids from bases. If a section of a gene is 30 bases long, how many amino acids does it code for?

Each amino acid needs 3 bases, so:

$\text{amino acids}=\frac{\text{number of bases}}{3}=\frac{30}{3}=10$

So 30 bases code for 10 amino acids.

Worked example 2 — working backwards. A short protein is made of 8 amino acids. How many bases of code are needed?

$\text{bases}=\text{amino acids}\times 3=8\times 3=24\text{ bases}$

In the two strands of the double helix, the bases pair up in a fixed way:

(Higher Tier only) In the complementary strands, a C is always linked to a G on the opposite strand, and a T always pairs with an A. So A–T and C–G are the only allowed pairings.

This complementary pairing means that if you know the bases on one strand, you can work out the other: opposite an A write T, opposite a T write A, opposite a C write G, and opposite a G write C.

(Higher Tier only — Biology only. AQA requires only a simple outline; you do not need the detailed structure of mRNA, tRNA, amino acids or proteins.)

The order of bases in a gene controls the order of amino acids in a protein. Building that protein happens in two outline stages:

1. Transcription — copying the gene. The base sequence of the gene is copied into a messenger molecule (mRNA). This copy carries the instructions out of the nucleus to a ribosome.

2. Translation — building the protein. Proteins are synthesised on ribosomes, according to the template. Carrier molecules bring specific amino acids to add to the growing protein chain in the correct order set by the triplet code.

A simple outline of the whole process:

When the amino acid chain is complete, it folds up into a unique 3D shape. This shape is what lets the protein do its job — as an enzyme, a hormone, or a structural protein such as collagen.

Remember the order: a gene's base sequence → a sequence of amino acids → a folded protein. The shape of the finished protein decides its function.

(Higher Tier only — Biology only.)

Mutations are changes to the base sequence of DNA. They occur continuously.

The effect of a mutation depends on whether it lands in DNA that codes for a protein:

Mutation type	Effect on the protein
Most mutations	Do not alter the protein, or alter it only slightly so its appearance or function does not change.
A few mutations (coding DNA)	Code for an altered protein with a different shape.
Mutations in non-coding DNA	May change how genes are switched on or off, affecting how genes are expressed.

If a mutation changes the shape of a protein, the consequences can be serious:

• An enzyme may no longer fit its substrate at the active (binding) site, so the reaction it controls is affected.
• A structural protein (such as collagen) may lose its strength.

Genetic variants influence the phenotype (an organism's observable characteristics) in two ways:

1. In coding DNA — by altering the activity of a protein.
2. In non-coding DNA — by altering how genes are expressed.

Not all DNA codes for proteins. Non-coding sections can switch genes on and off, so variation here can still affect an organism even though no protein sequence is changed directly.

1. Confusing gene, chromosome and genome

A gene codes for one protein; a chromosome is a long DNA molecule carrying many genes; the genome is all the genetic material. If a question asks for the genome, "all the DNA / all the genes of an organism" is the key idea — not just one chromosome.

2. Getting the base pairs wrong

The pairs are A with T and C with G. A common slip is pairing A with C or G with T. Opposite an A, write a T; opposite a C, write a G — and never mix the pairs up.

3. Forgetting the triplet code is in threes

A sequence of three bases codes for one amino acid. To find the number of amino acids, divide the number of bases by 3 (e.g. 30 bases → 10 amino acids); to find bases, multiply amino acids by 3.

4. Saying a gene "is" a protein

A gene codes for a sequence of amino acids that makes a protein — the gene is not itself the protein. The base order is the instruction; the protein is the product.

5. Overcomplicating protein synthesis (Higher Tier)

AQA wants only a simple outline: the gene is copied to mRNA (transcription), then a protein is built on a ribosome with carrier molecules bringing amino acids in order (translation). You are not required to describe the detailed structure of mRNA, tRNA or amino acids — adding that detail wastes time and earns no extra marks.

6. Assuming every mutation changes the protein (Higher Tier)

Most mutations have no effect or only a slight one. Only a few change the protein's shape enough to alter its function, and mutations in non-coding DNA may instead affect whether genes are switched on or off.