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Inside the nucleus of every one of your cells sits a remarkable molecule that carries the instructions to build and run your whole body: DNA. This lesson, part of Topic B1 of OCR Gateway Science A, covers what DNA is, its famous double-helix structure, the four bases that spell out its code, how DNA is organised into chromosomes and genes, and what we mean by the genome. These ideas underpin the later topics on inheritance, variation and genetic engineering, so a secure understanding now pays off across the whole course.
By the end you should be able to state that DNA is the genetic material, describe the double-helix structure and the four bases, define gene, chromosome and genome, and explain why scientists have worked to read the human genome.
DNA stands for deoxyribonucleic acid. It is the molecule that carries the genetic information in all living organisms — the instructions for making the proteins (especially enzymes) that build and control a cell. Because DNA can be copied exactly when a cell divides, these instructions are passed on faithfully from cell to cell and from parents to offspring.
A few key facts to anchor everything else:
Exam Tip: Be precise with the hierarchy of terms. Genome → chromosomes → genes → DNA bases. A genome is all of an organism's DNA; a chromosome is one long DNA molecule; a gene is a short section of that molecule; the bases are the individual "letters". Mixing these up is a common way to lose marks.
DNA has a famous shape: a double helix, often described as a twisted ladder. It is made of two strands coiled around each other. Each strand is a chain of repeating units, and the two strands are held together by the bases that pair up across the middle — these pairs are the "rungs" of the ladder.
The two outer "rails" are the sugar–phosphate backbone; the paired letters across the middle are the bases. In reality the whole ladder is twisted into a spiral — a helix — and because there are two strands, it is a double helix.
The "code" of DNA is written using just four bases:
| Base | Symbol | Pairs with |
|---|---|---|
| Adenine | A | T |
| Thymine | T | A |
| Guanine | G | C |
| Cytosine | C | G |
The rule that A always pairs with T, and G always pairs with C, is called complementary base pairing. This pairing is why the two strands fit together so precisely, and why DNA can be copied accurately: each strand acts as a template for building its partner.
The order (sequence) of these four bases along a gene is the actual instruction — it determines the order of amino acids in a protein, and therefore which protein is made.
It is worth seeing, in outline, how a sequence of just four bases can specify something as complex as a protein. The bases are read in groups of three. Each triplet of bases codes for one amino acid, and amino acids are the building blocks of proteins. So a gene — a particular run of bases — spells out a particular order of amino acids, which fold into a particular protein. Because proteins include all the body's enzymes, as well as structural proteins like those in muscle and hormones such as insulin, the base sequence of your DNA ultimately controls almost everything about how your body is built and how it works.
This connects directly to the enzymes you meet later in this topic: the active site of an enzyme has its precise shape because the gene for that enzyme specified a precise order of amino acids. Change the gene and you may change the protein. You do not need the full detail of protein synthesis for B1, but holding onto the chain "base sequence → amino acid order → protein → function" makes many later ideas click into place.
Occasionally the base sequence of DNA changes. A change to the sequence of bases is called a mutation. Mutations happen naturally at a low rate every time DNA is copied, and their frequency is increased by factors such as certain chemicals and ionising radiation. Because the base sequence is the code, a mutation can change the order of amino acids in the protein the gene codes for — which may change the protein's shape and stop it working properly. Many mutations have little or no effect (some fall in regions that do not code for proteins, and the code has some built-in redundancy), but a few can be harmful and, very rarely, beneficial. Mutations are the ultimate source of the genetic variation on which natural selection acts, an idea you will return to in later topics.
One strand of a short piece of DNA reads: A – G – T – C – A. Write the base sequence of the complementary strand.
Apply the pairing rule to each base (A with T, T with A, G with C, C with G):
| Strand 1 | A | G | T | C | A |
|---|---|---|---|---|---|
| Strand 2 | T | C | A | G | T |
Answer: T – C – A – G – T.
Common error: pairing A with G or C — only A–T and G–C pairings occur. A useful memory aid: the pairs are the two "thin" letters (A, T) and the two letters in "Great Company" (G, C).
These three terms describe DNA at different scales, and OCR expects you to use them correctly.
flowchart TD
A["Genome<br/>(all the DNA of an organism)"] --> B["Chromosome<br/>(one long coiled DNA molecule)"]
B --> C["Gene<br/>(a section of DNA coding for one protein)"]
C --> D["Bases A, T, G, C<br/>(the sequence is the code)"]
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