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Tucked inside the nucleus of nearly every one of your cells is a molecule that carries the full set of instructions to build and run your body: DNA. This lesson, part of Topic B1 of OCR Gateway Combined Science, covers what DNA is, its famous double-helix shape, the four bases that spell out its code, and how DNA is organised into chromosomes and genes to make up the genome. Along the way it looks at the key biological molecules — sugars, amino acids and fatty acids — that cells are built from and that DNA's instructions help make. These ideas run right through the later topics on inheritance, variation and genetic engineering, so getting them secure now pays off across the whole course.
By the end of this lesson 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, explain the importance of sugars, amino acids and fatty acids, and explain why scientists have worked to read the human genome.
This lesson builds AO1 (knowledge of DNA structure and the key terms gene, chromosome and genome) and AO2 (applying the base-sequence idea to explain how genes determine proteins).
DNA stands for deoxyribonucleic acid. It is the molecule that carries the genetic information in every living organism — the instructions for making the proteins (and especially the enzymes) that build and control a cell. Because DNA can be copied exactly whenever a cell divides, those instructions are passed on faithfully from cell to cell, and from parents to their offspring.
A few facts 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". Muddling these is a common way to drop marks.
DNA has a famous shape — a double helix, often pictured as a twisted ladder. It is made from two strands wound around each other. Each strand is a chain of repeating units, and the two strands are joined together across the middle by the bases, which pair up to form the "rungs" of the ladder.
The two outer "rails" form the sugar–phosphate backbone, and the paired letters across the middle are the bases. In a real molecule the whole ladder is twisted into a spiral — a helix — and because it has two strands, it is a double helix.
The "code" of DNA is written with 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 is why the two strands fit together so exactly, and why DNA can be copied so 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 sets the order of amino acids in a protein, and therefore decides which protein gets made.
It is worth seeing, in outline, how a sequence of just four bases can specify something as complicated 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 — one particular run of bases — spells out a particular order of amino acids, which then fold into a particular protein. Because proteins include all of 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 links straight to the enzymes you meet later in this topic: an enzyme's active site 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 combined science, but holding onto the chain "base sequence → amino acid order → protein → function" will make many later ideas fall into place.
DNA is not the only important large molecule in a cell, and its instructions ultimately help to make the others. Living things are built from a small number of key molecules, each assembled from smaller units:
| Large molecule | Made from (its units) | Why it matters |
|---|---|---|
| Carbohydrates (e.g. starch, glucose) | Simple sugars | Main energy source for respiration; storage (starch) and structure (cellulose) |
| Proteins | Amino acids | Enzymes, muscle, antibodies, some hormones |
| Lipids (fats and oils) | Fatty acids and glycerol | Energy store; make up cell membranes |
So sugars are important because glucose is the fuel for respiration and can be linked up into storage and structural carbohydrates; amino acids are important because they are joined together (in the order set by DNA) to build every protein, including all enzymes; and fatty acids are important because, with glycerol, they build the lipids that store energy and form membranes. These are exactly the molecules that the digestive enzymes you meet later break large foods down into, so keeping them in mind ties several parts of the topic together.
Every so often 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 raised 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 a 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 are harmful, and very rarely one is beneficial. Mutations are the ultimate source of the genetic variation on which natural selection acts, an idea you will return to in a later topic.
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 with C — only A–T and G–C pairings ever happen. One useful memory aid is that the pairs are the two "thin" letters (A, T) and the two letters in "Great Company" (G, C).
These three terms describe DNA at three different scales, and you are expected to use them correctly.
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