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Inside the nucleus of every one of your cells sits the molecule that carries the instructions to build and run your whole body: DNA. To understand inheritance, variation and natural selection — the rest of Topic B5 of OCR Gateway Science A — you first need to be secure on what DNA is, how it is organised into chromosomes and genes, and what we mean by the genome. This lesson covers the structure of DNA at GCSE level, the four bases and how they pair, the all-important idea that a gene codes for a protein, and why scientists have worked to read the human genome. For Higher tier it also gives a simple account of protein synthesis.
By the end of this lesson you should be able to describe the structure of DNA, define gene, chromosome and genome, explain that a gene codes for a sequence of amino acids in a protein, discuss the value of understanding the genome, and (Higher) outline how a protein is made.
DNA stands for deoxyribonucleic acid. It is the molecule that carries the genetic information in all living organisms — the instructions for making the proteins 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 called nucleotides, 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. Each repeating unit (a sugar, a phosphate and a base) is a nucleotide, so DNA is described as a polymer of nucleotides.
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.
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 letters in "Apple Tart" (A, T) and "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)"]
To put the scale in perspective: a single human cell contains about two metres of DNA if you could stretch it all out, packaged into those 46 chromosomes inside a nucleus only a few micrometres across. Along that DNA lie roughly twenty thousand genes, each a section that codes for a protein, separated by long stretches that do not code for proteins. The genome is therefore not one continuous instruction but a vast library of individual genes — which is why finding the gene linked to a particular characteristic or disease is such a major scientific task.
Exam Tip: A common definition question is "What is the genome of an organism?" The mark-worthy answer is "the entire genetic material / all of the DNA of that organism". For "gene", say "a section of DNA that codes for a (particular) protein".
Why does the order of bases matter so much? Because it determines which proteins a cell makes, and proteins do almost everything in the body. Proteins include all the enzymes that control the cell's reactions, structural proteins such as those in muscle and skin, and hormones such as insulin. The shape and function of each protein depend on the order of its amino acids, and that order is spelled out by the sequence of bases in a gene.
Most of your visible characteristics — eye colour, the ability to roll your tongue, blood group and so on — are determined (at least partly) by which versions of genes you inherit, because those genes control which proteins your cells make. Some characteristics are controlled by a single gene, but most are influenced by many genes working together, and often by the environment as well. You will study how single-gene characteristics are inherited in the next lessons; the key idea here is simply that a gene codes for a protein, and proteins shape the characteristics of the organism.
Higher tier only: You should be able to give a simple account of how the base sequence of a gene leads to a protein. Keep this at GCSE depth — you need the outline, not the fine biochemical detail.
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