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Inside the nucleus of almost every one of your cells sits the molecule that carries the instructions to build and run your entire body: DNA. To make sense of inheritance, variation and natural selection — the rest of Topic B5 of your OCR Gateway Combined Science course — you first need to be secure on what DNA is, how it is organised into chromosomes and genes, and what scientists mean by the genome. This lesson covers the structure of DNA at the level you need, the four bases and how they pair, the crucial idea that a gene codes for a protein, and why researchers have worked so hard to read the human genome.
By the end of this lesson you should be able to describe the structure of DNA, define the terms gene, chromosome and genome, explain that a gene codes for a sequence of amino acids in a protein, and discuss the value of understanding the human genome.
This lesson develops AO1 (recall and understanding of DNA, genes and the genome) and AO3 (evaluating the benefits and wider implications of genome research).
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 anchor everything else in this topic:
Exam Tip: Be precise with the hierarchy of terms. The order of scale runs 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 up is a common way to lose marks.
DNA has a famous shape: a double helix, often pictured as a twisted ladder. It is made of two strands wound 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 form the "rungs" of the ladder.
The two outer "rails" of the ladder 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, which is why 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 sets the order of amino acids in a protein, and therefore decides 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).
The base-pairing rule does more than build the second strand: it lets you predict the proportions of the bases in a piece of DNA. Because A always pairs with T, a molecule must contain the same amount of A as T; and because G always pairs with C, it must contain the same amount of G as C.
In a sample of DNA, 20% of the bases are adenine (A). What percentage of the bases are (a) thymine, (b) guanine and (c) cytosine?
Step 1 — use the A–T rule. A pairs with T, so there must be as much T as A: thymine is also 20%.
Step 2 — find how much is left for G and C. A and T together make 20%+20%=40%, so G and C together make 100%−40%=60%.
Step 3 — use the G–C rule. G pairs with C, so the 60% is shared equally: guanine is 30% and cytosine is 30%.
Answer: (a) thymine =20%; (b) guanine =30%; (c) cytosine =30%. (Check: 20+20+30+30=100%.)
Common error: assuming all four bases are always present in equal amounts (25% each). That is only true if A happens to equal G. The reliable rules are simply that A equals T and G equals C; the split between the A–T pair and the G–C pair varies from one organism to another.
These three terms describe DNA at different scales, and you are expected 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 picture the scale: a single human cell holds 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".
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