You are viewing a free preview of this lesson.
Subscribe to unlock all 10 lessons in this course and every other course on LearningBro.
This lesson covers the structure and function of DNA (deoxyribonucleic acid) as required by the Edexcel GCSE Combined Science specification (1SC0). You need to describe the structure of DNA, explain how complementary base pairing works and understand the relationship between DNA, genes and chromosomes.
DNA stands for deoxyribonucleic acid. It is the molecule that carries the genetic information in all living organisms. DNA determines the characteristics of an organism by providing the instructions for making proteins.
DNA is found inside the nucleus of eukaryotic cells. In prokaryotic cells (bacteria), DNA is found free in the cytoplasm as a circular loop.
Exam Tip: DNA is not the same as a gene. DNA is the entire molecule; a gene is a small section of DNA that codes for a specific protein.
DNA is a polymer — a long molecule made up of many repeating units called nucleotides. Each nucleotide consists of three parts:
The nucleotides join together through bonds between the sugar of one nucleotide and the phosphate of the next, forming a long sugar-phosphate backbone.
There are four bases in DNA:
| Base | Abbreviation | Pairs with |
|---|---|---|
| Adenine | A | Thymine (T) |
| Thymine | T | Adenine (A) |
| Cytosine | C | Guanine (G) |
| Guanine | G | Cytosine (C) |
The bases always pair in the same way — this is called complementary base pairing:
The base pairs are held together by weak hydrogen bonds.
Exam Tip: Remember the base-pairing rules with "Apple Tree" (A-T) and "Car Garage" (C-G). The examiners will test this frequently.
Two polynucleotide strands wind around each other to form a shape called a double helix. Think of it like a twisted ladder:
graph TD
A["DNA Double Helix"] --> B["Two polynucleotide strands"]
B --> C["Sugar-phosphate backbone (sides)"]
B --> D["Complementary base pairs (rungs)"]
D --> E["A — T (two hydrogen bonds)"]
D --> F["C — G (three hydrogen bonds)"]
A --> G["Twisted into a helical shape"]
The two strands run in opposite directions — they are described as antiparallel. The double helix structure was famously described by Watson and Crick in 1953, building on X-ray crystallography work by Rosalind Franklin and Maurice Wilkins.
Understanding the hierarchy of genetic material is essential:
| Term | Definition |
|---|---|
| DNA | The entire molecule of deoxyribonucleic acid |
| Gene | A short section of DNA that codes for a specific protein (or polypeptide) |
| Chromosome | A long, tightly coiled molecule of DNA found in the nucleus |
| Genome | The entire set of genetic material in an organism |
graph LR
A["Nucleus"] --> B["Chromosomes (46 in humans)"]
B --> C["DNA molecule"]
C --> D["Genes (sections of DNA)"]
D --> E["Code for proteins"]
The Human Genome Project (completed in 2003) mapped the entire human genome — approximately 3 billion base pairs and around 20,000–25,000 genes. This has allowed scientists to:
Exam Tip: If asked about the importance of DNA structure, always connect it to protein synthesis — the order of bases determines the order of amino acids in a protein, which determines its shape and function.
The specific sequence of bases in a gene determines:
A change in even a single base can alter the protein produced — this is called a mutation (covered in Lesson 3).
Although not a required practical, you should know that DNA can be extracted from cells (e.g. from kiwi fruit or strawberries) using:
A nucleotide is the monomer from which DNA is built. Each nucleotide contains exactly three components: a phosphate group (PO4), a deoxyribose sugar (a pentose — five-carbon sugar), and one nitrogenous base (A, T, C or G). The sugar links to the phosphate of the next nucleotide by a phosphodiester bond, producing the long sugar-phosphate backbone. Because the backbone is on the outside of the double helix, it is the hydrophilic part that interacts with water, while the bases are tucked inside and are relatively hydrophobic.
The strict pairing rules (A–T and C–G) are not arbitrary — they arise from the size and shape of the bases and the number of hydrogen bonds each pair can form. A and G are purines (two-ring bases); T and C are pyrimidines (one-ring bases). A purine always pairs with a pyrimidine so that the width of the ladder stays constant.
| Pair | Number of hydrogen bonds | Why it matters |
|---|---|---|
| A–T | 2 | Slightly easier to separate |
| C–G | 3 | Stronger bond; DNA regions rich in C–G are more heat-stable |
Common mistake: Students often write that bases are joined by ionic or covalent bonds. They are joined by hydrogen bonds — weak individually, but collectively strong along a long DNA molecule.
graph TD
N["Nucleotide"] --> P["Phosphate group"]
N --> S["Deoxyribose sugar"]
N --> B["Nitrogenous base"]
B --> AD["Adenine (purine)"]
B --> GU["Guanine (purine)"]
B --> TH["Thymine (pyrimidine)"]
B --> CY["Cytosine (pyrimidine)"]
Suppose one strand of DNA reads:
5' A T G C C T A G G T 3'
Because A pairs with T and C pairs with G, the complementary strand (read in the opposite direction, antiparallel) is:
3' T A C G G A T C C A 5'
Counting base pairs is a common short-answer exam task. In the example above there are ten base pairs — four A–T pairs and six C–G pairs. If 40% of the bases in a DNA molecule are A, then 40% must also be T (because of the pairing rule), and the remaining 20% is shared between C and G, giving 10% C and 10% G. This relationship is known as Chargaff's rule and is frequently tested.
Exam Tip: If you are told the percentage of one base, use Chargaff's rule to work out the percentages of the others. %A = %T, and %C = %G, and all four must add up to 100%.
Although not the focus of this lesson, you must be able to distinguish DNA from RNA because the next lesson (protein synthesis) relies on it.
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Bases | A, T, C, G | A, U, C, G |
| Strands | Double (helix) | Single |
| Location | Nucleus (and mitochondria) | Nucleus and cytoplasm |
| Role | Long-term information store | Carries code to ribosomes (mRNA), delivers amino acids (tRNA) |
Grade 3–4 answer: "DNA is made of nucleotides. It is a double helix. The bases A, T, C and G pair up. A gene is a piece of DNA that makes a protein."
Grade 5–6 answer: "DNA is a polymer of nucleotides, each made of a phosphate, a deoxyribose sugar and a base. The two strands are held together by hydrogen bonds between complementary bases (A–T, C–G). A gene is a section of DNA coding for a specific protein, and a chromosome is a long coiled DNA molecule."
Grade 7–9 answer: "DNA is a polymer of nucleotides arranged as a double helix with two antiparallel sugar-phosphate backbones. Complementary base pairing links purines (A, G) to pyrimidines (T, C) via hydrogen bonds — two between A and T, three between C and G. The linear sequence of bases along a gene provides the template for a specific polypeptide, and the full set of genes across all 46 chromosomes makes up the human genome, sequenced by the Human Genome Project."
The grade 7–9 response uses precise terms such as antiparallel, purine, pyrimidine, polypeptide, and genome, and explains the structure-function relationship. Lower grade responses state facts; higher grade responses connect structure to function and use technical vocabulary accurately.
Q1. A sample of DNA contains 30% adenine. Using Chargaff's rule, calculate the percentage of each of the other three bases.
Answer: Because A pairs with T, thymine is also 30%. That leaves 100 − 60 = 40% to be shared equally between C and G (because C pairs with G), so cytosine is 20% and guanine is 20%.
Q2. Explain why C–G base pairs are stronger than A–T base pairs.
Answer: C–G pairs are held by three hydrogen bonds, whereas A–T pairs are held by two. More hydrogen bonds mean more energy is needed to separate the pair, so C–G is stronger and regions of DNA rich in C–G are more heat-stable.
Q3. State one similarity and one difference between a gene and a chromosome.
Answer: Similarity — both are made of DNA. Difference — a chromosome is a whole long molecule of DNA carrying many genes, while a gene is just a short section of DNA coding for a specific polypeptide.
The double helix was proposed by James Watson and Francis Crick in 1953. Crucial experimental evidence came from the X-ray crystallography work of Rosalind Franklin and Maurice Wilkins at King's College London — Franklin's famous "Photograph 51" revealed the helical shape. Earlier, Erwin Chargaff had shown that in every DNA sample A = T and C = G, foreshadowing the base-pairing rule.
Knowing how DNA is structured was only the start. The Human Genome Project finished in 2003 and sequenced all 3 billion base pairs of the human genome, identifying roughly 20,000 protein-coding genes. This has enabled modern applications such as:
Exam Tip: You may be asked to name scientists. The safest ones for DNA structure are Watson and Crick. For X-ray evidence, Franklin (with Wilkins). For base proportions, Chargaff.
Edexcel alignment: This content is aligned with Edexcel GCSE Combined Science (1SC0) Biology Topic 3 Genetics — specifically CB3.3 DNA structure/protein synthesis and CB3.6 Human Genome Project. Assessed on Biology Paper 1.