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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: