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This lesson covers the structure of DNA and the concept of the genome as required by the AQA GCSE Biology specification. Understanding DNA is the foundation for the entire Inheritance, Variation and Evolution topic. You need to know what DNA is, where it is found, how it is organised, and why the Human Genome Project was such a landmark achievement.
DNA stands for deoxyribonucleic acid. It is a polymer made up of two strands forming a double helix structure. DNA carries the genetic instructions for the development, functioning, growth, and reproduction of all known organisms.
DNA is found in the nucleus of eukaryotic cells. A small amount of DNA is also found in mitochondria and, in plant cells, in chloroplasts.
| Feature | Detail |
|---|---|
| Full name | Deoxyribonucleic acid |
| Location | Nucleus (main), mitochondria, chloroplasts |
| Structure | Double helix (two strands wound together) |
| Monomer | Nucleotide |
| Function | Carries the genetic code for making proteins |
Exam Tip: You must be able to describe DNA as a polymer made up of two strands forming a double helix. This is a very common 1-2 mark question.
Each strand of DNA is made up of repeating units called nucleotides. A nucleotide consists of three parts:
The sugar and phosphate groups alternate to form the backbone of each DNA strand, while the bases point inwards and pair with bases on the opposite strand.
graph TD
A[Nucleotide] --> B[Phosphate Group]
A --> C[Deoxyribose Sugar]
A --> D[Base]
D --> E[Adenine - A]
D --> F[Thymine - T]
D --> G[Cytosine - C]
D --> H[Guanine - G]
E --- F
G --- H
The four bases in DNA are:
| Base | Abbreviation | Pairs with |
|---|---|---|
| Adenine | A | Thymine (T) |
| Thymine | T | Adenine (A) |
| Cytosine | C | Guanine (G) |
| Guanine | G | Cytosine (C) |
The bases are held together by complementary base pairing:
This is called complementary base pairing because the shape of A fits with T, and the shape of C fits with G, like pieces of a jigsaw.
Exam Tip: Remember base pairing with the phrase: "Apples go in Trees, Cars go in Garages." This simple mnemonic can save you marks in the exam.
DNA in the nucleus is organised into structures called chromosomes. Humans have 46 chromosomes arranged in 23 pairs.
| Term | Definition |
|---|---|
| Chromosome | A long, coiled molecule of DNA found in the nucleus |
| Gene | A short section of DNA on a chromosome that codes for a specific protein |
| Genome | The entire set of genetic material (all DNA) in an organism |
| Allele | A version of a gene; different alleles code for different variations of a characteristic |
A single chromosome contains many genes. Each gene occupies a specific position on the chromosome, called its locus.
graph LR
A[Cell] --> B[Nucleus]
B --> C[Chromosomes 23 pairs]
C --> D[DNA double helix]
D --> E[Genes sections of DNA]
E --> F[Code for proteins]
The Human Genome Project (HGP) was a major international scientific research project that ran from 1990 to 2003. Its aim was to identify and map all of the approximately 20,000-25,000 genes in the human genome and to determine the sequence of the 3 billion base pairs that make up human DNA.
| Benefit | Explanation |
|---|---|
| Identifying genes linked to diseases | Scientists can now identify genes that increase the risk of certain disorders, such as breast cancer (BRCA genes) |
| Developing new medicines | Understanding the genetic basis of disease allows drugs to be targeted more precisely |
| Personalised medicine | Doctors may be able to tailor treatments to an individual's genetic makeup (pharmacogenomics) |
| Tracing human migration | Comparing genomes from different populations reveals how humans spread across the globe |
| Understanding evolution | Comparing human DNA with other species shows evolutionary relationships |
| Forensic science | DNA profiling relies on knowledge of the genome for criminal investigations and paternity testing |
Exam Tip: The HGP is a very common exam topic. Be prepared for questions asking you to evaluate its benefits and risks. Use specific examples (e.g. BRCA genes, pharmacogenomics) to show depth of knowledge.
It is important to understand that DNA is organised differently in prokaryotic and eukaryotic cells:
| Feature | Eukaryotic cells | Prokaryotic cells |
|---|---|---|
| DNA location | Nucleus | Cytoplasm (no nucleus) |
| DNA structure | Linear chromosomes | Single circular chromosome |
| Additional DNA | Mitochondrial DNA | Plasmids (small rings of extra DNA) |
| Chromosomes | Multiple pairs (e.g. 23 pairs in humans) | Usually one circular chromosome |
Plasmids in bacteria are particularly important because they can carry genes for antibiotic resistance and can be transferred between bacteria.
Understanding DNA is crucial for:
Exam Tip: When describing DNA structure, always mention three things: (1) it is a double helix, (2) it is made of nucleotides, and (3) bases pair A-T and C-G. These three points together will secure full marks on a typical 3-mark question.
Because adenine always pairs with thymine, and cytosine always pairs with guanine, the number of A bases in a DNA molecule is always equal to the number of T bases, and the number of C bases is always equal to the number of G bases. This is known as Chargaff's rule.
Worked example: A sample of double-stranded DNA is analysed and found to contain 22% adenine. Calculate the percentage of each of the other three bases.
Step 1 — Apply complementary base pairing: %A = %T, so %T = 22%. Step 2 — Total so far: %A + %T = 22 + 22 = 44%. Step 3 — Remaining bases: 100 - 44 = 56% must be split equally between C and G. Step 4 — Divide: 56 / 2 = 28%. So %C = 28% and %G = 28%.
Final answer: A = 22%, T = 22%, C = 28%, G = 28%.
Common mistake: Students sometimes forget that Chargaff's rule only applies to double-stranded DNA. In single-stranded nucleic acids (e.g. mRNA), the bases do not have to pair up in equal numbers.
These three terms appear in almost every exam paper and are frequently confused.
| Term | What it is | Scale |
|---|---|---|
| Chromosome | A coiled molecule of DNA containing many genes | Largest |
| Gene | A section of a chromosome that codes for one polypeptide (protein) | Medium |
| Allele | A specific version of a gene, differing by a few bases | Smallest |
Common mistake: Saying "everyone has the same alleles" — this is wrong. Everyone has the same genes (humans share roughly the same set of ~20,000 genes), but individuals differ in which alleles they carry at each gene locus. That is why people look different despite sharing identical gene lists.
Exam-style question: The Human Genome Project mapped all of the genes in the human genome. Explain what is meant by the term "genome" and describe one benefit and one concern of having this information available. (4 marks)
Grade 4-5 answer: The genome is all the DNA in an organism. A good thing about the Human Genome Project is that scientists can find genes that cause diseases. A bad thing is that insurance companies might use the information unfairly.
Grade 8-9 answer: The genome is the entire set of genetic material — every gene and every section of non-coding DNA — found in an organism's cells. A major benefit of the Human Genome Project is that identifying disease-linked alleles (for example the BRCA1 and BRCA2 alleles associated with breast cancer risk) allows earlier screening and targeted pharmacogenomic treatments tailored to a patient's genotype. A significant concern is genetic discrimination: if employers or insurers gain access to an individual's genotype they may treat carriers of high-risk alleles unfavourably, even where the phenotype has not yet expressed, raising serious ethical and privacy issues.
Notice how the Grade 8-9 response uses precise terminology (genome, allele, genotype, phenotype, pharmacogenomic) and gives a concrete named example (BRCA genes). Grade 4-5 responses tend to use everyday language and stay general.
Biology-only students are expected to describe DNA structure in greater molecular detail. Key points:
A single copy of the human genome contains approximately 3.0 x 10^9 base pairs and roughly 20,000 protein-coding genes. If the DNA from a single cell were stretched out it would be about two metres long — yet it fits into a nucleus only a few micrometres across because it is wound tightly around proteins called histones to form chromatin, which condenses further to form chromosomes during cell division.
Exam Tip: AQA has moved away from the old phrase "junk DNA". Refer to sections that do not code for proteins as non-coding DNA — this term is used in the specification and in the mark schemes.
AQA alignment: This content is aligned with AQA GCSE Biology (8461) specification section 4.6 Inheritance, variation and evolution — specifically 4.6.1.3 DNA and the genome, 4.6.1.4 DNA structure [biology only] and 4.6.3.3 The understanding of genetics. Assessed on Paper 2.