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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.