DNA, Genes and the Genome

This lesson covers the structure of DNA, the relationship between genes and proteins, and the importance of the human genome project. These are key topics in the AQA GCSE Combined Science Trilogy specification (8464) — Inheritance, Variation and Evolution.

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What Is DNA?

DNA stands for deoxyribonucleic acid. It is a large molecule found in the nucleus of cells and carries the genetic code — the instructions that control how an organism develops and functions.

DNA has a distinctive structure:

• It is a polymer made up of two strands twisted into a double helix
• Each strand is made up of repeating units called nucleotides
• Each nucleotide consists of a sugar (deoxyribose), a phosphate group and a base
• The two strands are held together by complementary base pairing

Complementary Base Pairing

There are four bases in DNA. They always pair in the same way:

Base	Pairs With
Adenine (A)	Thymine (T)
Cytosine (C)	Guanine (G)

$A \longleftrightarrow T \quad \text{and} \quad C \longleftrightarrow G$A⟷TandC⟷G

This means if you know the sequence of bases on one strand, you can work out the sequence on the other strand.

Exam Tip: A common exam question gives you a sequence on one strand and asks you to write the complementary sequence. Remember: A pairs with T, C pairs with G. For example, if one strand is A-T-G-C-C-A, the complementary strand is T-A-C-G-G-T.

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What Is a Gene?

A gene is a small section of DNA on a chromosome. Each gene codes for a particular sequence of amino acids, which are joined together to make a specific protein.

The order of bases in a gene determines the order of amino acids in the protein:

graph LR
    A["DNA base sequence<br/>(gene)"] --> B["Sequence of<br/>amino acids"]
    B --> C["Protein"]
    C --> D["Characteristic<br/>(e.g. eye colour,<br/>enzyme function)"]
    style A fill:#bbdefb,stroke:#1565c0
    style B fill:#c8e6c9,stroke:#2e7d32
    style C fill:#fff9c4,stroke:#f9a825
    style D fill:#ffccbc,stroke:#d84315

Key Definitions

Term	Definition
Gene	A small section of DNA that codes for a specific protein
Chromosome	A long, coiled molecule of DNA; found in the nucleus
Genome	The entire set of genetic material in an organism
Allele	A different form (version) of the same gene

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Chromosomes

• DNA is packaged into structures called chromosomes inside the nucleus
• Human body cells have 23 pairs of chromosomes — 46 in total
• One chromosome in each pair comes from the mother and one from the father
• Chromosome pair 23 are the sex chromosomes: XX = female, XY = male

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The Human Genome

The genome is the entire set of genetic material in an organism. The Human Genome Project (completed in 2003) identified the sequence of all 3 billion base pairs in human DNA.

Why Is the Genome Important?

Benefit	Explanation
Understanding genetic disorders	Identifying genes linked to inherited diseases like cystic fibrosis and sickle cell disease
Personalised medicine	Developing treatments tailored to an individual's genetic makeup
Tracing human migration	Comparing genomes from different populations reveals how humans spread across the world
Understanding evolution	Comparing genomes of different species shows evolutionary relationships
Identifying gene function	Knowing the sequence helps scientists work out what each gene does

Exam Tip: The AQA (8464) specification specifically mentions the importance of the human genome. Be prepared to give at least two reasons why understanding the genome is useful.

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How Genes Code for Proteins

The process of making a protein from a gene involves two main steps:

1. Transcription — the DNA code is copied onto a molecule of messenger RNA (mRNA) in the nucleus
2. Translation — the mRNA travels to a ribosome in the cytoplasm, where it is read and amino acids are assembled in the correct order

Each group of three bases (a triplet or codon) codes for one amino acid. Since there are 20 different amino acids, the sequence of triplets determines which protein is made.

DNA triplet (example)	Amino acid coded
ATG	Methionine
GCA	Alanine
TAC	Tyrosine

Exam Tip: At GCSE level you do not need to memorise specific codons, but you must understand that the order of bases in a gene determines the order of amino acids, which determines the shape and function of the protein.

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Proteins and Their Functions

Proteins are essential for life. Different proteins perform different functions depending on their shape:

Type of Protein	Function	Example
Structural	Provides support and shape	Collagen (in skin and bone)
Enzymes	Speed up chemical reactions (biological catalysts)	Amylase (digests starch)
Hormones	Chemical messengers carried in the blood	Insulin (controls blood glucose)
Antibodies	Defend against pathogens	Immunoglobulins
Receptors	Detect stimuli	Rhodopsin (in the eye)

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Mutations

A mutation is a change in the DNA base sequence. Most mutations have no effect on the protein produced, but some can:

• Change the amino acid sequence of the protein
• Alter the protein's shape and function
• Lead to a new characteristic or a genetic disorder

Mutations occur continuously and randomly. They can be caused by:

• Errors during DNA replication
• Environmental factors such as UV radiation, X-rays or certain chemicals

Exam Tip: Not all mutations are harmful. Most have no effect because they do not change the amino acid coded for (due to the redundancy of the genetic code). Some mutations can even be beneficial, providing an advantage in a particular environment.

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Common Mistakes

Mistake	Correction
Saying genes are "made of chromosomes"	Genes are on chromosomes; chromosomes are made of DNA
Confusing gene and allele	A gene is a section of DNA coding for a protein; an allele is a version of that gene
Saying DNA "is" a protein	DNA codes for proteins; DNA itself is a nucleic acid
Saying all mutations are harmful	Most mutations have no effect; some are beneficial

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Summary