DNA and the Genome

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.

What Is DNA?

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.

The Structure of a Nucleotide

Each strand of DNA is made up of repeating units called nucleotides. A nucleotide consists of three parts:

A sugar molecule (deoxyribose)
A phosphate group
A base (one of four types)

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

Base Pairing Rules

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:

A always pairs with T
C always pairs with G

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.

Chromosomes and Genes

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

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.

Benefits of the Human Genome Project

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

Concerns about the Human Genome Project

Genetic discrimination — employers or insurance companies could discriminate based on genetic information
Privacy — who should have access to an individual's genetic data?
Psychological impact — knowing you carry a gene for a serious disease can cause anxiety, especially if there is no cure
Ethical issues — concerns about genetic modification and "designer babies"

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.

DNA in Prokaryotic Cells

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.

Why Understanding DNA Matters

Understanding DNA is crucial for:

Medicine — diagnosing genetic disorders, gene therapy, drug development
Agriculture — selective breeding and genetic engineering of crops and livestock
Forensics — DNA profiling to identify individuals
Evolution — comparing DNA sequences to determine how closely related species are
Conservation — understanding genetic diversity within populations to prevent inbreeding

Summary

DNA is a polymer made of two strands forming a double helix, found in the nucleus of cells
Each strand is made of nucleotides, each containing a sugar, phosphate, and one of four bases (A, T, C, G)
Complementary base pairing means A pairs with T, and C pairs with G
A gene is a section of DNA that codes for a specific protein
The genome is the entire set of genetic material in an organism
The Human Genome Project mapped all human genes and has huge implications for medicine, forensics, and understanding evolution
DNA in prokaryotic cells is arranged differently (circular chromosome, plasmids) compared to eukaryotic cells

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.