DNA and the genome — AQA GCSE Biology
Test yourself on DNA and the genome with AQA GCSE practice questions.
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DNA and the genome explained
DNA is the molecule that stores genetic information.
Read the full explanation
Its structure is a polymer of nucleotides, each containing a sugar, a phosphate group and a base. Two strands of nucleotides pair up by complementary bases (A with T, C with G) and twist around each other to form a double helix. The genome is the entire genetic material of an organism: in humans this is the complete set of DNA in the nucleus, including all genes and non-coding regions. When describing structure, link the repeating nucleotide units to the polymer backbone and the base pairing that holds the two strands together. When defining genome, state that it is the whole genetic material, not just the genes, and give a named example such as the human genome.
The genetic material in the nucleus of a cell is composed of a chemical called DNA.
In eukaryotic cells, most genetic material is found in the nucleus. That material is the chemical DNA, which stands for deoxyribonucleic acid. DNA carries the instructions that control the characteristics and activities of the cell. The nucleus therefore acts as the control centre because it contains DNA. You should be able to state the location and the chemical identity of the genetic material, and explain that DNA is a molecule, not a cell structure. For example, in a human body cell, the nucleus contains DNA organised into chromosomes; the DNA sequence provides the coded information needed to make proteins. This links the nucleus to inheritance and to the control of cell activity.
DNA is a polymer made up of two strands forming a double helix.
DNA is a polymer, meaning it is a large molecule built from many repeating smaller units called nucleotides. Each nucleotide has a sugar, a phosphate group and a base. In DNA, two strands of nucleotides run in opposite directions and are held together by complementary base pairing: adenine pairs with thymine, and cytosine pairs with guanine. The two strands twist around a shared axis to form a double helix, which looks like a twisted ladder. The sugar and phosphate groups form the uprights of the ladder, and the paired bases form the rungs. This structure allows the base sequence to store information and allows the molecule to be copied accurately.
The DNA is contained in structures called chromosomes.
Within the nucleus, DNA is packaged into structures called chromosomes. A chromosome is a long molecule of DNA that is associated with proteins, which help to coil and condense the DNA so it can fit inside the nucleus. Humans have 46 chromosomes in a normal body cell, arranged as 23 pairs. Each chromosome carries many genes, and the sequence of bases along the DNA determines the proteins that the cell can make. When you describe this, state that chromosomes are found in the nucleus, that they are made of DNA, and that they carry genetic information. For example, a human body cell has 23 pairs of chromosomes, with one chromosome of each pair inherited from each parent.
A gene is a small section of DNA on a chromosome. Each gene codes for a particular sequence of amino acids, to make a specific protein.
DNA is a long polymer stored in chromosomes. A gene is one short stretch of that DNA, so a chromosome carries many genes. The base sequence within a gene is read in triplets; each triplet specifies one amino acid, and the order of triplets fixes the order of amino acids. That order determines how the chain folds, so each gene makes one particular protein with a specific job, such as an enzyme or a structural protein. A useful model is a recipe book: the chromosome is the book, a gene is one recipe, bases are the letters, and amino acids are the ingredients added in the written order. Because the sequence is specific, a change in it can alter the protein produced.
The genome of an organism is the entire genetic material of that organism.
The genome is the complete set of genetic material in an organism, not just the genes. It includes all the DNA in the chromosomes, plus the DNA in mitochondria and, in plant cells, in chloroplasts. In humans the genome is the whole of the nuclear DNA together with mitochondrial DNA. The word 'entire' matters: a single gene, or even all the protein-coding genes, is only part of the genome. Studying a genome means reading the full base sequence. This matters because non-coding DNA regulates genes, and comparing genomes between species or individuals reveals evolutionary relationships and variation. A useful check is to ask whether the description covers every piece of DNA in the organism, not merely the genes.
The whole human genome has now been studied and this will have great importance for medicine in the future.
Scientists have determined the complete base sequence of human DNA, so the whole human genome has been studied. This reference sequence lets researchers compare an individual's DNA with the standard and spot differences linked to disease. Medical importance includes predicting risk of conditions, choosing treatments that suit a person's genetic make-up, designing new medicines, and understanding inherited disorders. It can also support tracing the spread of disease and developing gene-based therapies. The benefits must be weighed against issues such as privacy of genetic data, insurance and employment discrimination, cost, and unequal access to testing. A balanced answer states a medical use, explains how the genome sequence enables it, and notes a limitation or ethical concern.
Students should be able to discuss the importance of understanding the human genome.
To discuss importance, you must present more than one side and reach a judgement. Understanding the human genome matters scientifically because it shows how genes work and how humans compare with other species. Medically, it supports predicting disease risk, selecting treatments, developing medicines and understanding inherited conditions. Socially, it raises questions about privacy, consent, insurance and fair access. A strong answer names a benefit, explains the mechanism, gives a concern, and weighs them. For example, knowing a variant raises the risk of a condition could allow earlier monitoring, but the same information could lead to discrimination if shared. Use connectives such as 'however' and 'therefore' to build the discussion, and finish with a clear overall view.
This is limited to the: • search for genes linked to different types of disease • understanding and treatment of inherited disorders • use in tracing human migration patterns from the past.
The human genome is the complete set of genetic material in a human cell. Its study has three main applications. First, searching for genes linked to different types of disease: comparing genomes of affected and unaffected people can reveal gene variants associated with conditions such as cancer or heart disease, aiding diagnosis and prevention. Second, understanding and treatment of inherited disorders: identifying the faulty allele behind a disorder such as cystic fibrosis helps explain how it is inherited and supports treatments such as gene therapy. Third, tracing human migration patterns from the past: comparing DNA between populations shows how humans moved around the world over thousands of years. For example, mitochondrial DNA and Y-chromosome markers act like inherited tags that track maternal and paternal lineages.
Your focus
- Describe DNA as a polymer of nucleotides forming a double helix.
- State the complementary base pairing rule A–T and C–G.
- Define genome as the entire genetic material of an organism.
Show all 27 objectives
- Identify the nucleus as the main location of genetic material in eukaryotic cells.
- Name DNA as the chemical that makes up the genetic material.
- Explain that DNA carries coded instructions that control the cell.
- Describe DNA as a polymer of nucleotides.
- Explain how two strands form a double helix through complementary base pairing.
- Relate the double helix structure to the storage of genetic information.
- State that DNA is contained in chromosomes.
- Describe chromosomes as DNA associated with protein, located in the nucleus.
- Recall that human body cells contain 46 chromosomes arranged as 23 pairs.
- Define a gene as a small section of DNA on a chromosome.
- Describe how a gene codes for a particular sequence of amino acids.
- Relate the amino acid sequence of a protein to its specific function.
- State that the genome is the entire genetic material of an organism.
- Identify where genetic material is found within a cell.
- Distinguish between a gene and the genome.
- Describe how the human genome has been studied.
- Explain medical benefits that may arise from understanding the human genome.
- Discuss ethical and practical concerns surrounding human genome data.
- Present benefits and concerns about understanding the human genome.
- Use scientific ideas to explain how genome knowledge can be applied.
- Reach and justify a conclusion about the importance of the human genome.
- State the three limited applications of the human genome studied at GCSE.
- Explain how comparing DNA can identify genes linked to disease.
- Describe how DNA analysis can trace past human migration.
DNA and the genome exam tips
Marking Points
- DNA is a polymer made of repeating nucleotide units.
- Each nucleotide contains a sugar, a phosphate group and a base.
- The bases pair specifically: adenine with thymine, and cytosine with guanine.
- Two strands of nucleotides form a double helix.
- The genome is the entire genetic material of an organism.
- The human genome includes all the DNA in the nucleus, not only the coding genes.
- The nucleus contains the genetic material of the cell.
- The genetic material is composed of the chemical DNA.
- DNA is a molecule that carries coded instructions.
- DNA controls the characteristics and activities of the cell.
- In eukaryotic cells, DNA is found mainly in the nucleus.
- DNA is a polymer built from repeating nucleotide units.
- Each nucleotide contains a sugar, a phosphate group and a base.
- Two strands of nucleotides make up the DNA molecule.
- The strands are held together by complementary base pairing.
- The two strands twist to form a double helix.
- The sugar-phosphate backbone forms the sides and the paired bases form the rungs of the helix.
- DNA is contained in structures called chromosomes.
- Chromosomes are found in the nucleus of eukaryotic cells.
- A chromosome is made of a long DNA molecule associated with proteins.
- Chromosomes carry genetic information in the sequence of bases.
- Human body cells contain 46 chromosomes arranged as 23 pairs.
- State that a gene is a section of DNA located on a chromosome, and that a chromosome carries many genes.
- Explain that the base sequence of a gene determines the sequence of amino acids in the protein it codes for.
- Link the amino acid sequence to protein folding and function, giving a named example such as an enzyme.
- Use the idea that each gene codes for one particular protein, so different genes produce different proteins.
- Apply the concept to a simple scenario, such as explaining why a mutation in one gene affects only one protein.
- Define the genome as the entire genetic material of an organism, including all its DNA.
- Distinguish the genome from a single gene or from the protein-coding genes alone.
- Recognise that the genome includes DNA in chromosomes and, where present, in mitochondria and chloroplasts.
- Explain that the genome can be studied by determining the full base sequence of the DNA.
- Use the concept to compare organisms, for example noting that genome size and content differ between species.
- State that the complete human genome has been sequenced, giving a reference set of human DNA.
- Explain how comparing an individual's DNA with the reference genome can reveal variants linked to disease.
- Describe a medical application, such as predicting disease risk, matching treatments to individuals, or designing new drugs.
- Discuss limitations or concerns, such as data privacy, discrimination, cost, or access to genetic testing.
- Reach a supported judgement about the future importance of the human genome for medicine.
- Identify several areas of importance, such as medicine, understanding inheritance, and comparing species.
- Explain how genome knowledge produces a specific benefit, for example identifying risk variants or targeting treatments.
- Present concerns, such as privacy of genetic data, consent, cost, or possible discrimination.
- Weigh benefits against concerns and reach a justified overall judgement.
- Use accurate terminology, including genome, genetic variant, inherited disorder and personalised medicine.
- The human genome is the entire genetic material of an organism, and studying it allows identification of genes linked to disease.
- Comparing DNA of people with and without a condition can reveal gene variants associated with diseases such as cancer, aiding earlier diagnosis or prevention.
- Identifying the gene responsible for an inherited disorder, such as the allele behind cystic fibrosis, improves understanding of how the disorder is inherited.
- Knowledge of an inherited disorder's genetic cause can support treatment, for example gene therapy or screening programmes.
- Comparing DNA sequences between different human populations provides evidence for migration patterns over thousands of years.
- Mitochondrial DNA and Y-chromosome DNA are useful in migration studies because they are inherited in a traceable way.
Examiner Tips
- 💡Use the terms nucleotide, complementary base pairing and double helix when describing structure.
- 💡When defining genome, include the word entire or complete to show it is all the genetic material.
- 💡Sketch a simple labelled diagram of the double helix with base pairs to support your written description.
- 💡Name the nucleus as the location and DNA as the chemical when answering location questions.
- 💡Use the full name deoxyribonucleic acid once, then the abbreviation DNA.
- 💡Link DNA to the control of cell activity to show understanding of its function.
- 💡Use the word polymer and the phrase double helix in your answer.
- 💡State the base pairing rule precisely: A with T and C with G.
- 💡If asked to describe the structure, work from nucleotide to strand to double helix in a logical order.
- 💡Use the phrase chromosomes are found in the nucleus when describing location.
- 💡Link chromosomes to DNA and genes in the same answer to show the relationship.
- 💡Quote the human chromosome number as 46 in body cells and 23 in gametes.
- 💡Use the phrase 'sequence of amino acids' rather than 'sequence of proteins' when describing what a gene codes for.
- 💡If asked to explain, work through the chain: gene base sequence, amino acid sequence, protein folding, protein function.
- 💡Sketch a quick labelled diagram of chromosome, DNA and gene to organise your answer before writing.
- 💡Underline the word 'entire' in the question to remind yourself to include all DNA, not just genes.
- 💡When comparing organisms, refer to the genome as the full genetic material rather than to individual genes.
- 💡Use precise wording: 'the entire genetic material of that organism' is safer than 'all the chromosomes'.
- 💡For 'discuss' questions, give both benefits and concerns, then state a clear conclusion.
- 💡Link each medical use to how the genome sequence makes it possible, rather than listing uses alone.
- 💡Use terms such as 'genetic variant', 'risk' and 'personalised treatment' accurately and sparingly.
- 💡Plan two columns, benefits and concerns, then write a conclusion that follows from your points.
- 💡Use the word 'because' to link each point to its consequence, showing reasoning rather than assertion.
- 💡Keep ethical points specific, such as insurance discrimination or data privacy, instead of general worries.
- 💡Link each application to a named example, such as cancer for disease genes or cystic fibrosis for inherited disorders.
- 💡Use the phrase 'comparing DNA' when explaining both disease gene searches and migration tracing.
- 💡For migration questions, mention that DNA differences accumulate over time and can indicate how closely populations are related.
Common Mistakes
- Saying that DNA is made of protein: correct this by stating that DNA is a nucleic acid polymer and proteins are made from the instructions carried by DNA.
- Describing the genome as only the genes: correct this by stating that the genome is all the genetic material, including non-coding DNA.
- Writing that the two strands are identical: correct this by explaining that the strands are complementary because of specific base pairing.
- Saying that the nucleus is made of DNA: correct this by stating that the nucleus is a structure that contains DNA.
- Confusing DNA with a protein: correct this by stating that DNA is a nucleic acid and proteins are different molecules.
- Stating that DNA is found only in the nucleus of every cell type: correct this by noting that prokaryotes have DNA not enclosed in a nucleus and some DNA is in mitochondria and chloroplasts.
- Saying that DNA is made of two separate molecules: correct this by stating that DNA is one molecule made of two strands.
- Writing that the strands are joined by strong covalent bonds between all bases: correct this by stating that complementary bases pair by hydrogen bonds.
- Describing the double helix as a single strand that is twisted: correct this by stating that two strands twist together.
- Saying that chromosomes are made only of protein: correct this by stating that chromosomes contain DNA and associated proteins.
- Confusing chromosomes with genes: correct this by stating that a chromosome is a structure that carries many genes.
- Stating that human gametes have 46 chromosomes: correct this by stating that human gametes have 23 chromosomes.
- Saying a gene is a whole chromosome: correct this by describing a gene as a small section of DNA on a chromosome.
- Confusing amino acids with bases: correct this by stating that bases in DNA code for amino acids, which are joined to form a protein.
- Claiming one gene codes for many unrelated proteins: correct this by stating that each gene codes for a particular sequence of amino acids making a specific protein.
- Equating the genome with all the genes only: correct this by stating that the genome is all the genetic material, including non-coding DNA.
- Thinking the genome is found only in the nucleus: correct this by including mitochondrial DNA and, in plant cells, chloroplast DNA.
- Confusing genome with genotype: correct this by defining genotype as the alleles an organism has for particular genes, while the genome is all its genetic material.
- Claiming the genome project can cure all diseases now: correct this by describing current and potential future uses, such as risk prediction and treatment selection.
- Ignoring ethical issues: correct this by including concerns about privacy, consent and possible discrimination.
- Treating genome study as identical to genetic testing of one gene: correct this by explaining that the whole genome sequence allows many variants to be examined together.
- Writing a one-sided list of benefits: correct this by adding concerns and a balanced conclusion.
- Giving vague importance such as 'it helps doctors': correct this by naming a specific use and explaining how the genome sequence enables it.
- Confusing discussion with description: correct this by comparing points and stating which arguments are stronger and why.
- Thinking the genome is only the genes coding for proteins; correction: the genome is all the genetic material, including non-coding DNA.
- Confusing tracing migration with tracing disease inheritance; correction: migration studies compare DNA between populations, while disease studies compare affected and unaffected individuals.
- Assuming gene identification automatically cures a disorder; correction: it improves understanding and can inform treatment, but treatments such as gene therapy are still being developed.