DNA and the genome — AQA GCSE Combined Science
Test yourself on DNA and the genome with AQA GCSE practice questions.
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DNA and the genome explained
This statement requires two linked skills.
Read the full explanation
First, students must describe the structure of DNA. DNA is a polymer made up of two strands that coil around each other to form a structure known as a double helix. The DNA is contained in structures called chromosomes, which are found in the nucleus of a cell. Second, students must define the term genome. The genome is the entire genetic material of an organism. In humans, the genome includes all the DNA found in the chromosomes within the nucleus, as well as the DNA in mitochondria. A clear exam answer will state that DNA is a polymer forming a double helix and precisely define the genome as the entire genetic material of an organism, avoiding confusion with a single gene.
The genetic material in the nucleus of a cell is composed of a chemical called DNA. DNA is a polymer made up of two strands forming a double helix. The DNA is contained in structures called chromosomes.
DNA is the chemical that carries genetic material in the nucleus of a cell. It is a polymer, meaning a large molecule built from many repeating smaller units joined together. In DNA, two strands wind around each other to form a double helix, a shape like a twisted ladder. The DNA is packaged into structures called chromosomes, so a chromosome is a coiled molecule of DNA. A useful scale picture is a human body cell: its nucleus contains 46 chromosomes, and each chromosome contains one very long DNA molecule. When a cell divides, the chromosomes must be copied and shared accurately so that each new cell receives the full set of genetic material.
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.
A gene is a small section of DNA found on a chromosome. The order of the bases along that section of DNA is a code. Each gene codes for a particular sequence of amino acids, and the amino acids are joined together in that order to make a specific protein. Different genes have different base sequences, so they code for different amino acid sequences and therefore different proteins. For example, one gene may code for the protein haemoglobin, while another codes for an enzyme such as amylase. Because a chromosome contains many genes, one chromosome carries the instructions for making many different proteins. The proteins produced determine many of the characteristics of an organism.
The genome of an organism is the entire genetic material of that organism. The whole human genome has now been studied and this will have great importance for medicine in the future.
The genome is the complete set of genetic material in an organism. In humans this means all the DNA in the 46 chromosomes of a body cell, including the roughly 20 000 protein-coding genes plus non-coding DNA. The Human Genome Project sequenced the whole human genome, base by base, and the reference sequence is now available. Understanding it matters for medicine: it helps identify genes linked to inherited disorders such as cystic fibrosis, supports genetic testing and counselling, guides the design of medicines matched to a patient's genotype, and reveals how some diseases arise. It also raises ethical questions about privacy and insurance.
Students should be able to discuss the importance of understanding the human genome.
Discussing the human genome means weighing up benefits and limitations. Benefits include identifying genes linked to inherited disorders such as cystic fibrosis, enabling genetic testing and counselling, understanding how cancers arise from mutations, and developing medicines matched to a patient's genotype. Limitations include the role of environment, the fact that many conditions involve several genes, and ethical concerns about privacy, insurance and editing genes. A good discussion gives both sides and reaches a supported judgement, using accurate terms such as genome, allele, mutation and genetic testing.
search for genes linked to different types of disease
Scientists can compare the DNA of many people to find gene variants that are more common in people with a particular disease. This is a search for correlations: a gene variant is identified as linked to a disease when it appears more often in affected people than in unaffected people. For example, researchers might compare the genomes of thousands of people with type 2 diabetes and thousands without it, then look for variants that are over-represented in the first group. A link does not prove that the variant causes the disease, because other factors such as diet or environment may also differ. The search helps identify targets for new medicines and can support earlier diagnosis or risk assessment.
understanding and treatment of inherited disorders
Inherited disorders arise from alleles passed from parents to offspring. A recessive disorder, such as cystic fibrosis, only appears when both alleles are recessive (for example, both parents are carriers, Cc × Cc, giving a 1 in 4 chance of an affected child, cc). A dominant disorder, such as polydactyly, appears if at least one dominant allele is present (for example, Pp × pp gives a 1 in 2 chance). Understanding inheritance lets families use genetic diagrams and Punnett squares to predict probabilities, and allows screening, genetic counselling and informed decisions. Treatment may involve managing symptoms, for example physiotherapy and enzyme replacement for cystic fibrosis, or surgery for polydactyly; gene therapy is an experimental approach that aims to replace or repair faulty alleles.
use in tracing human migration patterns from the past.
DNA carries variations that accumulate slowly over generations, so comparing DNA from different populations can reveal how humans moved around the world in the past. Mitochondrial DNA is inherited only from the mother, and the Y chromosome only from the father, so each traces a single line of ancestry. By comparing these sequences, scientists build family trees of populations and estimate when groups separated. For example, mitochondrial DNA evidence supports the idea that modern humans originated in Africa and then migrated to other continents. The more similar the DNA sequences, the more closely related the populations are and the more recently they shared a common ancestor. This evidence is combined with archaeology and linguistics to reconstruct migration routes.
Your focus
- Describe DNA as a polymer made of two strands forming a double helix.
- State that DNA is contained in structures called chromosomes.
- Define genome as the entire genetic material of an organism.
Show all 24 objectives
- State that DNA is the chemical making up the genetic material in the nucleus.
- Describe DNA as a polymer of two strands forming a double helix.
- Identify chromosomes as the structures that contain DNA in the nucleus.
- Define a gene as a small section of DNA on a chromosome.
- Explain that a gene codes for a particular sequence of amino acids.
- Relate the amino acid sequence coded by a gene to the specific protein made.
- Define the genome as the entire genetic material of an organism.
- Describe that the whole human genome has been studied and sequenced.
- Explain how understanding the human genome can be important for medicine in the future.
- Describe medical benefits of understanding the human genome.
- Identify limitations and ethical concerns of using human genome information.
- Reach a supported judgement about the importance of understanding the human genome.
- Describe how genomes are compared to find gene variants linked to disease.
- Explain why a statistical link between a gene variant and a disease is not proof of causation.
- Apply the idea to a named disease and state a medical benefit of the finding.
- Describe how recessive and dominant alleles cause inherited disorders.
- Use genetic diagrams to predict the probability of offspring inheriting a disorder.
- Explain how knowledge of inheritance supports counselling, screening and treatment of inherited disorders.
- Describe how DNA sequences can be compared to trace human migration.
- Explain why mitochondrial DNA and the Y chromosome are useful for tracing ancestry.
- Interpret data on DNA similarities to identify relatedness between populations.
DNA and the genome exam tips
Marking Points
- State that DNA is a polymer.
- Describe DNA as consisting of two strands.
- Explain that the two strands coil together to form a double helix.
- State that DNA is contained in structures called chromosomes.
- Define the genome as the entire genetic material of an organism.
- DNA is the chemical of which the genetic material in the nucleus is composed.
- DNA is a polymer, built from many repeating units joined into a long molecule.
- DNA consists of two strands that wind around each other to form a double helix.
- DNA is contained in chromosomes, which are the structures visible in the nucleus.
- A chromosome is a coiled or packaged molecule of DNA, so chromosomes carry the DNA.
- The nucleus of a cell contains the DNA, and in human body cells there are 46 chromosomes.
- A gene is a small section of DNA located on a chromosome.
- Each gene codes for a particular sequence of amino acids.
- The sequence of amino acids determines the specific protein that is made.
- Different genes code for different proteins because they have different base sequences.
- A chromosome contains many genes, so it carries instructions for many proteins.
- The order of amino acids in a protein is controlled by the gene, and this affects the protein's shape and function.
- Defines the genome as the entire genetic material of an organism, not just the genes that code for proteins.
- States that the human genome is contained in the DNA of the chromosomes, 46 in a typical human body cell.
- Recalls that the whole human genome has been sequenced and that the sequence is stored as a reference.
- Explains at least one medical benefit, such as identifying genes linked to inherited disorders, enabling genetic testing, or matching medicines to a patient's genotype.
- Recognises that the genome includes non-coding DNA as well as protein-coding genes.
- Uses the term genome accurately rather than confusing it with a single gene or a chromosome.
- Gives at least one medical benefit of understanding the human genome, such as identifying genes linked to inherited disorders.
- Gives at least one further benefit, such as genetic testing, counselling, or matching medicines to a patient's genotype.
- Recognises a limitation or ethical concern, such as privacy of genetic data, insurance discrimination, or the influence of environment.
- Uses correct terminology, including genome, allele, mutation and genetic testing.
- Reaches a supported judgement rather than listing points without a conclusion.
- Describes comparing DNA sequences or gene variants between people with and without a disease.
- Explains that a gene is linked to a disease when a variant is more common in affected individuals.
- Recognises that a statistical link does not by itself prove causation.
- Gives a valid purpose, such as identifying drug targets, improving diagnosis or estimating risk.
- Uses a specific disease example, such as type 2 diabetes or breast cancer, to illustrate the method.
- State that inherited disorders are caused by alleles passed from parents to offspring, and that the disorder may be recessive or dominant.
- Use a genetic diagram or Punnett square with correct parental genotypes to work out the probability of offspring inheriting a disorder, for example Cc × Cc giving a 1 in 4 chance of cystic fibrosis.
- Distinguish recessive from dominant inheritance: a recessive disorder needs two recessive alleles, whereas a dominant disorder needs only one dominant allele.
- Describe how understanding inheritance supports genetic counselling, carrier testing and prenatal screening so families can make informed choices.
- Describe treatments that manage symptoms, such as physiotherapy and enzyme replacement for cystic fibrosis, or surgery for polydactyly, and note that gene therapy is still being developed.
- State that differences in DNA sequences between populations can be compared to work out how closely related they are.
- Describe how mitochondrial DNA is inherited only from the mother and the Y chromosome only from the father, so each traces ancestry along one line.
- Explain that the more similar the DNA sequences, the more recently the populations shared a common ancestor, and the more different they are, the longer ago they separated.
- Apply this to a named example, such as evidence that modern humans originated in Africa and then migrated to other parts of the world.
- Recognise that DNA evidence is used alongside other evidence, such as archaeology and linguistics, to trace past human migration.
Examiner Tips
- 💡When describing DNA structure, always mention that it is a polymer and forms a double helix.
- 💡For the genome definition, use the exact phrase 'entire genetic material of an organism' to ensure you get the mark.
- 💡Use the key terms polymer, two strands, double helix and chromosomes in your answer, because these are the ideas being checked.
- 💡If asked to describe the structure of DNA, work from the general to the specific: chemical, polymer, two strands, double helix, then chromosomes.
- 💡When a question shows a diagram of a nucleus, label the chromosomes and state that they contain DNA rather than writing only 'DNA'.
- 💡Link the three ideas in order: gene, sequence of amino acids, specific protein, so your answer shows the full chain of reasoning.
- 💡Use the phrase 'particular sequence of amino acids' rather than 'amino acids' alone, because the sequence is what makes each protein specific.
- 💡If asked why different cells make different proteins, refer to which genes are expressed rather than saying the genes are different, unless the question is about different organisms.
- 💡Define genome in one precise sentence before adding medical examples.
- 💡Link each medical benefit to a named situation, such as testing for a faulty allele linked to cystic fibrosis.
- 💡If asked about the future, use cautious wording such as 'could help' or 'may allow' rather than absolute claims.
- 💡Plan two benefits and one limitation before writing, then add a short concluding judgement.
- 💡Use connectives such as 'however' and 'therefore' to show balanced discussion.
- 💡Anchor each point to a named example, such as testing for the allele that causes cystic fibrosis.
- 💡Use comparative language such as more common in people with the disease than without it.
- 💡Name a disease and state what the researchers compare, so the method is clear.
- 💡Add a sentence about why the link is useful, for example to guide development of medicines.
- 💡Always define the letters you use, for example C for the dominant allele and c for the recessive allele, and keep the same letters throughout the genetic diagram.
- 💡Show the parental genotypes, the gametes and the offspring genotypes in a clear grid so the examiner can follow your reasoning.
- 💡When asked about treatment, link the treatment to the specific symptom or faulty protein rather than giving a general answer such as 'take medicine'.
- 💡Use the phrase 'common ancestor' when explaining why two populations have similar DNA sequences.
- 💡If a question gives data on DNA differences, compare the numbers directly and state which populations are most closely related and why.
- 💡Link the method to a specific type of DNA, such as mitochondrial DNA or the Y chromosome, to show precise understanding.
Common Mistakes
- Error: describing DNA as a single straight strand. Correction: DNA consists of two strands twisted into a double helix.
- Error: defining genome as a single gene or a single chromosome. Correction: the genome is the entire genetic material of an organism, including all its genes.
- Error: confusing a gene with a genome. Correction: a gene is a small section of DNA on a chromosome that codes for a specific sequence of amino acids, whereas the genome is the entire genetic material.
- Saying DNA is made of chromosomes: correct this by stating that chromosomes are structures that contain DNA, not the other way round.
- Describing DNA as a single strand: correct this by stating that DNA has two strands forming a double helix.
- Confusing the double helix with a double circle or a flat ladder: correct this by describing two strands twisted around each other.
- Saying a gene codes for an amino acid: correct this by stating that a gene codes for a sequence of amino acids that forms a protein.
- Saying a gene is a whole chromosome: correct this by stating that a gene is a small section of DNA on a chromosome.
- Saying a gene codes for a protein directly without mentioning amino acids: correct this by including the sequence of amino acids as the link between the gene and the protein.
- Saying the genome is a single gene or one chromosome; correction: it is the entire genetic material of the organism.
- Claiming the genome is only the protein-coding DNA; correction: it includes non-coding DNA too.
- Stating that studying the genome will cure all diseases; correction: it improves understanding, testing and treatment options, but does not guarantee cures.
- Listing only benefits and ignoring limitations; correction: include at least one limitation or ethical concern.
- Confusing genetic testing with gene therapy; correction: testing identifies alleles, whereas gene therapy aims to treat a disorder.
- Claiming a person's genome alone determines all their characteristics; correction: environment and lifestyle also contribute.
- Claiming that finding a linked gene proves the gene causes the disease; correction: state that the evidence shows an association that may involve other factors.
- Confusing the search for linked genes with gene therapy; correction: the search identifies candidate genes, whereas gene therapy aims to treat a disorder.
- Describing only one person's genome; correction: the method relies on comparing data from many people in different groups.
- Writing that a carrier of a recessive disorder is affected; correction: a carrier has one recessive allele and one dominant allele and is usually unaffected but can pass the recessive allele on.
- Confusing the probability of each child with the proportion of a family; correction: each pregnancy is independent, so a 1 in 4 chance applies to each child, not to a fixed number of children.
- Assuming a dominant disorder must be more common or more severe; correction: dominance describes which allele is expressed, not how common or serious the disorder is.
- Thinking that mitochondrial DNA is inherited from both parents; correction: mitochondrial DNA is inherited only from the mother, so it traces the maternal line.
- Assuming that DNA evidence alone proves a migration route; correction: DNA evidence is combined with other evidence such as fossils, tools and languages.
- Confusing similarity of DNA with similarity of appearance; correction: DNA sequence similarity shows relatedness, which may not match visible features.