Chromosomes — AQA GCSE Combined Science
Test yourself on Chromosomes with AQA GCSE practice questions.
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Chromosomes explained
The nucleus is the control centre of a eukaryotic cell and holds the genetic material.
Read the full explanation
That material is organised into chromosomes, and each chromosome is one very long DNA molecule folded and packaged with proteins. Along the DNA molecule are genes: short sections that code for particular proteins, and therefore influence characteristics. Because a single chromosome is enormous compared with one gene, each chromosome carries a large number of genes, not just one. For example, a human body cell has 46 chromosomes but around 20 000 genes, so genes are densely packed along each DNA molecule. Understanding this hierarchy — nucleus, chromosome, DNA, gene — lets you explain inheritance, protein synthesis and mutation.
In body cells the chromosomes are normally found in pairs.
Body cells are diploid: their chromosomes occur in matching pairs, one chromosome of each pair inherited from each parent. In humans there are 46 chromosomes, arranged as 23 pairs. The two chromosomes in a pair carry the same genes in the same order, but they may carry different versions of those genes, called alleles. This is why offspring show a mixture of parental characteristics. Gametes are the exception: sperm and egg cells are haploid, carrying one chromosome from each pair, so that fertilisation restores the diploid number. For example, a human sperm has 23 chromosomes and an egg has 23, giving a zygote with 46 chromosomes in 23 pairs.
Your focus
- State that the nucleus contains chromosomes made of DNA molecules.
- Describe a gene as a section of DNA that codes for a protein.
- Explain that each chromosome carries a large number of genes.
Show all 6 objectives
- State that body cells normally contain chromosomes in pairs.
- Explain that one chromosome of each pair is inherited from each parent.
- Distinguish diploid body cells from haploid gametes.
Chromosomes exam tips
Marking Points
- The nucleus contains the cell's genetic material in the form of chromosomes.
- Chromosomes are made of DNA molecules, so DNA is the molecule of inheritance.
- A gene is a section of a DNA molecule that codes for a protein.
- Each chromosome carries a large number of genes, not a single gene.
- The relationship is hierarchical: nucleus contains chromosomes, chromosomes contain DNA, and genes are sections of that DNA.
- Chromosomes are long DNA molecules, so many genes can be present along one chromosome.
- Body cells are diploid, meaning chromosomes are normally found in pairs.
- One chromosome of each pair comes from each parent.
- The two chromosomes in a pair carry the same genes but may carry different alleles.
- Human body cells contain 46 chromosomes arranged as 23 pairs.
- Gametes are haploid and contain one chromosome from each pair, so fertilisation restores the paired state.
- Pairing allows matching genes to be present twice in a body cell.
Examiner Tips
- 💡Use the hierarchy nucleus → chromosome → DNA → gene when explaining where genetic material is found.
- 💡Define a gene as a section of DNA that codes for a protein, then link it to a characteristic.
- 💡If asked to compare numbers, note that there are far more genes than chromosomes in a body cell.
- 💡Use the terms diploid for body cells and haploid for gametes, and link them to the paired or single chromosome number.
- 💡When explaining inheritance, state that one chromosome of each pair comes from each parent.
- 💡Check whether a question asks for chromosome number or pair number before writing your answer.
Common Mistakes
- Saying a chromosome is made of genes: correct this by stating a chromosome is made of DNA, and genes are sections along that DNA.
- Thinking one chromosome carries one gene: correct this by noting each chromosome carries a large number of genes.
- Confusing the nucleus with the chromosome: correct this by describing the nucleus as the structure that contains the chromosomes.
- Saying chromosomes are always in pairs in every cell: correct this by noting gametes are haploid and carry single chromosomes.
- Thinking the two chromosomes of a pair are identical: correct this by stating they carry the same genes but may have different alleles.
- Confusing the number of chromosomes with the number of pairs: correct this by stating humans have 46 chromosomes, which is 23 pairs.