Chromosomes
Revise Chromosomes for GCSE Biology with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Human body cells contain 23 pairs of chromosomes, and each chromosome carries a large number of genes.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Chromosomes are long molecules of DNA found in the nucleus of a cell. Each chromosome carries a large number of genes, and each gene codes for a particular protein.
Human body cells contain 46 chromosomes, arranged as 23 pairs. One chromosome of each pair comes from the mother and one from the father, which is why offspring resemble both parents.
Gametes are the exception. Sperm and egg cells contain 23 single chromosomes rather than 23 pairs, so that when they fuse at fertilisation the full number of 46 is restored rather than doubled.
Revision notes
Structure and content
A chromosome is a long molecule of DNA carrying many genes. Genes are sections of DNA that code for particular proteins.
Chromosomes are found in the nucleus of eukaryotic cells. They are only visible as distinct structures when the cell is dividing, because at other times the DNA is unwound.
Chromosome numbers
Human body cells contain 46 chromosomes in 23 pairs. The members of a pair carry genes for the same characteristics in the same order.
Gametes contain 23 single chromosomes — half the number — which is described as haploid. Body cells with the full 46 are diploid.
Why pairs matter
One chromosome of each pair is inherited from each parent, so an individual has two copies of every gene.
Those two copies may be the same or different, which is the basis of genetic inheritance and the reason some characteristics can be carried without being shown.
Key points
- Chromosomes are long molecules of DNA in the nucleus.
- Each chromosome carries many genes.
- A gene codes for a particular protein.
- Human body cells have 46 chromosomes in 23 pairs.
- Gametes have 23 single chromosomes.
- One chromosome of each pair comes from each parent.
Worked examples
Example 1
How many chromosomes are found in a human body cell? [1 mark]
Model answer
46, arranged as 23 pairs (1 mark).
Example 2
Explain why gametes contain half the number of chromosomes found in body cells. [2 marks]
Model answer
Gametes contain 23 single chromosomes rather than 23 pairs (1 mark), so that when a sperm and an egg fuse at fertilisation the full number of 46 is restored rather than doubled (1 mark).
Example 3
Describe the relationship between a chromosome, a gene and a protein. [3 marks]
Model answer
A chromosome is a long molecule of DNA (1 mark). A gene is a section of that DNA (1 mark), and each gene codes for a particular protein (1 mark).
Common mistakes
Saying a gene and a chromosome are the same thing.
A chromosome carries many genes; a gene is a section of the DNA within it.
Giving 23 as the number in a body cell.
There are 46 chromosomes, which is 23 pairs. 23 is the number in a gamete.
Saying chromosomes are always visible.
They are only visible as distinct structures during cell division.
Forgetting that both parents contribute.
One chromosome of each pair comes from each parent.
Exam tips
- Say 46 chromosomes in 23 pairs — give both figures.
- Keep chromosome, gene and protein clearly distinguished.
- Explain the halving in gametes by referring to fertilisation.
- Use the words diploid and haploid if you are confident with them.
Key terms
- Chromosome
- A long molecule of DNA carrying many genes.
- Gene
- A section of DNA coding for a particular protein.
- Gamete
- A sex cell containing half the usual number of chromosomes.
- Diploid
- Having chromosomes in pairs, as in a body cell.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.