AQA GCSE Combined Science Foundation (8464), Foundation tier · Biology › Cell biology › Cell division
Practise Chromosomes. 6 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
What chromosomes are, how they relate to DNA and genes, and how they are arranged in body cells. Questions are usually short: recall, putting structures in size order, and working out chromosome numbers.
Key facts
Chromosomes are found in the nucleus
Chromosome = a long, coiled molecule of DNA
Gene = a small section of DNA on a chromosome that codes for a particular protein
Each chromosome carries a large number of genes
Body cells: chromosomes in pairs (humans: 23 pairs = 46)
Gametes: one chromosome from each pair (humans: 23)
Size: cell > nucleus > chromosome > gene
Notes
Chromosomes, DNA and genes
diagram
The nucleus of a cell contains chromosomes.
Chromosomes are made of DNA molecules. Each chromosome is a very long DNA molecule, tightly coiled.
Each chromosome carries a large number of genes. A gene is a small section of DNA that codes for a particular protein.
Cell → nucleus → chromosome → gene. A gene (blue) is a small section of the DNA in a chromosome.
Chromosomes come in pairs
In body cells, chromosomes are normally found in pairs.
Human body cells contain 23 pairs of chromosomes: 46 in total.
One chromosome of each pair came from the mother (in the egg) and the other from the father (in the sperm).
Gametes (sperm and egg cells) contain only one chromosome from each pair: 23 in humans. When a sperm and an egg join at fertilisation, the pairs form again.
Different species have different numbers of chromosomes. For example, a dog body cell has 78 (39 pairs).
Why this matters
Before a cell divides by mitosis, each chromosome is copied, so each new cell gets a full set of chromosome pairs.
Genes on chromosomes are passed from parents to offspring, which is why offspring share features with their parents.
How to answer each type of question
Put structures in order of size
1 markGrade 4
Remember that each structure contains the next: a cell contains a nucleus, a nucleus contains chromosomes, a chromosome carries genes.
Check the question: largest first or smallest first?
Example. Put these structures in order of size, starting with the largest. chromosome, gene, nucleus, cell
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cell, nucleus, chromosome, gene (1)
Describe how DNA, genes and chromosomes are related
2 to 3 marksGrade 5
Say what chromosomes are made of (DNA).
Say what a gene is (a small section of DNA).
Say how many genes a chromosome carries (a large number).
Example. Describe how genes, chromosomes and DNA are related.
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Chromosomes are made of DNA (1). A gene is a small section of DNA (1). Each chromosome carries a large number of genes (1).
Work out chromosome numbers
1 to 2 marksGrade 5
Pairs × 2 = total number of chromosomes in a body cell.
A gamete has one chromosome from each pair, so it has the same number as there are pairs.
Example. A cat body cell has 19 pairs of chromosomes. (a) How many chromosomes are in a cat body cell? (b) How many chromosomes are in a cat sperm cell?
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(a) 38 (1) (b) 19 (1)
Don’t lose marks
Saying a chromosome is a gene, or that a gene contains chromosomes. Genes are sections of a chromosome.
Saying humans have 46 pairs of chromosomes. It is 23 pairs, 46 in total.
Saying gametes have 23 pairs. They have 23 single chromosomes.
Saying chromosomes are in the cytoplasm of animal or plant cells. They are in the nucleus.