Practise Chromosomes. 11 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.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
State where chromosomes are foundIn the nucleus of a cell.
4
Describe what chromosomes are made ofChromosomes are made of DNA, and each chromosome carries a large number of genes.
4
State the chromosome number in human cellsBody cells have 23 pairs (46 in total); sperm and egg cells have 23 single chromosomes.
5
Put cell, nucleus, chromosome and gene in orderLargest to smallest: cell, nucleus, chromosome, gene.
6
Explain why body cell chromosomes are pairedOne chromosome of each pair came from each parent, in the egg and the sperm.
6
Link genes to proteinsA gene is a small section of DNA that codes for a particular protein.
Notes
Chromosomes, DNA and genes
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.
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.
Cheatsheet
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
How to answer each type of question
Put structures in order of size
1 mark4
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
Show the model answer
cell, nucleus, chromosome, gene (1)
Describe how DNA, genes and chromosomes are related
2 to 3 marks5
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.
Show the model answer
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 marks5
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?
Show the model answer
(a) 38 (1) (b) 19 (1)
Explain why chromosomes are in pairs
2 marks6
Say where each chromosome of a pair came from.
Link this to the sperm and egg joining at fertilisation.
Example. Explain why the chromosomes in human body cells are found in pairs.
Show the model answer
One chromosome of each pair is inherited from each parent (1). The sperm and the egg each carry one chromosome from each pair, and they join at fertilisation (1).
Think of a chromosome as a book and its genes as the chapters: one book, many chapters, all written in DNA.
Pairs: one chromosome from mum, one from dad.
Where marks are lost
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.
Exam technique
Use 'DNA', 'gene' and 'chromosome' precisely: examiners do not accept them as meaning the same thing.
Read carefully whether the question asks for the number of pairs or the total number of chromosomes.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
The nucleus of a cell contains chromosomes. Name the molecule that chromosomes are made of.
DNA
Write these structures in order of size, starting with the smallest. cell, chromosome, gene, nucleus
gene, chromosome, nucleus, cell
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
The nucleus of a cell contains chromosomes.
(a) Name the molecule that chromosomes are made of.[1]
(b) Complete the sentence. Each chromosome carries a large number of ____________ .[1]
(c) How are chromosomes normally arranged in body cells? Tick (✓) one box.[1]
In groups of three
In pairs
One of each type
Joined into one long chromosome
(d) A human body cell contains 46 chromosomes. How many pairs of chromosomes is this?[1]
Show the answer and mark scheme
(a)Answer: DNA
DNA
(b)Answer: genes
genes
(c)Answer: In pairs
(d)Answer: 23
23
Question 2Medium6 marks
Chromosomes are found in the nucleus of cells.
(a) The body cells of a fruit fly contain 4 pairs of chromosomes. How many chromosomes are in a fruit fly body cell?[1]
(b) Explain why the chromosomes in body cells are found in pairs.[2]
(c) Mature human red blood cells do not contain any chromosomes. Suggest why.[1]
(d) Describe the relationship between chromosomes, DNA and genes.[2]
Show the answer and mark scheme
(a)Answer: 8
8
(b)
one chromosome of each pair comes from the mother / egg
the other comes from the father / sperm
(c)
they have no nucleus
(d)
chromosomes are made of (long molecules of) DNA
genes are (short) sections of the DNA on a chromosome / each chromosome carries many genes
Question 3Hard7 marks
{table} shows the number of chromosomes in a body cell of different organisms.
[object Object]
(a) A student said: 'The more complex an organism is, the more chromosomes its cells contain.' Evaluate the student's statement using {table}.[3]
(b) Chimpanzees and potatoes have the same number of chromosomes. Suggest why they are very different organisms.[2]
(c) Human body cells contain about 20 000 different genes. The 46 chromosomes form 23 pairs. Assume that both chromosomes in a pair carry the same genes. Estimate the mean number of different genes carried on one chromosome.[2]
Show the answer and mark scheme
(a)
the fruit fly (simplest animal shown) has the fewest chromosomes, which supports the statement
but the potato / dog / horse / chimpanzee has more chromosomes than a human, which does not support it
conclusion: there is no clear link between complexity and chromosome number
(b)
their chromosomes carry different genes / different DNA
so they have different characteristics / make different proteins