Why is "Saying a chromosome is the same thing as a gene." a common mistake in Chromosomes, Genes and Proteins?

Why it happens: Both are inheritance terms. How to avoid it: A chromosome contains MANY genes; a gene is a small section of a chromosome.

Why is "Saying body cells have 23 chromosomes." a common mistake in Chromosomes, Genes and Proteins?

Why it happens: 23 is the gamete (haploid) number. How to avoid it: Body cells have 46 (23 PAIRS); gametes have 23. Don't confuse the number with the number of pairs.

Why is "Saying different cell types contain different genes." a common mistake in Chromosomes, Genes and Proteins?

Why it happens: The cells look and act very differently. How to avoid it: All body cells contain the SAME genes (made by mitosis from the zygote); they differ because each uses only some of those genes.

InheritanceCambridge IGCSE Biology 0610 Extended

Chromosomes, Genes and Proteins

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Chromosomes, Genes and Proteins — Cambridge IGCSE 0610 Biology Extended (2026)

DNA → genes → proteins → traits. The hierarchy that links your genome to what you look like.

What you’ll learn

Mapped to the Cambridge IGCSE 0610 syllabus (2026-2028).

17.2 — Describe the relationship between DNA, chromosomes and genes.
17.2 — State that genes code for proteins.
17.2 — Outline protein synthesis (transcription + translation).
17.2 — State chromosome numbers in humans.

The hierarchy: DNA → chromosome → gene → protein

Each level zooms in. DNA molecule → chromosomes → genes (sections) → alleles (versions) → proteins → traits.

DNA.

Long molecule, double helix.
Made of nucleotides (sugar + phosphate + nitrogenous base).
Bases: adenine (A), thymine (T), guanine (G), cytosine (C).
A pairs with T; G pairs with C — the COMPLEMENTARY pairs.

Chromosome.

DNA wrapped around proteins (histones) → coiled tightly.
Visible under microscope when cell is dividing.
Humans: 23 PAIRS = 46 total in body cells.
Sex chromosomes: XX (female) or XY (male).

Gene.

A SECTION of DNA on a chromosome.
Codes for ONE specific protein (or part of one).
Different genes code for different proteins.
Humans have ~20,000-25,000 genes.

Allele.

A particular version of a gene.
Each gene typically has 2 alleles in a diploid cell (one from each parent).
Examples: blood type gene has alleles A, B, O.

Protein.

The PRODUCT of a gene.
Made of amino acids in a specific sequence.
Determines a TRAIT (e.g. enzyme catalysing a colour pigment → eye colour).

Zooming in: nucleus → chromosome → gene → the protein it codes for.

Worked qualitative. Why do CHROMOSOMES form pairs in body cells but not in gametes?

Body cells diploid (2n) — have one chromosome from each parent → forms pairs.
Gametes haploid (n) — one of each chromosome → no pairs.
This way, fertilisation (sperm + egg) restores the pairs.

Cambridge tip. State 'GENE = section of DNA coding for a protein'. Cambridge marks this directly.

DNA: double helix.
Chromosome: coiled DNA.
Gene: section of DNA → one protein.
Allele: version of a gene.
Protein: makes the trait.

How a protein is made — transcription and translation

Two stages. DNA stays in the nucleus; mRNA carries the message to ribosomes.

Stage 1 — TRANSCRIPTION (in the NUCLEUS).

The relevant gene 'unzips' (DNA opens).
An RNA copy of the gene is made — mRNA (messenger RNA).
mRNA detaches from DNA.
DNA closes back up (intact).

Stage 2 — TRANSLATION (at a RIBOSOME, in cytoplasm).

mRNA leaves the nucleus through nuclear pores.
mRNA attaches to a ribosome.
Ribosome reads mRNA in groups of THREE bases (called codons).
Each codon specifies one AMINO ACID.
Amino acids brought in by tRNA (transfer RNA), joined together in order.
Chain of amino acids = the PROTEIN.

DNA stays in the nucleus; mRNA carries the message out to a ribosome where the protein is built.

Why mRNA?

DNA stays safe in the nucleus.
mRNA = a temporary copy that can be sent out to where work is done.
Multiple ribosomes can read the same mRNA → many proteins from one gene.

Worked qualitative. Why does a mutation in DNA usually change the protein?

Mutation = change in DNA sequence.
Changed DNA → changed mRNA → changed codons → different amino acid → different protein.
A different protein may not work properly → genetic disease.
Example: sickle-cell anaemia is caused by a single base change in the haemoglobin gene.

Cambridge tip. Cambridge IGCSE wants the OUTLINE — DNA → mRNA → ribosome → protein. Don't go too deep into the molecular detail.

Transcription: DNA → mRNA (nucleus).
mRNA exits nucleus.
Translation: mRNA → protein (ribosome).
Codons (3 bases) = one amino acid.
Amino acids join → protein.

Chromosome numbers and sex chromosomes

Body cells: 46. Gametes: 23. Sex: XX (female) or XY (male).

In humans:

Cell	Chromosomes	Notes
Body cell (diploid)	46	23 PAIRS
Sperm or egg (gamete, haploid)	23	One of each pair
Zygote	46	23 from each parent — diploid restored

Sex chromosomes (the 23rd pair).

XX = female (two X chromosomes).
XY = male (one X, one Y).
Eggs always carry an X.
Sperm carry either X or Y → determines baby's sex.
50:50 chance.

The father's sperm (X or Y) decides the baby's sex; the ratio is always 50:50.

Worked qualitative. Why is the SEX of a baby determined by the FATHER, not the mother?

Mother provides only X (in eggs).
Father provides X OR Y (50:50 in sperm).
If father's sperm has X → baby = XX = girl.
If father's sperm has Y → baby = XY = boy.

Cambridge tip. Memorise 46 (body) / 23 (gamete). Cambridge often gives a chromosome count and asks 'is this body cell or gamete?'.

Body: 46 (23 pairs).
Gametes: 23.
Female XX, male XY.
Father determines sex.

How it’s examined

Chromosomes/genes/proteins appear Paper 4 (5-7 marks: hierarchy, protein synthesis outline, chromosome numbers). Examiner reports flag students saying chromosomes ARE genes, and missing 'codes for protein'.

Sources: Cambridge IGCSE Biology 0610 syllabus 2026-2028 (17.2); 0610/42 Oct/Nov 2024 — Q17 (DNA, proteins); 0610 Examiner Reports 2022-2024. Last reviewed 2026-05-07.

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Step-by-step worked examples — Chromosomes, Genes and Proteins

Step-by-step solutions to past-paper-style questions on chromosomes, genes and proteins, written exactly the way a tutor would explain them at the board.

1DNA, chromosomes and genes
Getting started• DNA hierarchy
Question
Describe the relationship between DNA, a chromosome, a gene and an allele.
Step-by-step solution
1. Step 1
  A chromosome is a long, coiled molecule of DNA (with protein).
2. Step 2
  A gene is a section of DNA on a chromosome that codes for one protein.
3. Step 3
  An allele is a particular version of a gene.
4. Step 4
  So: a chromosome is made of DNA; it carries many genes; each gene can exist as different alleles.
Answer
A chromosome is a coiled DNA molecule; a gene is a section of that DNA coding for a protein; an allele is a version of a gene.
2Chromosome numbers
Getting started• chromosome counts
Question
State the number of chromosomes in a human (a) body cell, (b) gamete and (c) zygote.
Step-by-step solution
1. Step 1
  (a) A body cell is diploid and has 46 chromosomes (23 pairs).
2. Step 2
  (b) A gamete is haploid and has 23 chromosomes (one of each pair).
3. Step 3
  (c) A zygote is formed when two gametes fuse, so it has 23 + 23 = 46 chromosomes (diploid restored).
Answer
Body cell: 46. Gamete: 23. Zygote: 46.
Examiner tip
46 is the diploid number (23 PAIRS); 23 is the haploid number. Do not confuse the number with the number of pairs.
3Genes control characteristics through proteins
Building confidence• genes, proteins
Question
Explain how a gene controls a characteristic of an organism.
Step-by-step solution
1. Step 1
  A gene is a length of DNA that carries the code for making a particular protein.
2. Step 2
  The protein made could be a structural protein (e.g. part of a cell) or an enzyme that controls a chemical reaction.
3. Step 3
  These proteins then produce the characteristic — for example, an enzyme that makes a pigment gives a flower its colour.
4. Step 4
  So genes control characteristics by controlling which proteins are made in the cells.
Answer
A gene carries the code for a protein; the protein (structural or an enzyme) produces a characteristic. So genes control characteristics by controlling which proteins are made.
4Why all body cells have the same genes
Building confidence• mitosis, genes
Question
All the body cells of an organism contain the same genes, yet they do different jobs. Explain how the cells come to have the same genes, and why they are still different.
Step-by-step solution
1. Step 1
  All the body cells are produced from the original zygote by mitosis, which copies the DNA exactly.
2. Step 2
  So every body cell contains the same complete set of genes.
3. Step 3
  However, only some of the genes are 'switched on' (used) in each type of cell.
4. Step 4
  Different cells use different genes, so they make different proteins and become specialised for different jobs, even though they share the same genes.
Answer
All cells are made by mitosis from the zygote, so they have the same genes. They differ because each cell type only uses (switches on) some of those genes, making different proteins.
5How a protein is made from a gene
Stretch• Supplement 17.2• protein synthesis
Question
Outline how the information in a gene is used to make a protein.
Step-by-step solution
1. Step 1
  The sequence of bases in the gene (DNA) is the code for the order of amino acids in a protein.
2. Step 2
  A copy of the gene is made as a molecule of messenger RNA (mRNA) in the nucleus.
3. Step 3
  The mRNA passes out of the nucleus to a ribosome in the cytoplasm.
4. Step 4
  At the ribosome the code is read in groups of three bases, and amino acids are joined in the correct order to build the protein.
Answer
The base sequence of the gene codes for the amino acid order. The gene is copied into mRNA, which goes to a ribosome, where amino acids are joined in the coded order to make the protein.
Examiner tip
Cambridge wants: DNA base sequence codes for amino acid order; mRNA copy carries the code to a ribosome; amino acids joined in order.
6How a mutation can change a protein
Stretch• mutation
Question
Explain how a change (mutation) in the base sequence of a gene could change the protein the gene makes.
Step-by-step solution
1. Step 1
  The sequence of bases in the gene determines the order of amino acids in the protein.
2. Step 2
  A mutation is a change in this base sequence.
3. Step 3
  This can change which amino acids are joined, so the protein has a different sequence of amino acids.
4. Step 4
  A different sequence can give the protein a different shape, so it may not work properly — for example, an enzyme whose active site has changed shape may no longer fit its substrate.
Answer
A mutation changes the gene's base sequence, which changes the order of amino acids, so the protein has a different shape and may not work properly (e.g. an enzyme's active site changes).
Examiner tip
Link the base change → amino acid order → protein shape → altered function.

Model Answers — Chromosomes, Genes and Proteins

High-scoring sample answers for chromosomes, genes and proteins on the Cambridge IGCSE 0610 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.

Question 1
Paper 4 short-answer style1 mark
State the number of chromosomes in a normal human body cell. (1 mark)
Model answer
46 chromosomes (23 pairs).
Why this scores
One mark for '46'.
Question 2
Paper 4 short-answer style2 marks
Explain the relationship between a gene and a chromosome. (2 marks)
Model answer
A chromosome is a long molecule of DNA, and a gene is a section (length) of the DNA on a chromosome. Each chromosome carries many genes.
Why this scores
Two marks: gene is a section of DNA on a chromosome; a chromosome carries many genes.
Question 3
Paper 4 structured style3 marks
Explain how a gene controls a characteristic of an organism. (3 marks)
Model answer
A gene is a length of DNA that carries the code for making a particular protein. The protein produced — which may be a structural protein or an enzyme — then produces the characteristic (for example, an enzyme that makes a pigment determines colour). So a gene controls a characteristic by controlling which protein is made.
Why this scores
Three marks: gene codes for a protein; protein is structural or an enzyme; the protein produces the characteristic.
Question 4
Paper 4 (Extended) structured style4 marks
All the body cells of a person contain the same genes, but they are specialised for different jobs. Explain how this is possible. (4 marks)
Model answer
All the body cells are produced from the original zygote by mitosis, which makes exact copies of the DNA, so every cell contains the same complete set of genes. However, in each type of cell only some of the genes are switched on (used). Because different cell types use different genes, they make different proteins, which makes them specialised for different jobs — even though they all carry the same genes.
Why this scores
Four marks: cells made by mitosis from the zygote; same genes in all; only some genes used/expressed in each; different proteins → different specialised cells.
Question 5
Paper 4 (Extended) structured style5 marks
Describe how the information in a gene is used to make a protein. (5 marks)
Model answer
The sequence of bases in the gene (DNA) is a code that determines the order of amino acids in the protein. A copy of the gene is made as a molecule of messenger RNA (mRNA) in the nucleus. The mRNA leaves the nucleus and attaches to a ribosome in the cytoplasm. At the ribosome, the base sequence of the mRNA is read in groups of three bases, and amino acids are joined together in the order specified by the code to build the protein.
Why this scores
Five marks: base sequence codes for amino acid order; mRNA copy made; mRNA goes to a ribosome; read in threes; amino acids joined in the coded order to make the protein.
Question 6
Paper 4 (Extended) extended-response style6 marks
Explain how a mutation in a gene can lead to the production of a non-functioning enzyme. (6 marks)
Model answer
A gene carries a code in the form of a sequence of bases, and this base sequence determines the order of amino acids in the protein the gene makes. A mutation is a change in the base sequence of the gene. Because the base sequence has changed, when the gene is used to make the protein, the amino acids may be joined in a different order, so the protein has a different sequence of amino acids. The order of amino acids determines the way the protein folds and the shape it takes. If the protein is an enzyme, a change in its shape can change the shape of its active site, so that it is no longer complementary to its substrate. The substrate can then no longer fit into the active site, so the enzyme cannot catalyse its reaction and does not work properly. In this way a mutation in a gene can lead to a non-functioning enzyme.
Why this scores
Up to 6 marks: gene base sequence codes for amino acid order; mutation changes the base sequence; changes the order of amino acids; changes the protein's shape; for an enzyme, the active site shape changes; substrate no longer fits / enzyme does not work.

Key Definitions and Keywords — Chromosomes, Genes and Proteins

Definitions to memorise and the exact keywords mark schemes credit for chromosomes, genes and proteins answers — sharpened from recent examiner reports for the 2026 0610 sitting.

Chromosome
Examiner keyword
A long, coiled molecule of DNA. Humans have 46 (23 pairs) in each body cell.
DNA
Examiner keyword
The genetic material, made of two strands of nucleotides in a double helix; carries the code for making proteins.
Gene
Examiner keyword
A length of DNA that codes for a protein. Its base sequence determines the amino acid order of the protein.
mRNA (messenger RNA)
A molecule that carries a copy of a gene's code from the DNA in the nucleus to a ribosome.
Ribosome
The structure where mRNA is read and amino acids are joined together to make a protein.

Common Mistakes and Misconceptions — Chromosomes, Genes and Proteins

The traps other students keep falling into on chromosomes, genes and proteins questions — taken from recent Cambridge IGCSE 0610 examiner reports and mark schemes — and how to avoid them.

✕Saying a chromosome is the same thing as a gene.
Why it happens
Both are inheritance terms.
How to avoid it
A chromosome contains MANY genes; a gene is a small section of a chromosome.
✕Saying body cells have 23 chromosomes.
Why it happens
23 is the gamete (haploid) number.
How to avoid it
Body cells have 46 (23 PAIRS); gametes have 23. Don't confuse the number with the number of pairs.
✕Saying different cell types contain different genes.
Why it happens
The cells look and act very differently.
How to avoid it
All body cells contain the SAME genes (made by mitosis from the zygote); they differ because each uses only some of those genes.

Practice questions

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Chromosomes, Genes and Proteins

Short Study Notes in the form of Slides

Short Study Notes — Chromosomes, Genes and Proteins

Detailed Notes

What you’ll learn

How it’s examined

1DNA, chromosomes and genes

2Chromosome numbers

3Genes control characteristics through proteins

4Why all body cells have the same genes

5How a protein is made from a gene

6How a mutation can change a protein

Chromosome

DNA

Gene

mRNA (messenger RNA)

Ribosome

✕Saying a chromosome is the same thing as a gene.

✕Saying body cells have 23 chromosomes.

✕Saying different cell types contain different genes.

Practice questions

Past paper style quiz

Assess your understanding

Video lesson