DNA is the chemical that stores instructions, a gene is one section of DNA that carries one instruction, and a chromosome is a long packed piece of DNA. The three are not different materials, only different levels.
This lesson is part of SPM Science genetics. The next lesson, completing basic inheritance diagrams, uses these words.
How do genes, chromosomes and DNA fit together?
Think from the cell inwards. A cell contains a nucleus, the nucleus contains chromosomes, each chromosome is one long molecule of DNA, and a gene is one section along that molecule.
So the size order, biggest to smallest, is nucleus, chromosome, gene. DNA is the material itself, so it runs through the chromosome and the gene.
Picture a long road (the chromosome) built from one material (DNA). A gene is one stretch of that road with its own sign.
Worked example: read the statement
An original question says: “A scientist marks one short stretch on a chromosome. This stretch carries the instruction for making one protein.” What is the marked stretch, and which statement is correct?
- The stretch is bigger than the chromosome.
- The stretch is a section of the chromosome.
- The stretch and the chromosome are the same.
A stretch that carries the instruction for one protein is a gene. A gene sits along the chromosome, so statement 2 is correct.
Each chromosome carries many genes, so a gene is always smaller than the chromosome it sits on. Look for a clue like this in the question before choosing a statement.
The wording mistake that loses the mark
The common slip is to write “a chromosome is made of genes, and genes are made of chromosomes”. It sounds fluent, but the second half reverses the order.
| Question | Wrong | Right |
|---|---|---|
| What is a gene? | A small chromosome | A section of DNA that carries the instruction for one characteristic |
| What is a chromosome? | Made of genes only | A long DNA molecule, with proteins, carrying many genes |
| What is DNA? | A type of gene | The chemical that stores genetic information |
The fix is to say the order aloud each time: DNA forms a gene, many genes sit on a chromosome, and chromosomes are in the nucleus.
Where does each term appear in questions?
Questions rarely ask for a bare definition. They ask you to choose the correct term for a situation.
- “Where is the genetic material stored?” asks for nucleus and chromosomes.
- “What carries the instruction for eye colour?” asks for a gene.
- “What is the chemical that contains the instruction?” asks for DNA.
Match the term to what the question asks for: a place, a section or a chemical.
Check yourself
Put these in order from smallest to largest: chromosome, gene, nucleus. Then state, in one sentence, what DNA is.
Answer
Smallest to largest: gene, chromosome, nucleus.
DNA is the chemical that stores the genetic information in a cell. A gene is one section of it, and a chromosome is a long packed molecule of it.
What to study next
Move on to completing basic inheritance diagrams, then try the genetics practice set. The mistake log is a good place to record any term that you swap.
If you want a teacher to listen to you explain these terms aloud, see online one-to-one Science tuition.