A level-readyAll subjects · Biology · WBI12 (Unit 2)
Cell specialisation and gene expression
Every body cell has the same genes, yet a nerve cell and a muscle cell are very different. The difference is which genes are switched on.
Differential gene expression
Cells become specialised because only some of their genes are expressed (switched on). An active gene is transcribed to mRNA, which is translated into proteins. These proteins (for example enzymes, structural proteins or hormones) determine the cell's structure and what it does.
For example, cells that become red blood cells express the genes for haemoglobin; β cells in the pancreas express the insulin gene. Other genes in those cells are switched off.
Gene expression is controlled by proteins called transcription factors, which bind to DNA and switch transcription on or off.
One gene, many proteins
In eukaryotes, a gene is first transcribed into pre-mRNA, which contains exons (coding sections) and introns (non-coding sections).
In splicing, the introns are removed and the exons joined. In alternative splicing, exons are joined in different combinations, or some exons are left out. So one gene can produce several different mRNA molecules and therefore several different proteins.
These post-transcriptional changes help explain how humans make far more proteins than they have genes.
Worked example
A muscle cell and a nerve cell from one person contain the same DNA. Explain why they make different proteins.
- Different genes are switched on in each cell type.
- Only active genes are transcribed to mRNA and translated.
- So each cell type makes its own set of proteins.
Answer: Differential gene expression: each cell type transcribes and translates a different set of genes.
Check yourself
- Why are cells in the same body specialised in different ways?
- Each cell type has different DNA
- Some cells lose all their genes
- Different genes are expressed in different cells
- Cells swap chromosomes
Show answer
Different genes are expressed in different cells. All cells have the same genes, but different ones are active.
- What happens when a gene is expressed?
- It is transcribed to mRNA, which is translated into a protein
- It is copied by DNA polymerase
- It is removed from the chromosome
- It mutates
Show answer
It is transcribed to mRNA, which is translated into a protein. Expression means making the gene's product.
- What happens to introns during splicing?
- They are translated
- They are added to DNA
- They are removed from pre-mRNA
- They join to tRNA
Show answer
They are removed from pre-mRNA. Introns are non-coding sections cut out of pre-mRNA.
- How can one gene produce several different proteins?
- Its introns are translated
- Exons in its pre-mRNA are spliced together in different combinations
- It is copied into several chromosomes
- Its codons change randomly each time
Show answer
Exons in its pre-mRNA are spliced together in different combinations. Alternative splicing gives different mRNAs.
- What is the role of transcription factors?
- They carry amino acids to ribosomes
- They join nucleotides in replication
- They remove introns
- They bind to DNA and switch transcription of genes on or off
Show answer
They bind to DNA and switch transcription of genes on or off. Transcription factors control which genes are transcribed.
- Insulin is made only in β cells of the pancreas, yet the insulin gene is in all body cells. Why?
- Other cells have lost the gene
- The gene is switched on only in β cells
- Only β cells have ribosomes
- Insulin mRNA is made in all cells and destroyed in most
Show answer
The gene is switched on only in β cells. Differential gene expression.
Sources
Last updated 7 November 2026. Spotted a mistake? Tell us.