What you'll learn
- Why most cells in a multicellular organism contain the same DNA but look and behave differently.
- How gene expression leads to protein production, and how this controls cell structure and function.
- How transcription factors and chromatin can switch genes on or off.
- How to interpret simple gene expression data in an exam.
The starting point: most body cells have the same genome
A genome is the complete set of DNA in an organism or cell. In eukaryotic cells, DNA is found as chromosomes inside the nucleus.
A gene is a length of DNA that codes for a functional product, usually a polypeptide, which folds to form a protein. Proteins include enzymes, membrane channels, receptors, structural fibres and hormones.
A somatic cell is a body cell, not a gamete. Most somatic cells in an organism are produced by mitosis, so they contain the same genetic information as the fertilised egg cell they came from.
That creates a big question: if a neurone, muscle cell and liver cell contain the same DNA, why are they so different? The answer is that they use different parts of the genome.
Same genome, different use
Most specialised cells contain the same DNA, but different genes are expressed. This means they make different mRNA and proteins, giving them different structures and functions.
Do not overextend the rule
Mature mammalian red blood cells have no nucleus, so they cannot transcribe new genes. Gametes are haploid, so they do not have the same genome as somatic cells. In most exam questions, use normal nucleated body cells when explaining differential gene expression.
Gene expression: from DNA to protein
Gene expression means using the information in a gene to make its functional product. For protein-coding genes, this involves two main stages:
- Transcription: a complementary mRNA copy of the gene is made from DNA in the nucleus.
- Translation: ribosomes use the mRNA sequence to join amino acids into a polypeptide.
The polypeptide then folds and may be modified to form a functional protein.
The set of proteins made by a cell is sometimes called its proteome. A muscle cell and a liver cell have the same genome, but they have different proteomes.
Why proteins matter for cell specialisation
Proteins directly affect what a cell can do. For example:
| Cell type | Highly expressed genes may code for | Resulting specialisation |
|---|---|---|
| Neurone | Ion channels, neurotransmitter receptors | Rapid electrical signalling |
| Muscle cell | Actin and myosin | Contraction |
| Ciliated epithelial cell | Tubulin, dynein and other cilia proteins | Movement of mucus across surfaces |
| Liver cell | Detoxification enzymes, plasma proteins | Metabolism and secretion into blood |
Linking active genes to cell function
A ciliated epithelial cell in the trachea has high expression of genes coding for tubulin, dynein and enzymes used in aerobic respiration. Explain how this helps the cell carry out its function.
-
Tubulin gene expression leads to mRNA production and translation of tubulin proteins, which form microtubules inside cilia.
-
Dynein proteins interact with these microtubules and use ATP to produce bending movements, so the cilia can beat.
-
Increased expression of respiratory enzyme genes supports a high rate of aerobic respiration in mitochondria, helping to supply the ATP needed for ciliary movement.
-
Beating cilia move mucus along the airway, helping remove trapped dust and pathogens.
Differential gene expression
Differential gene expression
Differential gene expression is when different cells express different genes, or express the same genes at different rates, even though they contain the same DNA sequence.
“Switched on” and “switched off” are useful shorthand, but in reality gene expression is often about level as well as yes/no. A gene might be highly expressed, weakly expressed or not expressed at all.
Some genes are active in almost all cells because they are needed for basic cell survival. These are often called housekeeping genes. They may code for proteins needed in respiration, DNA replication, transcription, translation or membrane transport.
Other genes are expressed only in particular cell types. These cell-specific genes are what make cells specialised.
Thinking unused genes are removed
A specialised cell usually does not lose the genes it is not using. Those genes are still present in the DNA, but they are not being transcribed, or they are transcribed at a very low rate.
How are genes switched on or off?
A gene is not expressed just because it exists. The cell has to allow transcription to happen.
A promoter is a DNA sequence near the start of a gene where RNA polymerase and other proteins bind to begin transcription. RNA polymerase is the enzyme that joins RNA nucleotides together to make mRNA.
A transcription factor is a protein that binds to DNA and affects the rate of transcription. Some transcription factors help RNA polymerase bind, increasing transcription. Others block transcription or recruit proteins that make the DNA less accessible.
DNA in eukaryotic cells is wrapped around proteins called histones, forming chromatin. If chromatin is tightly packed, RNA polymerase cannot easily reach genes, so transcription is reduced. If chromatin is more open, genes are more accessible and can be transcribed.
Epigenetic control
Epigenetic modifications are changes that affect gene expression without changing the DNA base sequence.
Examples include:
- adding methyl groups to DNA, often reducing transcription;
- modifying histone proteins, which can make chromatin more open or more condensed.
These changes can be long-lasting, so a cell can “remember” its identity after it divides by mitosis.
Best answer chain
For a strong explanation, link the levels clearly: gene switched on → mRNA made → protein produced → cell structure or process changes → specialised function.
Differentiation: becoming specialised
Cell differentiation is the process by which an unspecialised cell becomes specialised for a particular function. Cell specialisation is the result: the cell has structural and biochemical features suited to its role.
Early embryos contain stem cells, which are unspecialised cells that can divide and differentiate. Their ability to form different cell types is called potency.
- Totipotent cells can form all cell types, including extra-embryonic tissues such as the placenta.
- Pluripotent cells can form many body cell types, but not all extra-embryonic tissues.
- Multipotent cells can form a limited range of related cell types, such as blood stem cells forming different blood cells.
As development continues, signals from neighbouring cells and the environment affect transcription factors. This changes which genes are expressed. The proteins produced can then reinforce the cell’s identity, sometimes by switching on more cell-specific genes.
In multicellular organisms, specialised cells with similar functions are grouped into tissues. Different tissues work together in organs, and organs work together in organ systems.
Interpreting gene expression data
Scientists can investigate differential gene expression by measuring mRNA or protein abundance. If a gene is highly transcribed, more mRNA for that gene is usually present.
In practical contexts, expression data should be compared carefully. Good investigations use repeats, standardised tissue samples, and often compare the gene of interest with a housekeeping gene as a control.
Equating mRNA directly with protein
A high mRNA concentration suggests a gene is being transcribed, but it does not always prove there is lots of functional protein. Translation rate, protein modification and protein breakdown also matter.
Calculating fold change in gene expression
A gene coding for a muscle contraction protein has a normalised expression of 48 arbitrary units in cardiac muscle cells and 3 arbitrary units in liver cells. Calculate how many times higher the expression is in cardiac muscle.
-
Use a ratio because the question asks “how many times higher”:
fold change=normalised expression in cardiac musclenormalised expression in liver\text{fold change} = \frac{\text{normalised expression in cardiac muscle}}{\text{normalised expression in liver}}fold change=normalised expression in livernormalised expression in cardiac muscle -
Substitute the values:
fold change=48 arbitrary units3 arbitrary units\text{fold change} = \frac{48\ \text{arbitrary units}}{3\ \text{arbitrary units}}fold change=3 arbitrary units48 arbitrary units -
The arbitrary units cancel, giving:
fold change=16\text{fold change} = 16fold change=16 -
Expression is 16 times higher in cardiac muscle cells, supporting the idea that this gene is important for muscle cell specialisation.
Pulling it together
Differential gene expression explains how one genome can produce many cell types. A fertilised egg divides by mitosis to produce cells with the same DNA. During development, signals affect transcription factors and chromatin structure. This changes which genes are transcribed, which changes which proteins are made. Those proteins give cells their specialised structures and functions.
In the exam
-
Start with the core idea: most specialised body cells have the same DNA, but different genes are expressed.
-
Always link gene expression to protein production, then link the protein to a named structure or function.
-
If given expression data, compare like with like, use ratios carefully, and avoid saying a gene has “changed” unless the question describes a mutation.
Check yourself
- Why can a neurone and a liver cell contain the same genome but produce different proteins?
- What is the role of transcription factors in differential gene expression?
- Why is mRNA abundance useful evidence for gene expression, but not perfect evidence for protein abundance?
