Why do cells need to regulate genes?
A gene is a piece of DNA that holds the recipe for a protein. Gene expression means using that recipe: DNA → mRNA (transcription) → protein (translation).
Making protein costs energy. So a cell makes a protein only when and where it is needed. Turning genes on or off is called gene regulation.
Control can happen at many points: whether DNA is open, whether transcription starts, how mRNA is processed, how fast it is translated, and how long the protein lasts. The most common control point is the start of transcription.
Genes that are always on (e.g. for respiration enzymes) are called housekeeping genes. Others are switched on only by a signal.
The lac operon: a switch in bacteria
In bacteria, genes that work together often sit side by side and share one switch. This unit is an operon.
The lac operon of E. coli has:
- Promoter – where RNA polymerase lands.
- Operator – the on/off switch, next to the promoter.
- Structural genes (lacZ, lacY, lacA) – recipes for enzymes that take in and break down lactose (e.g. β-galactosidase).
A separate regulator gene (lacI) makes a repressor protein.
No lactose → genes OFF
The repressor binds the operator and blocks the polymerase. No mRNA, no enzymes, no waste.
Lactose present → genes ON
Lactose (as allolactose) binds the repressor and changes its shape. The repressor lets go of the operator. Polymerase transcribes the genes and the enzymes are made. Because a substance switches it on, the lac operon is called an inducible operon; lactose is the inducer.
When glucose is also present, the bacterium prefers glucose and the lac genes stay low, because an activator protein (CAP) is only active when glucose is scarce.
Gene control in eukaryotes
Plant and animal cells rarely use operons. Each gene has its own promoter and is controlled by many proteins called transcription factors.
- Activators bind DNA regions called enhancers, which can be far from the gene. The DNA loops so the activator touches the polymerase and helps it start.
- Repressors bind silencers and stop transcription.
Signals from outside (hormones, growth factors) bind receptors. A chain of reactions inside the cell (a signal cascade) switches on transcription factors. Example: the hormone oestrogen enters a cell, binds its receptor, and the pair acts as a transcription factor.
Later controls: introns are cut out of the pre-mRNA (splicing, which can join exons in different ways), small RNAs can block mRNA, and proteins can be broken down.
Epigenetics and cell differentiation
DNA in eukaryotes is wound round proteins called histones. Tightly packed DNA is hard to read; loosely packed DNA is easy to read.
Epigenetic marks change how a gene is used without changing its base sequence:
- DNA methylation – small methyl (–CH₃) tags on DNA usually switch a gene off.
- Histone changes – tags such as acetyl groups loosen packing and switch genes on.
Some marks are copied when a cell divides, so daughter cells keep the same on/off pattern. Diet, stress, temperature and toxins can change marks: this is how the environment affects genes.
Cell differentiation: a fertilised egg divides into many cells. Different signals switch on different sets of genes, so cells become muscle, nerve or blood cells. All keep the same DNA; only their gene expression pattern differs. Stem cells still have many genes available, which is why they can become different cell types.
Try it: the 3D and at home
In the 3D: on the last step, predict first, then test: (1) lactose ON, methyl OFF; (2) lactose ON, methyl ON; (3) lactose ON plus activator. Which gives the most mRNA?
At home: make a "gene switch" card game. Write PROMOTER, OPERATOR and GENE on three cards in a row. A coin is the repressor; a sweet is lactose. Put the coin on OPERATOR: gene off. Put the sweet on the coin and lift both: gene on.
Exam focus
Expect: label a lac operon diagram; explain what happens with and without lactose; say why it is called inducible; compare prokaryote and eukaryote control; explain how cells with the same DNA become different; define epigenetics with an example.
Key formulas and definitions
- Operon = promoter + operator + structural genes (one switch)
- Repressor on operator → transcription blocked
- Inducer (lactose) + repressor → repressor leaves → genes ON
- Activator + enhancer → more transcription
- Methylated DNA / tight histones → gene OFF
- Same DNA + different genes ON = different cell types
Worked examples
1. E. coli grows in a medium with lactose but no glucose. What happens to the lac operon?
Lactose binds the repressor, which leaves the operator. CAP is active because glucose is low, so it helps polymerase. The lac genes are transcribed strongly and the enzymes are made.
2. A mutant repressor cannot bind the operator. What happens?
The operator is never blocked, so the lac genes are transcribed all the time, with or without lactose (constant expression). The cell wastes energy.
3. A mutant repressor cannot bind lactose. What happens?
Lactose cannot remove it, so the repressor stays on the operator. The genes stay off even when lactose is present, and the cell cannot use lactose.
4. A liver cell and a nerve cell from the same person have identical DNA. Why are they different?
Different transcription factors and epigenetic marks switch on different sets of genes. The liver cell expresses liver enzymes; the nerve cell expresses ion channels and neurotransmitter genes.
Common mistakes
- Saying lactose binds the operator. Lactose binds the repressor, not the DNA.
- Thinking different cells have different DNA. They have the same DNA; they express different genes.
- Thinking epigenetic marks change the base sequence. They do not; they change how readable the DNA is.
- Mixing up promoter and operator: the promoter is where polymerase binds; the operator is where the repressor binds.