How do cells switch genes on and off so that the right proteins are made at the right time?
Explain the control of gene expression in prokaryotes (the lac operon) and the principles of eukaryotic gene control
A focused answer to the H2 Biology Molecular Genetics outcome on gene control. The lac operon as a model of prokaryotic regulation, transcription factors and chromatin in eukaryotes, and why differential gene expression underlies cell specialisation.
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What this dot point is asking
SEAB wants you to explain how cells regulate which genes are expressed, using the lac operon as the model for prokaryotic control and the principles of transcription factors and chromatin for eukaryotic control. The big idea is that all cells of a body share the same genes, so differential gene expression is what makes a nerve cell different from a muscle cell.
The answer
Why control is needed
A cell does not make every protein all the time. Making an unneeded enzyme wastes energy and resources, and in multicellular organisms cells must specialise. Control therefore happens mostly at the level of transcription: switching genes on or off.
Prokaryotic control: the lac operon
In bacteria, related genes are often grouped into an operon: a promoter, an operator, and structural genes transcribed together. The lac operon controls enzymes for lactose metabolism, and a separate regulatory gene makes a repressor protein.
- Lactose absent. The repressor binds the operator and blocks RNA polymerase, so the structural genes are not transcribed and the enzymes are not made.
- Lactose present. Lactose (converted to an inducer) binds the repressor and changes its shape so it leaves the operator. RNA polymerase can now transcribe the genes, and the enzymes are made.
This is an efficient, responsive switch: the enzymes appear only when their substrate is present.
Eukaryotic control
Eukaryotes use several layers:
- Transcription factors bind regulatory sequences and increase (activators) or decrease (repressors) transcription of specific genes.
- Chromatin structure. Tightly packed (condensed) chromatin cannot be transcribed; loosening it allows access. Chemical tags (DNA methylation, histone modification) switch genes off or on without changing the sequence; these are epigenetic controls.
- Post-transcriptional control such as alternative splicing lets one gene make several proteins.
Examples in context
Example 1. Cell specialisation. A liver cell and a neurone carry the same genome but express different genes, so they make different proteins and perform different jobs. This is differential gene expression, controlled largely by which transcription factors are active and which regions of chromatin are accessible.
Example 2. Steroid hormones as gene switches. A steroid hormone enters a target cell, binds an intracellular receptor, and the complex acts as a transcription factor that switches specific genes on. This links the cell signalling dot point directly to the control of gene expression.
Try this
Q1. State what the repressor binds to in the lac operon. [1 mark]
- Cue. The operator.
Q2. Explain why it is advantageous for a bacterium to make lactose-metabolising enzymes only when lactose is present. [2 marks]
- Cue. Making the enzymes when there is no lactose would waste energy, amino acids and other resources; producing them only when needed conserves resources.
Q3. Describe one way in which the packaging of DNA can control gene expression in a eukaryotic cell. [2 marks]
- Cue. When chromatin is tightly condensed, RNA polymerase and transcription factors cannot access the genes, so they are not transcribed; loosening the chromatin allows transcription.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original6 marksExplain how the lac operon controls the expression of the genes for lactose metabolism in a bacterium, in the absence and in the presence of lactose.Show worked answer →
Examiners want both states explained with the named components.
The lac operon has a promoter, an operator, and structural genes coding for enzymes that metabolise lactose. A separate regulatory gene constantly produces a repressor protein.
In the absence of lactose, the repressor binds the operator. This blocks RNA polymerase from transcribing the structural genes, so the enzymes are not made. This avoids wasting resources making enzymes that are not needed.
In the presence of lactose, some lactose is converted to an inducer molecule that binds the repressor and changes its shape so it can no longer bind the operator. The operator is now free, RNA polymerase transcribes the structural genes, and the enzymes are produced to metabolise the lactose.
Markers reward the named components (promoter, operator, structural genes, repressor, inducer), the bound-repressor state when lactose is absent, the release of the repressor by the inducer when lactose is present, and the link to efficient use of resources.
Original4 marksDescribe two ways in which gene expression is controlled in eukaryotic cells, other than by an operon.Show worked answer →
The answer should give two distinct eukaryotic mechanisms.
First, transcription factors. Proteins called transcription factors bind to specific regulatory sequences near a gene and either help RNA polymerase bind and increase transcription (activators) or hinder it (repressors). The combination of transcription factors present in a cell determines which genes are transcribed.
Second, chromatin remodelling and epigenetic control. The way DNA is packaged with histone proteins affects access. When chromatin is tightly condensed, the genes within it cannot be transcribed; when it is loosened, transcription can occur. Chemical tags such as methylation of DNA or modification of histones switch genes off or on without changing the base sequence.
Other acceptable answers include alternative splicing and control of mRNA stability. Markers reward two clearly described, distinct mechanisms with correct terminology.
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