How do cells control which genes are expressed and when?
Explain how gene expression is regulated, including the lac operon in prokaryotes and differential expression in eukaryotes
Cells regulate which genes are expressed; the lac operon is the prokaryotic model and differential gene expression explains how identical eukaryotic cells specialise.
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What this dot point is asking
You need to explain why gene regulation is necessary and describe a prokaryotic example (the lac operon) and the eukaryotic idea of differential gene expression leading to cell differentiation.
Why regulate genes?
A cell contains far more genes than it needs at any one time. Making every protein continuously would waste energy and resources, and many proteins are only useful in particular conditions. Gene regulation lets a cell switch genes on or off in response to its needs and environment.
Prokaryotic regulation: the lac operon
In bacteria such as E. coli, related genes are often grouped into an operon controlled together. The lac operon controls the enzymes needed to digest the sugar lactose.
Key parts of the operon:
- Structural genes that code for the lactose-digesting enzymes.
- A promoter, where RNA polymerase binds to start transcription.
- An operator, a control sequence next to the promoter.
- A separate regulatory gene that codes for a repressor protein.
How it switches:
- Lactose absent. The repressor binds the operator, physically blocking RNA polymerase. The genes are switched off, so the enzymes are not made.
- Lactose present. Lactose (as allolactose) binds the repressor and changes its shape so it can no longer bind the operator. RNA polymerase is now free to transcribe the genes, so the enzymes are made and the lactose is digested.
This is an efficient, responsive switch: the bacterium only makes the enzymes when their substrate is available.
Eukaryotic regulation: differential gene expression
Eukaryotes do not generally use operons. Instead, regulation occurs at several points, and the central concept for SACE is differential gene expression.
Every body cell in a multicellular organism contains the same complete genome, yet a nerve cell, a muscle cell and a skin cell look and behave very differently. This is because each cell type expresses a different subset of its genes - some genes are switched on and others off. The pattern of genes expressed determines which proteins are made, and therefore the cell's structure and function.
Differential gene expression is the basis of cell differentiation, the process by which unspecialised cells become specialised during development. Regulatory proteins (transcription factors) and chemical signals during development determine which genes a cell switches on.
Exam-style practice questions
Practice questions written in the style of SACE Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2019 SACE Stage 22 marksScientists found different DNA methylation patterns in the genes required for milk production in dairy cows. Explain how methylation of the genes that are required for the production of milk could affect milk production.Show worked answer →
For 2 marks, link methylation to gene expression, then to the phenotype.
DNA methylation adds methyl groups to the gene (typically at cytosine bases), which switches the gene off or reduces its transcription, so less or no mRNA is made.
With the milk-production genes silenced, less of the protein needed for milk production is made, so the cow produces a lower volume of milk. This is epigenetic regulation: the base sequence is unchanged, only its expression is altered.
2018 SACE Stage 21 marksIn some patients there is more DNA methylation of the CDKN1C gene than in individuals who do not have the syndrome. State the effect of increased DNA methylation of the CDKN1C gene on its expression.Show worked answer →
Increased methylation decreases (switches off, or reduces) expression of the CDKN1C gene, so less CDKN1C protein is produced. The mark is for stating that methylation lowers or silences expression.
2018 SACE Stage 21 marksThe product of gene 1 is a protein that binds to a promoter region on the DNA near gene 2, resulting in the expression of gene 2. A mutation in which one of the following will most likely prevent the expression of gene 2: an intron of gene 1, an exon of gene 1, the product of gene 1, or the product of gene 2?Show worked answer →
An exon of gene 1. The protein product of gene 1 is a regulatory (transcription) factor that must bind the promoter to switch on gene 2. A mutation in an exon of gene 1 changes the coding sequence, so the gene 1 protein may be misfolded or unable to bind the promoter, preventing expression of gene 2. An intron mutation is usually removed during splicing, and gene 2's own product acts after expression has already occurred.