Botany · Ch 7 — Molecular Basis of Inheritance
Regulation of Gene Expression
Regulation of Gene Expression
The Need for Regulation
A cell does not express all its genes at once. That would be wasteful and chaotic. Instead, gene expression is tightly controlled so that a bacterium, for example, makes the enzymes to digest lactose only when lactose is actually present in its environment. This saves energy and resources. The same principle applies to all organisms: genes are switched on or off in response to internal and external signals.
The Operon Model in Prokaryotes
The classic model for understanding gene regulation is the lac operon in E. coli. An operon is a cluster of genes that are transcribed together as a single mRNA molecule. The lac operon contains three structural genes — lacZ, lacY, and lacA — which code for enzymes needed to break down lactose.
The operon also has regulatory elements:
- Promoter: The site where RNA polymerase binds to begin transcription.
- Operator: A short DNA sequence located between the promoter and the structural genes. It acts as a switch.
- Regulator gene (lacI): Located elsewhere on the DNA, it codes for the lac repressor protein.
How the Switch Works: The Repressor
The lac repressor protein is always being produced. In the absence of lactose, this repressor binds tightly to the operator region. When the repressor sits on the operator, it physically blocks RNA polymerase from moving past the promoter and transcribing the structural genes. The operon is switched off.
Induction: Turning the Operon On
When lactose enters the cell, a small amount of it is converted into allolactose. Allolactose acts as an inducer. It binds to the lac repressor protein and changes its shape. This altered repressor can no longer bind to the operator. With the operator free, RNA polymerase can now transcribe the structural genes. The operon is switched on, and the enzymes for lactose digestion are produced.
The lac operon is an inducible operon — it is normally off and is turned on by the presence of an inducer (allolactose).
Negative Control (and a Note on Positive Control)
The lac operon, as described here, works by negative control — the default state is repression. The repressor protein, made constantly from the regulatory (i) gene, actively prevents transcription by sitting on the operator; removing the repressor (by the inducer) is what allows expression to proceed.
The lac operon is also subject to a separate layer of positive control, involving how the cell responds when glucose (its preferred energy source) is available alongside lactose. This additional layer of regulation is beyond the scope of discussion at this level.
Regulation in Eukaryotes
Eukaryotic gene regulation is far more complex than in prokaryotes. It can occur at several levels:
- Transcriptional level: Controlling when and how often a gene is transcribed into RNA. This is the primary control point.
- Post-transcriptional level: Processing the primary RNA transcript (capping, tailing, splicing) to form a mature mRNA.
- Translational level: Regulating how efficiently the mRNA is translated into protein.
- Post-translational level: Modifying the protein after it is made (e.g., phosphorylation, cleavage) to activate or inactivate it.
Key Features of Eukaryotic Regulation
- Chromatin structure: DNA in eukaryotes is wrapped around histone proteins to form chromatin. Tightly packed chromatin (heterochromatin) is generally inaccessible to transcription factors and RNA polymerase. Loosening the chromatin (euchromatin) is necessary for gene expression. Chemical modifications to histones (acetylation, methylation) and to DNA itself (methylation) play a major role in this. …