Polar mutation

Polar mutation is a type of genetic mutation that affects the expression of downstream genes in an operon. This phenomenon is particularly significant in prokaryotes, where genes are often organized into operons, allowing for coordinated regulation of gene expression.

Mechanism

Polar mutations typically occur due to the insertion of a transposon, the presence of a nonsense mutation, or the introduction of a frameshift mutation within the coding sequence of an upstream gene in an operon. These mutations can lead to premature termination of transcription or translation, thereby affecting the expression of downstream genes.

Transcriptional Polarity

In the case of transcriptional polarity, a mutation in an upstream gene can cause the RNA polymerase to terminate transcription prematurely. This results in the failure to transcribe downstream genes, leading to a reduction or complete loss of their expression.

Translational Polarity

Translational polarity occurs when a mutation leads to the premature termination of translation. This can happen due to the introduction of a stop codon within the coding sequence of an upstream gene. The premature stop codon can cause the ribosome to dissociate from the mRNA, preventing the translation of downstream genes.

Examples

Polar mutations are often studied in the context of the lac operon in Escherichia coli. Mutations in the lacZ gene, which encodes β-galactosidase, can affect the expression of downstream genes such as lacY and lacA, which encode lactose permease and thiogalactoside transacetylase, respectively.

Significance

Understanding polar mutations is crucial for comprehending the regulation of gene expression in prokaryotes. These mutations can provide insights into the mechanisms of gene regulation, genetic control, and the functional organization of operons.

Related Pages

• Operon
• Gene expression
• Transcription (genetics)
• Translation (biology)
• Mutation
• Lac operon
• Escherichia coli