Question

Record what you expect the expression pattern of the structural gene (s) to be, given the absence/presence of the environmental signal, and the genotype of the regulatory gene (r) or the operator (o). write "on" when you expect the structural gene to be expressed, and "off" when you expect it to be repressed.

Answer 1

The expression pattern of structural genes in prokaryotic operons is determined by the state of a regulatory gene and operator; genes are 'off' when a repressor is bound to the operator, and 'on' when the repressor is inactivated by an environmental signal or an activator is present.

Step-by-step explanation:

In the context of gene expression in prokaryotic cells, particularly using operons such as the lac operon in E. coli, the expression pattern of structural genes in the presence or absence of an environmental signal depends on the functional state of the regulatory gene (r) and the operator (o). If the regulatory gene product (repressor) is active and the environmental signal (such as an inducer or corepressor) is absent, the repressor binds to the operator and prevents transcription of the structural genes; this means the genes are "off". Conversely, if the environmental signal inactivates the repressor or an activator is present, the structural genes are transcribed, or "on".

For example, with the lac operon, in the absence of lactose, the regulatory gene product (repressor) is bound to the operator, and the structural genes for lactose metabolism are "off". In the presence of lactose, it acts as an inducer, inactivating the repressor, which allows RNA polymerase to bind to the promoter and transcribe the structural genes, turning the operon "on". Other operons, like the trp operon, work in the opposite manner, being "on" by default and turning "off" in the presence of tryptophan, which acts as a corepressor.