Question:
A) Explain succinctly how the expression of a structural gene is regulated. Define and include the following in your answer: activator, promoter, regulatory gene, repressor, structural gene, transcription factor.

B) Explain why the expression of a structural gene might be altered by a mutation in its promoter region.

Answer:
A) A structoral gene es a gene that encodes a protein that contributes to a biological trait. Upstream of these genes sit the regulatory sequences known as promoters that serve as the on/off switch for expression (transcription) of the gene. Proteins known as transcription factors are encoded by regulatory genes that have their own promoter elements and genomic location. They function to recognize promoter elements of structural genes or other regulatory genes and bind to them. Transcription factors that encourage the transcription of DNA to mRNA upon binding are known as activators, while those that prevent transcription are known as repressors.

B) A mutation in the promoter region of a structural gene can change both the type of transciption factors that bind, and the strength of their binding. As a result, the timing and levels of structoral gene expression will be altered.


Not necessary for full-credit answer, but important point: Changing the timing and levels of expression of this gene may lead to this gene being expressed in a new context (cell type, developmental stage, etc.). This new context can be good from an evolutionary perspective if the trait helps the animal survive in its repective environment.


*It is important to realize that a mutation in the promoter region does not alter the actual protein that is encoded in the gene. The start codon (ATG) is a specific amino acid sequence encoded at the beginning of genes that tell the transcription machinery where the actual coding sequence starts. Therefore, a mutation in the promoter region will not affect the protein sequence, which is entirely upstream of the mutation in question. Rather, it will affect the timing or levels of protein expression, which them allows new interactions, and possibly a new/altered biological trait.