Question

You hypothesize that a DNA sequence located 3.0 kb upstream of the promoter of a gene you are studying

contains an enhancer. To test your hypothesis, you use recombinant DNA techniques to insert a DNA

fragment containing this DNA sequence into a plasmid also containing a “reporter gene.”

Note:A reporter gene encodes for a protein that you can detect when expressed. For example, you could use the lacZ gene that encodes for the enzyme β-Galactosidase as a reporter gene. As discussed in the “Prokaryotic gene expression” section, β-Galactosidase expression can be monitored using the dye, X-Gal.You place the DNA sequence in different positions relative to the reporter gene. In which positions do you expect an enhancer would stimulate expression of the reporter gene?

A) 3.0 kb upstream of the start codon of the reporter gene construct.

B) in an intron of the reporter gene construct.

C) 1 kb downstream of the stop codon of the reporter gene construct.

D) only A and B are correct

E) A, B and C are all correct.

Answer 1

A) 3.0 kb upstream of the start codon of the reporter gene construct.

An enhancer always induce the expression of a particular gene. An enhancer sequence always present ahead of the promoter (or, upstream of the promoter) and facilitate binding of some trans- acting protein (inducer) to activate the gene expression. It can not present in between the gene (i.e. in a intron) or after gene (i.e. downstream of the stop codon) as these situation will hamper the gene expression by incorporating mechanical constrain. (like if the enhancer is present inside the gene, the bound trans- acting protein will hamper the movement of RNA polymerase through the gene, and if the enhancer is present after stop codon, the DNA have to bend such a way that the trans- acting protein can interact with the RNA polymerase and transcription factor complex and this bending will incorporate mechanical constrain to the multi-protein complex)