The DOG gene is expressed in cats and gives them their fear of large dogs. Studi

Question

The DOG gene is expressed in cats and gives them their fear of large dogs. Studies suggest that the DOG gene is regulated by several proteins, two of which are termed protein 1 and protein 2. These bind to regions in the DOG gene termed region 1 and region 2. A schematic of the DOG gene is shown below.

The bar represents a double-stranded DNA molecule. The length of the bar serves only to illustrate the relative positions of the regulatory regions of this gene; it does NOT imply distances along the DNA.

To determine how these proteins control transcription of the DOG gene, you isolate the gene from various cats and determine the DNA sequences of the binding sites; you also measure the level of DOG mRNA produced in these cats’ cells. Interestingly, different cats have different versions of the gene (alleles) that vary in the nucleotide sequence of the binding sites. If the binding site is normal (wild type) it is indicated by a (+) in the table, and this will mean protein 1 or 2 can bind to its site. If the binding site is mutated it will be indicated with a (), and this will mean protein 1 or 2 cannot bind to the DNA. Here are your results:

SAMPLE SITE 1 SITE 2 LEVEL OF DOG mRNA
CAT 1 + + 200
CAT 2 - + 50
CAT 3 + - 50
CAT 4 - - 0
CAT 5 ? ? 50

Things to consider before you begin the online quiz:
• What is the function of proteins 1 and 2 in the regulation of the DOG gene? (Are they activators,
repressors, or some combination of the two?)
• How do Proteins 1 and 2 affect each other’s function?
• What might happen to the regulation of DOG if an additional protein (Protein 3) bound to site 3?
• How many base-pairs of DNA would be present in the binding sites? In the regions marked A and B?
• What is the expected status of Site 1 and Site 2 in Cat #5? (+ or -)
• Would you predict that Cats #2-5 have less fear of dogs? (Why or why not?)

Explanation / Answer

Answer:

1. Proteins A and B are activators since their presence increases the amount of mRNA transcribed.

2. Synergism refers to a greater than additive increase in transcription when two or more activators are present simultaneously or when additional binding sites for a single activator are introduced.

Proteins A and B have synergistic effect on each other, i.e, there is more than additive increase in the expression of the DOG mRNA when both are present.

3. (Answer in absence of the figure)

Considering that site 3 is a regulatory gene, protein 3 may either increase the expression of the DOG gene if its an activator like Proteins 1 and 2, or may decrease the expression if it is a repressor protein.

4. (The figure is missing which is required to answer the question)

5. Expected status of Site 1: Either + or -

Expected status of Site 2: + if site 1 is -, and - if site 1 is +

6. It can be predicted that Cats 2 to 5 have less fear of dogs. This is because the level of the DOG mRNA (which is responsible for inducing fear of dogs in cats) is maximum in cat 1, but it decreases in cats 2, 3, 4 and 5. Cat 4 should have the least fear of dogs since it has 0 expression for the DOG gene.

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