 | Correct! |
49% of students answered correctly.
Q: | You run three different proteins on an SDS-PAGE gel under reducing conditions. What must be true, given the results of the gel.
|
|
A |
The protein in lane A has two subunits
|
|
B |
The total size of the protein in lane B is larger than the protein in lane A
|
|
C |
The protein in lane B is more negatively charged than the protein in lane C
|
|
D |
The protein in lane C is either a monomer or a protein with subunits of the same size
|
| Why? |
In an SDS-PAGE gel with reducing conditions, disulfide bonds within the protein are reduced. This means that proteins with multiple subunits held together by disulfide bonds will run in their monomeric form on an SDS-PAGE gel under reducing conditions. The SDS will coat the entire protein in a uniform negative charge, so the only thing determining how far it will run on the gel in a given time is its size. A protein with multiple subunits of the same size (such as a homodimer), will appear as one band on an SDS-PAGE gel under reducing conditions, because the subunits will dissociate and run at the same speed. Therefore, we cannot be sure that the protein in lane A has two subunits, because it is possible that it has multiple subunits of each size shown on the gel. We also cannot be sure that the protein in lane B is larger than the protein in lane A, because again, it could be possible that the protein contains multiple subunits of one size that adds up to be larger than the size of the protein in lane B. SDS-PAGE gels tell us nothing about the charge of the protein, because the negative charge coated on the entire protein ensures the only factor contributing to the migration is the size of the protein. Answer choice (D) must be true, because the only two reasons we could get one band on an SDS-PAGE gel under reducing conditions is if the protein is either a monomer, or a protein with multiple subunits of the same size. Thus, (D) is the correct answer!
|
|
We can teach you how to crush the MCAT!
Learn More
|
