Which protein will migrate through a SDS-PAGE gel the fastest? a. a positively charged protein b. a globular protein c. a protein with a low net charge d. an elongated protein

Answer: In SDS-PAGE (sodium dodecyl sulfate polyacrylamide gel electrophoresis), proteins are separated primarily based on their molecular weight rather than their charge or shape. SDS is a detergent that denatures proteins and binds to them, giving them a uniform negative charge relative to their mass. Due to this, the charge differences between proteins do not affect their migration through the gel.

Considering the options given: a. A positively charged protein - The charge will be masked by SDS, thus it is not relevant. b. A globular protein - The shape is made irrelevant by SDS, as it denatures the protein into a linear form. c. A protein with a low net charge - Again, the net charge is neutralized by SDS, making this a non-factor. d. An elongated protein - This pertains to the shape, which is altered by SDS, but might imply a higher molecular weight.

The key factor in SDS-PAGE migration speed is the molecular weight of the protein. Since SDS coats proteins with negative charges proportional to their length (and hence their mass), larger proteins will move more slowly through the gel matrix, while smaller proteins will move more quickly. Therefore, none of the characteristics mentioned directly determines the speed of migration through an SDS-PAGE gel. However, if we consider "an elongated protein" to imply a longer, and therefore possibly heavier, protein, it will generally migrate slower than a shorter, smaller protein. So by elimination, the fastest migrating protein would likely be the one with the smallest molecular weight, which is not explicitly mentioned in the options. But if we were to infer which characteristic might indirectly suggest a smaller protein, none of the options provided (a, b, c) give a clear indication of molecular weight. That being said, all else being equal, a protein that maintains a lower molecular weight regardless of its native charge or shape would migrate the fastest.