Question

A membrane spanning protein that is a channel for potassium ion transport is made up of six copies of a single polypeptide. Potassium ions are transported across the membrane using the channel. In your experiments with the gene encoding the channel proteins, you have isolated a mutant of the potassium channel that transports very small amounts of potassium compared to the wild type. You perform SDS-PAGE electrophoresis with proteins from the mutant and proteins from the wild type. In the Coomassie blue stained gel, the lane for the wild type protein contains a protein band of 240 kDa and the lane for the mutant protein contains a protein band of 80 kDa. We can conclude that each mutant protein is______________________________the wild type subunits and that the molecular weight of each subunit in the mutant protein is ______________________________.

	a.	Shorter than, different
	b.	The same length, the same
	c.	There is not enough information to make a conclusion.
	d.	Shorter than, the same

If each of the three major classes of RNA found in a cell were hybridized to denatured DNA from the same cell and the presence of RNA-DNA hybrids were tested, which of the classes would be retained on the filter?

	a.	tRNA
	b.	All of these
	c.	mRNA
	d.	rRNA

Answer 1

1. Given that the membrane channel protein is made of 6 copies of a polypeptide, and the size of the wild type protein is determined as 240 KDa. This means that the size of each polypeptide is 240 KDa and the actual size of the channel protein is 240 * 6 = 1440 KDa.

Since the mutant protein shows a band at 80 KDa, it means that the subunit size is reduced from 240 KDa to 80 KDa in the case of the mutant. Possibly because of a truncated protein resulting due a non-sense mutation.

The mutant protein subunit is therefore, definitely shorter than the wild type subunit, The mutant protein showed a singe band at 80 KDa. This means that the molecular weights of each of the subunits is the same, if not there would be two or more additional size bands in the lane. So the right answer here is option d.Shorter than, the same.

options a and b are wrong, as the mutant protein is not of the same length as the wild type and the subunits size cannot be different as there is a single band of 80 KDa in the gel.

2. Ribosomal RNA binds to mRNA during translation and would not hybridize with the DNA strand. Also, tRNA would recognize the codons in mRNA, not DNA. So the right answer here would be option c. mRNA.