Question

How many bands are seen when immunoglobulin G analyzed on a sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) under reducing conditions.
a) $6$
b) $1$
c) $2$
d) $4$

Answer 1

Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) is used to separate mainly proteins and as we know that immunoglobulins are made of proteins. It has two heavy chains and two light chains which are joined together by disulfide bonds.

Complete answer :
As we know from our knowledge that an immunoglobulin contains two light chains and two heavy chains. The densities and sizes of these are different. Any kind of gel electrophoresis depends upon size only. The SDS-PAGE is very useful for small proteins and peptides. It differentiated various molecules based on their molecular mass.
Since immunoglobulin contains only two types of chain. One heavy chain and one light chain. Therefore the number of bands formed will be two.

So the correct answer is (C) that is $2$.

Additional information:
The reducing agent used in SDS-PAGE is $2$-mercaptoethanol.
The normal biochemistry behind this is we denature the proteins in the presence of SDS and the reducing agent by heating. SDS will bind to the protein causing it to unfold, whereas the reducing agent will reduce the disulfide bonds and various molecules are separated. Therefore, polypeptide and peptide of different molecular masses are separated. The SDS provides a negative charge to these molecules and is placed on an acrylamide gel. Then these negatively charged molecules placed against the anode.
The mobility of these molecules will be inversely proportional to their molecular mass and therefore we get separate bands.

Note:
Always remember in gel electrophoresis the mobility of molecules does not depend on their or the charge provided to them. Their mobility depends upon their size.
The sieving effect is provided by the gel used.
The smaller the molecule further it will be from the loading site.