You have prepared actin filament seeds in a microfluidic chamber through which you then pass either unlabeled or fluorescently labeled actin subunits in alternation several times, keeping the total monomer concentration constant. You then observe the resulting filaments under a fluorescence microscope. If the filaments appear as drawn schematically in the example below (in which fluorescence is indicated by black color), which end do you think is the plus end-(a) or (b)? Was the concentration of free actin subunits below or above the critical concentration (Cc) for polymerization at the barbed end?
A) (a); below
B) (b); below
C) (a); above
D) (b); above

Question

You have prepared actin filament seeds in a microfluidic chamber through which you then pass either unlabeled or fluorescently labeled actin subunits in alternation several times, keeping the total monomer concentration constant. You then observe the resulting filaments under a fluorescence microscope. If the filaments appear as drawn schematically in the example below (in which fluorescence is indicated by black color), which end do you think is the plus end-(a) or (b)? Was the concentration of free actin subunits below or above the critical concentration (Cc) for polymerization at the barbed end? 
A) (a); below 
B) (b); below 
C) (a); above 
D) (b); above

Quizplus · Accepted Answer

Since the polymerization rate is higher at the plus (barbed) end, the fluorescent segments at this end are longer than those at the minus (pointed) end. Both ends have grown (at different rates), which means the subunit concentration has been above the critical concentration.

You Have Prepared Actin Filament Seeds in a Microfluidic Chamber