Actin filaments have polarity. this means that the two ends can be identified due to structural differences. the plus end is the end to which subunits are added more rapidly, or the end of polymerization. which of the following would enable you to identify the plus end of actin filaments?
-add radiolabeled actin subunits to a mixture of actin filaments in which conditions are favorable for polymerization.
-add radiolabeled actin subunits to a mixture of actin filaments in which conditions favor depolymerization.
-determine the ionic charge of the ends of the actin filaments.

adding radiolabeled actin subunits to a mixture of actin filaments in which conditions are favorable for polymerization

Explanation:

The basic unit of actin filaments is G-actin or monomeric, globular form of actin. Actin polymerization is a process in which subunits are linking together in order to form filament. Monomers are rapidly added to the + end. Polymerization process can be divided into three stages:

•the nucleation phase-formation of actin nucleus which is composed of three actin monomers

•the elongation phase-adding of actin monomers to the + end (usually helped by elongation factors such as formin) which must be exposed, capp-free

•steady phase

Favorable in vitro condition that induces polymerization can be achieved by addition of salts to a solution of G-actin.