Fibroblasts can be placed on thin sheets of silicone rubber. Under normal circumstances, fibroblasts exert sufficient tension on the rubber so that it visibly wrinkles. Explain how the ability of fibroblasts to wrinkle rubber would change if the cells were treated with a permeable form of C3 transferase, a toxin (from the bacterium Clostridium botulinum) that covalently modifies an amino acid in Rho. The modification renders Rho unable to bind to proteins that it would normally bind to when active.

C3 transferase decreases Rho activity and contractility by decreasing the number of stress fibers and thus also reducing wrinkles

Explanation:

Rho protein is a small GTPase, i.e., an enzyme that binds GTP to hydrolyze it to GDP, which is known to mediate the formation of stress fibers and focal adhesion. Rho protein is involved in diverse cellular functions including, among others, adhesion, motility, and division by reorganization of the actin cytoskeleton. The bacterial toxin C3 transferase derived from Clostridium botulinum is able to lock Rho in an inactive state. C3 transferase adds an ADP-ribose to Rho, thereby leading to the accumulation of a Rho-guanine nucleotide dissociation inhibitor (GDI) complex in the cytoplasm. In consequence, C3 transferase functions to decrease contractility by blocking Rho activity, decreasing the number of stress fibers and leading to a reduction in wrinkles.

genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).