You are studying swimming motility in a pathogenic bacillus. You create mutations in random genes and then test which mutations effect swimming motility by looking at the mutant cells under the microscope. One of the mutant bacteria cannot swim anymore, but still rotates around in one spot when you watch them. Using electron microscopy you discover that some parts of the flagella are still present in the cell wall, but no long flagella are visible. Which gene do you think is mutated (i. e., missing) and which motility-related parts are still present in this mutant?

The correct answer is "The gene that is mutated is the gene that encodes for flagellin, and the motility-related parts that are still present are most likely the elements of the basal body: the central rod and the L, P, C, and MS rings".

Explanation:

Swimming motility for pathogenic bacilli is mediated by flagellin, the protein that forms the filaments needed for the proper arrangement of the flagellum. If a mutant bacteria has no long flagella it is very likely that the gene that is mutated is the one that encodes for flagellin, and the flagella can no be long because it lacks filaments. The parts of the flagella that most likely are still present in the mutated bacteria are the ones no related with flagellin, which are the elements of the basal body: the central rod and the L, P, C, and MS rings.