Why do bones heal on their own after a fracture, but damaged cartilage has difficulty repairing itself ?

Describe the role of proto-oncogenes and tumor suppressor genes in regulating the cell cycle. How can mutations in these genes contribute to cancer development? Provide specific examples of genes involved in cancer.  

Differentiate between the three types of muscle tissue (skeletal, cardiac, and smooth) in terms of structure, function, and location in the body. How does the nervous system control each type of muscle?