Final answer:
Mutations in proto-oncogenes increase the rate of cell division and cause them to become oncogenes, which disrupts balance with tumor suppressor genes, leading to changes in cell signaling and uncontrolled cancer cell growth.
Step-by-step explanation:
In tumor development, mutations in proto-oncogenes further increase the rate of cell division, resulting in clonal selection and amplification of tumorigenic cells. Proto-oncogenes normally play a role in the regulation of the cell cycle, but when mutated they become oncogenes that disrupt the delicate homeostatic balance with tumor suppressor genes and promote uncontrolled cell growth.
Cancer cells that continue to divide despite being defective often exhibit changes in cell signaling pathways. These pathways include the response to growth factors which are mediated by cell-surface receptors linked to tyrosine kinases. The disruption of signals from proto-oncogenes and tumor suppressor genes, which normally control the cell cycle, leads to the unregulated proliferation of cancer cells.
Mutations in these genes can lead to the inactivation of DNA repair genes, allowing cells to accumulate further mutations and contributing to the progression towards cancer. For example, mutations can lead to the loss of 'contact inhibition', a process where extensive cellular contact stops further cell division, thus enabling tumor growth.