12.5 Cancer and the cell cycle

Introduction

While there are several chronic diseases more destructive to life than cancer, none is more feared.

Mayo's words are still true today; a diagnosis of cancer is a fearful thing. But what is cancer? Cancer is a collective name for many different diseases caused by a common mechanism: uncontrolled cell division. Despite the redundancy and overlapping levels of control of cell division, errors occur. One of the critical processes monitored by the cell-cycle checkpoint surveillance mechanism is the proper replication of DNA during the S phase. Even when all of the cell-cycle controls are fully functional, a small percentage of replication errors (mutations) will be passed on to the daughter cells. If one of these changes to the DNA nucleotide sequence occurs within a gene, a gene mutation results. All cancers begin when a gene mutation gives rise to a faulty protein that participates in the process of cell reproduction. The change in the cell that results from the malformed protein may be minor. Even minor mistakes, however, may allow subsequent mistakes to occur more readily. Over and over, small, uncorrected errors are passed from parent cell to daughter cells and accumulate as each generation of cells produces more non-functional proteins from uncorrected DNA damage. Eventually, the pace of the cell cycle speeds up as the effectiveness of the control and repair mechanisms decreases. Uncontrolled growth of the mutated cells outpaces the growth of normal cells in the area, and a cancerous tumor can result.

Some definitions

Characteristics of cancer cells

What are the characteristics of a cancer cell, and how does it differ from a normal cell? Over the decades scientists have discovered many morphological and physiological differences ( [link] ), and studying those differences led to many of the advances in our knowledge of the cell cycle and its regulation. Cancer biologists have summarized and analyzed many of these known differences. It is known that cancer can result from mutations in many genes, and that cancers in different organs differ in their physiology, appearance, growth rate, and many other parameters. But when they filtered through all the data, they concluded that there are six essential alterations in cell physiology that are important hallmarks of the malignant state.

Cancer and normal cells