A cancer can be described as a group of rapidly growing cells that lose the ability to be controlled by the command center of the cell. The command center of the cell is the nucleus. DNA, a component of the nucleus, provides the cell with the directives to grow and divide and also the inhibitory signals to stop growth and even self-destruct, if things go awry. The cell cycle is the natural and orderly progression that cells take to undergo proper division.
The Cell Cycle
The cell cycle is a series of four steps. The four steps are known as G1, S, G2 and M. The steps conclude in the creation of a new cell. The cell cycle stages represent the activity of the nucleus and other cellular components. The G1 phase prepares the cell for copying, by increasing its growth. The instruction provided by the nucleus in the S phase, also known as the synthesis phase, directs the copying of the DNA. Once the DNA is replicated, the Institute notes that G2, otherwise known as gap 2, leads to further cell growth and reorganization in preparation for division. The M phase is the last stage of the cell cycle. Also known as mitosis phase, M represents the stage where the single mother cell divides into two daughter cells.
The relay of information that allows one level to proceed to the next is monitored by a system of checks and balances, according to a 2007 "Proceedings of the National Academy of Sciences" article. The checkpoint system ensures that each phase is thoroughly completed and no errors have occurred, before permitting movement into the next phase. The article also notes that cancer influences the cell cycle by reducing the sensitivity of the cell to its innate checkpoint system. Alterations in this system will allow damaged cells to proceed with growth that would have otherwise been signaled to stop growing and await better conditions, or simply die. The deterrents to optimal cell cycle progression are damaged DNA, incomplete replication of DNA during the S-phase, and insufficient nutrients to support growth.
Growth Without Stimulation
Only a small percentage of cells are going through the cell cycle at any one time. The cells that are replicating are responding to external stimuli. Cancer cells appear to be able to grow without the presence of external stimuli. Under normal conditions, cells will grow or regenerate after injury, pre-programmed cell death or enticing environmental conditions. The usual chemical stimulants are hormones, proteins or other specialized growth factors. The stimulating substances bind to the receptors on the cell and signal the nucleus to turn on the genes that start the cell cycle. Cancer cells are uniquely able to signal cell cycle progression without the binding of external stimuli. This action is likened to driving a car without first pressing the gas pedal.
Ignore Stop Signals
The cell cycle utilizes the checkpoint system to immediately halt the process of cell division if a problem is encountered. The system has inborn stop signals. A primary deterrent to cell division is damaged DNA. A normal cell will not continue to advance through the G1, M and G2 phases if its DNA became damaged. Prevention of this action will stop stop the damaged DNA from being passed down to the next generation of cells. A cancer cell, however, can ignore the stop signal, complete cell division and continue to proliferate and propagate the faulty DNA further along. This faulty DNA is a mutation -- these genes cause the cells to act inappropriately and encourage increased tumor growth.
Neighboring cells also help dividing cells monitor their need. The neighboring cells maintain a supply-demand relationship between the cells undergoing replication. Chemicals are sent between neighbors to help keep the level of new cell growth measure in close proximity to the rate of cell death. Crowding of cells, trigger warning signals that should slow down or stop the rate of growth in an area that is becoming overpopulated. Cancer cells also evade these warnings. They continue to grow within a limited space and create a pile-up of cells, often described as a tumor or cancer mass.