In the tissue culture, they usually proliferate indefinitely. The normal constraints which limit the growth of the cells absent in the cancerous state and are also characterized by the division ability for number of generations which is unlimited.
Cell cycle and cancer
With millions of chemical reactions taking place concurrently and in specific areas, the human body can be thought of as a small laboratory. It is the only "machine" with the ability to save fuel when fed in excess and also know to bring out the reserves when facing starvation, capable to protect itself from attacks by viruses and bacteria, ability to make adjustments to withstand changes of weather and the ability to learn, think and create on its own. The human body system is well integrated and organized, able to perform vital functions important to its survival. Malfunctions in the body can have damaging results that range in severity from discomfort to life threatening illnesses. Many processes and reactions taking place within the cells are tightly controlled, an example being cell division. Cell division involves a series of reactions and changes like DNA replication and protein synthesis involving cooperation between arrays of proteins to achieve a common goal.
Cell division is divided into four phases where normal cell division progresses from one phase to the next with strictly controlled "checkpoints." These serve as safety measures for the cell that prevent the control system from dictating the start of another cell cycle event before the previous one as finished or before any damage to the cell has been repaired accordingly. The cell division cycle also depends on external cues. In case cell division is unregulated, and is independent of the external cues, there is possibility of one developing cancer, one of the most devastating diseases in the world (Michor & Nowak, 2004, p.203).
Cell division cycle starts with the growth phase, followed by synthesis, second growth and mitotic phases. On receiving external cues, from growth factors released from neighboring cells, so as to initiate division, the cells move into the growth phase. Here, cells prepare for division by producing more proteins. DNA replication by the cells takes place in the synthesis phase. This brings about creation of identical copies to enable daughter cells inherit an exact copy of the DNA. In the second growth phase, synthesis of the proteins needed for the growth of the daughter cells takes place. The cell separates its DNA and divides into two in the mitotic phase.
Cell division requires the accurate replication and segregation of chromosomes. All the tasks are accomplished in an orderly manner if all the events related to cell division are coordinated throughout the cycle. An example is where cell division occurs before it attains its optimum size and as a result, smaller daughter cells result from the subsequent division. A set of interacting proteins forms the cell control system which is responsible for regulating the process of cell division. This system of proteins directs and coordinates other proteins that are involved in particular tasks like DNA replication. The control system still has to follow feedback signals from the cell cycle itself in spite of its ability to act on other proteins. There are other proteins in the cell cycle involved in surveillance control mechanisms. They are able to stop or delay the control systems' progress at the cycle checkpoints. The loss or inactivation of a gene encoding a protein in the surveillance system can result in susceptibility of developing cancer.
The cell cycle's control system is based on two families of proteins: namely the cyclins and cyclin-dependent kinases (CDK). The CDKs phosphorylate (add a phosphate group) key amino acid residues thus induce other proteins to perform their functions. Cyclins control the CDKs ability to phosphorylate by binding to the CDKs.
Many proteins involved in the surveillance system can delay or terminate the cell cycle progress. Some promote rapid degradation of cyclins and others prevent entry of CDK-cyclin complexes into the cell compartments where they are needed for cell cycle progression. The first checkpoint a cell encounters before entering a cycle is at transition between the dormant and growth phases. Retinoblastoma (Rb) gene is involved in encoding the protein involved in this stage which inhibits the passage of the cell past the cycle's starting point through shutting of transcription of genes necessary for cell division and sequestering the proteins involved in regulation of DNA replication. Rb gene importance is evident in the fact that many common types of cancers miss both its functional copies.
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