The Biology of Cancer - Vibrational Spectroscopy for Tissue Analysis - page 43

6. Senescence: When normal human cells are grown in culture, they
stop dividing after about 50-60 doublings of the population [3].
This phenomenon is called senescence . However, cancer cells do not
senesce, and grow indefinitely, like bacteria, if given suitable grow-
ing conditions. Some cellular products can cause the cancerous cells
to senesce. This will lead to the suppression of the effect of cancer
promoting genes.
The Cell Cycle
All cells reproduce by dividing in two, with each parental cell giving rise to
two daughter cells on completion of each cycle of cell division. These newly
formed daughter cells can themselves grow and divide, giving rise to a new
cell population formed by the growth and division of a single parental cell
and its progeny. In meiosis, however, a cell is permanently transformed and
cannot divide again [1,63].
Cell division is usually a small segment of a larger cell cycle, and is the
biological basis of life. The division cycle of most cells consists of four coor-
dinated process: cell growth, DNA replication, distribution of the duplicated
chromosomes to daughter cells, and cell division. The primary concern of
cell division is the maintenance of the original cell's genome. Although cell
growth is usually a continuous process, DNA is synthesized during only
one phase of the cell cycle, and the replicated chromosomes are then distrib-
uted to daughter nuclei by a complex series of events preceding cell division.
Progression between these stages of the cell cycle is controlled by a conserved
regulatory apparatus, which not only coordinates the different events of the
cell cycle, but also links the cell cycle with extracellular signals that control
cell proliferation. In other words, the cell cycle machinery is itself regulated
by the growth factors that control cell proliferation, allowing the division of
individual cells to be coordinated with the needs of the organism as a whole.
Not surprisingly, defects in cell cycle regulation are a common cause of the
abnormal proliferation of cancer cells, so studies of the cell cycle and cancer
have become closely interconnected [64].
Phases of the Cell Cycle
A typical eukaryotic cell cycle for human cells in culture takes place approxi-
mately every 24 hours. The cell cycle is divided into two basic parts: mito-
sis and interphase . Mitosis (nuclear division) is the most dramatic stage of