The Biology of Cancer - Vibrational Spectroscopy for Tissue Analysis - page 45

shortest subphase during interphase within the cell cycle. At the end of this
gap phase is a control checkpoint (G2 checkpoint) to determine if the cell can
proceed to enter the M phase and divide. The G2 checkpoint prevents cells
from entering mitosis with DNA that was damaged since the last division,
providing an opportunity for DNA repair and stopping the proliferation of
damaged cells. Because the G2 checkpoint helps to maintain genomic stabil-
ity, it is an important focus in understanding the molecular causes of cancer.
Prophase is a stage of mitosis in which chromatin condenses into a highly
ordered structure called a chromosome . Since the genetic material has been
duplicated, there are two identical copies of each chromosome in the cell
(sister chromosomes). When chromosomes are paired up and attached, each
individual chromosome in the pair is called a chromatid , while the whole unit
is called a chromosome . When the chromatids separate, they are no longer
called chromatids, but are called chromosomes again [65].
Metaphase is a stage of mitosis in the eukaryotic cell cycle in which con-
densed chromosomes, carrying genetic information, align in the middle
of the cell before being separated into each of the two daughter cells. The
analysis of metaphase chromosomes is one of the main tools of cancer cyto-
genetics. In anaphase chromosomes separate, and each chromatid moves to
opposite poles of the cell. Telophase is a stage reversing the effects of pro-
phase and prometaphase events. For instance, a new nuclear envelope, using
fragments of the parent cell's nuclear membrane, forms around each set of
separated sister chromosomes. Finally, cytokinesis takes place. This stage is
the process whereby the cytoplasm of a single cell is divided to spawn two
daughter cells.
The duration of these cell cycle phases varies considerably in different
kinds of cells. For a typical rapidly proliferating human cell with a total cycle
time of 24 hours, the G 1 phase might last about 11 hours, the S phase about
8 hours, the G 2 about 4 hours, and the M phase about 1 hour. Some cells
in adult animals cease division altogether (e.g., nerve cells) and many other
cells divide only occasionally, as needed to replace cells that have been lost
because of injury or cell death. Cells of the latter type include skin fibroblasts,
as well as the cells of some internal organs, such as the liver. As mentioned,
these cells exit G 1 to enter a quiescent stage of the cycle called G 0 , where they
remain metabolically active but no longer proliferate unless called on to do
so by appropriate extracellular signals.
Regulation of the Cell Cycle
If the cell cycle is considered to be like an automobile in motion under nor-
mal conditions, proto-oncogenes are like the accelerator and tumour suppres-
sor genes are like brakes. The progression of cells through the division cycle is