Submitted by: Ruby Lynn G. GenelsaAssignment

Cell Cycle and Proliferation

The cell cycle is a sequence of events that occur leading to cell stimulation for growth

and division. Cells generally enter the cell cycle through two distinct processes, namely,

fertilization and cell proliferation as activated by growth factors.

When resting cells (zero growth, G0 phase) are stimulated by growth factors, they enter

the cell cycle marked by the first period of growth (G1 phase) and prepare for DNA synthesis (S

phase). A restriction point in the G1 phase changes the events from the growth-factor mediated

signaling to the internal cell cycle signaling system, hence the progress towards the S phase,

and subsequently, the G2 phase. In the G2 phase, the cells prepare for cell division – the period

of mitosis (M phase). Mitosis is comprised of a series of events: prophase, prometaphase,

metaphase, anaphase, and telophase. The focus on this write-up summary is however not on

this series of mitotic events as this has been constantly mentioned in many texts.

One very interesting aspect is the existence of what is termed, the cell cycle network.

When newly divided cells exit from the cell cycle, there are four possibilities: they undergo

senescence (aging and maturation), apoptosis (cell death), and differentiation (develop specific

function), and cell proliferation (growing in number). For cells to undergo proliferation, they have

to follow a dynamic signaling pathway which is composed of what they call “positive” (aka proto-

oncogenes) and “negative” (aka tumor suppressors) elements. The first and the later are

mutated and inactivated, respectively, in cases where tumor cells are formed. Whether cells

proliferate or undergo the other three fates mentioned, it all depends on the interface between

the growth signaling pathway and the cell cycle signaling pathway.

The diagram below is adapted unmodified from M. Bridge (Cell Signaling Biology, 2012)

and shows the different exit points for cells after the cell cycle. The portion within the yellow

circle shows the proliferation