THE CELL CYCLE
A cell cycle is the series of events that takes place in a cell as it grows and ultimately causes it to divide into two cells .
A cell spends most of its time in what is called Interphase - a continuum of three distinct stages :
● G 1 - The cell grows and prepares for DNA replication .
● S - Synthesis stage - DNA replication is the process by which a double-stranded DNA molecule is copied to produce two identical DNA molecules .
● G 2 - The cell replicates its organelles and prepares for cell division .
The cell then leaves interphase to enter the M Phase in which the cell and its contents divide to create two genetically identical daughter cells . This phase is comprised of two distinct stages :
● Mitosis - Nuclear division , whereby DNA is separated into two identical nuclei .
● Cytokinesis - Cytoplasmic division , whereby cellular contents are separated and the cell splits into two distinct cells .
A cell can leave the cell cycle ( G 0 ) at one of two points :
● Cells exit the cell cycle right after mitosis and enter G 0 in order to differentiate into specialized cells . Terminally differentiated cells are those that never again enter the cell cycle and so never divide . Other cells can be triggered to leave G 0 and re-enter G 1 , allowing them to divide again .
● A cell in G 1 can enter the G 0 phase . In this state of quiescence , it does not divide but retains the ability to re-enter cell proliferation .
After each of the key stages of the cycle , there are control mechanisms known as cell cycle checkpoints that determine if the cell can progress to the next stage or phase .