The cell cycle describes the stages through which a cell passes through as it replicates. The length of time a cell may take to complete the cell cycle is highly variable; in mammals the time varies from 10 hours to 10 days. A commonly used hamster cell line has a cell cycle time of 24 hours.
The basic division of the cell cycle is into that of mitosis and interphase. Cells may also be in a special state known as G0 or ‘resting phase’, where the cell is not making any effort to divide. Most terminally differentiated cells are in this phase.
Phases of the Cell Cycle
Interphase is the period between when a cell has just divided until it divides again. It has three phases – Gap 1 (G1), Synthesis (S) and Gap 2 (G2). These are represented in the diagram below:
G1 is the phase following mitosis and preceding the synthesis phase. It is the most variable part of the cell cycle – lasting at least one hour and over 100 hours in some dividing cells. It is the main contributor to the length of the cell cycle. Part of this gap is to allow the cell to duplicate its organelles (such as ribosomes, endoplasmic reticulum). The other reason the gap exists is to monitor external and internal signals to identify whether division should proceed. A checkpoint exists at the end of the G1 phase which is designed to delay progression into the S phase until the environment is ideal for replication.
The synthesis phase is when DNA is duplicated into two sister chromatids. The cell requires two complete sets of all 46 chromosomes (one set for each daughter cell). This is performed by DNA polymerase and other enzymes, which divide the DNA helix into two separate strands and then assemble a new set of base pairs for each. The copying of DNA takes between 6-10 hours in most cells.
The G2 gap, which occurs after DNA synthesis and before mitosis, is usually short (1-3 hours). It allows the cell to confirm that the DNA has been replicated correctly and that other conditions have been met to allow it to proceed to M phase. A G2 checkpoint exists to facilitate this.
Mitosis and Cytokinesis
M phase is the period where the cell actively divides into two daughter cells. It takes approximately one hour in nearly all cells, regardless of the total cell cycle time (Hall & Giaccia, 2005). There are two related events – mitosis and cytokinesis. Mitosis is subdivided into several events:
- Prophase – The cell begins to assemble the mitotic spindle, a set of microtubules extending from the centromeres which will later attach to the chromosomes.
- Prometaphase – The nuclear envelope disintegrates, and the microtubules of the mitotic spindle attach to the chromosomes.
- Metaphase – The chromosomes are aligned on the mitotic spindle. There is a pause here to allow all chromosomes to become attached.
- Anaphase – The cohesion proteins which bind the sister chromatids together are cleaved and the chromosomes are pulled apart by the mitotic spindle.
- Telophase – The nuclear membrane reconstitutes around each set of chromosomes.
Cytokinesis completes the M phase. It begins during anaphase, with the cell membrane forming a contractile ring perpendicular to the metaphase plate. Organelles are distributed more or less equally to the two daughter cells. During the end of telophase, the cell completes division and two daughter cells are created. Cells do not always undergo cytokinesis, instead forming multinucleate giant cells. This can be a normal event or can be due to errors in mitosis.