The Phases of Cell Cycle
A typical eukaryotic cell cycle is illustrated by human cells in culture. These cells divide once in approximately every 24 hours.
Duration of cell cycle may vary from organism to organism and from cell-type to cell-type.
For example yeast can progress through the cell cycle in about 90 minutes.
The cell cycle is divided into two basic phases:
- Interphase
- M Phase (Mitosis phase)
Stages of the cell cycle
The M Phase represents the phase when the actual cell division or mitosis occurs.
The interphase represents the phase between two successive M phases.
24 hour is the average duration of cell cycle of a human cell, cell division proper lasts for only about an hour. The interphase lasts more than 95% of the duration of cell cycle.
The M Phase starts with the nuclear division, corresponding to the separation of daughter chromosomes (karyokinesis) and ends with division of cytoplasm (cytokinesis).
The interphase though called the resting phase, is the time during which the cell is preparing for division by undergoing both cell growth and DNA replication in an orderly manner.
The interphase is further divided into three phases:
- G1 phase (Gap 1)
- S phase (Synthesis)
- G2 phase (Gap 2)
G1 phase corresponds to the interval between mitosis and initiation of DNA replication.
During G1 phase the cell is metabolically active and continuously grows but does not replicate its DNA.
S or synthesis phase marks the period during which DNA synthesis or replication takes place.
During this time the amount of DNA per cell doubles.
If the initial amount of DNA is denoted as 2C then it increases to 4C. However, there is no increase in the chromosome number, if the cell had diploid or 2n number of chromosomes at G1, even after S phase the number of chromosomes remains the same (2n).
In animal cells, during the S phase, DNA replication begins in the nucleus and the centriole duplicates in the cytoplasm.
During the G2 phase, proteins are synthesised in preparation for mitosis while cell growth continues.
Some cells in the adult animals do not appear to exhibit division (heart cells) and many other cells divide only occasionally, as needed to replace cells that have been lost because of injury or cell death.
These cells that do not divide further exit G1 phase to enter an inactive stage called quiescent stage (G0) of the cell cycle.
Cells in this stage remain metabolically active but no longer proliferate unless called on to do so depending on the requirement of the organism.
In animals, mitotic cell division is only seen in the diploid somatic cells. However, there are few exceptions to this where haploid cells divide by mitosis for example- male honey bees. Against this, the plants can show mitotic divisions in both haploid and diploid cells.