A typical eukaryotic cell cycle is illustrated byhuman cells in culture.
Yeast can progressthrough the cell cycle in only about 90 minutes
The cell cycle is divided into two basicphases:
1) Interphase
represents the phase between twosuccessive M phases. The interphase lastsmore than 95% of the cell cycle duration. The interphase though calledthe resting phase is the time during which the cell is preparing for divisionby undergoing both cell growth and DNA replication in an orderly manner.
The interphase is divided into three further phases:
l G1 phase (Gap 1)
G1 phase corresponds to the interval between mitosis and initiationof DNA replication. During the G1 phase, the cell is metabolically active andcontinuously grows but does not replicate its DNA.
l S phase (Synthesis)
S or synthesis phasemarks the period during which DNA synthesis or replication takes place.During this time the amount of DNA per cell doubles. However, there is noincrease in the chromosome number. In animal cells, during the S phase, DNA replication begins in thenucleus, and the centriole duplicates in the cytoplasm
l G2 phase (Gap 2).
During the G2phase, proteins are synthesized in preparation for mitosis while cell growthcontinues.
quiescent stage (G0)
do not appear to exhibit division (e.g.,heart cells) and many other cells divide only occasionally, as needed toreplace cells that have been lost because of injury or cell death.
Thesecells that do not divide further exit the G1 phase to enter an inactive stage.
Cells in this stage remainmetabolically active but no longer proliferate unless called on to do sodepending on the requirement of the organism.
2) M Phase (Mitosis phase).
The M Phase represents the phase when theactual cell division or mitosis occurs. It is significant to notethat in the 24-hour average duration of the cellcycle of a human cell, cell division proper lastsfor only about an hour. The M Phase starts with the nuclear division, corresponding to theseparation of daughter chromosomes (karyokinesis), and usually endswith the division of cytoplasm (cytokinesis).
POINTS TO REMEMBER
- In animals, mitotic cell division is only seen in the diploid somaticcells
EXCEPTIONS
#haploid cells divideby mitosis, for example, male honey bees
#the plants canshow mitotic divisions in both haploid and diploid cells
