The life cycle of a cell begins with the formation of daughter cells at the end of telophase.
The newly formed cell undergoes growth and proceeds through interphase and divides to form two daughter cells.
The life-cycle of a cell involves two distinct phases (Fig. 2.3).
1. Interphase (non-dividing period)
2. Cell division or Mitosis (period of division)
1. Interphase:
A series of changes takes place in a newly formed cell to prepare it for division again. At the end of interphase the cell becomes ready for equitable division into two daughter cells. Interphase into three distinct phases on the basis of activities.
(i) G1 phase:
The young daughter cell grows in size during this period. During this phase synthesis of proteins and RNA takes place.
(ii) S-phase:
At this stage major part of DNA synthesis and replication takes place. A diploid cell during this phase has double the amount of DNA. The time required by this phase is 30-50%.
(iii) G2 period:
This phase is characterized by increased nuclear volume. During its phase certain metabolic activities occur as a prerequisite of cell division. Nucleolar RNA, ribosomal RNA and messenger RNA are all synthesized during G2 phase. The time required by this phase is 10- 20%.
2. M- phase:
This is the phase of cell division by mitosis. The chromosomes become morphologically distinct. This phase is divided into four stages: prophase, metaphase, anaphase and telophase. The time required by this phase is 5-10% of the total mitotic cycle.