The cell cycle comprises two main phases: Interphase and Mitosis. The interphase consists of Gap 1 (G1), synthesis (S), Gap (G2), and Mitosis as the M phase (Figure 1 below). Collectively, the three phases of Interphase prepare the cell for Mitosis.
During the G1, the cell gets ready for the S phase by making sufficient amounts of proteins. Ribosomes are most predominant during this phase. In fact, a sufficient supply of ribosomes during G1 is a prerequisite for the cell's readiness to exit G1 and enter the S phase.
During the S phase, the cell duplicates its DNA to get ready for Mitosis. At the end of the S phase, the cell's 23 pairs of chromosomes become 46 pairs. Besides DNA, histone proteins are also copied. Histone proteins are DNA "organizers." Specifically, duplicated DNA loops around histones to form nucleosomes. Nucleosomes further coil to form chromatin fibers. Chromatin fibers are condensed into chromosomes.
The G2 phase is a period of protein synthesis and rapid cell growth to prepare the cell for mitosis. Microtubules begin to reorganize to form a spindle (preprophase). Also, the cells must be checked at this phase for any DNA damage within the chromosomes.
During Mitosis (M), a mother cell divides into two daughter cells with the exact "carbon copies" of all the genetic materials from the mother cell.
