Phase of Cell Cycle

Each member of a particular species possesses a distinctive count of chromosomes within its bodily cells. The regular chromosomal compositions are termed karyotypes.

Ordinarily, when cells attain a specific size, they must opt to cease growing or initiate division. Certain cells such as nerve, skeletal muscle, and red blood cells ordinarily abstain from dividing once they attain maturity. The activities of proliferating and dividing cells can be elucidated through the cell's lifecycle, known as the cell cycle. These cycles commence whenever fresh cells are generated and conclude once these cells complete their division. The cycle reinitiates for each subsequent daughter cell. The events transpiring during the division of eukaryotic cells are conveniently delineated as a cell cycle: G1 → S → G2 → M → C.

Interphase

The predominant duration of the cell cycle is spent in interphase. Chromosomes are replicated during this period, numerous cell components are synthesized, and the cell undergoes substantial growth. Interphase typically persists for at least 90% of the overall time requisite for the cell cycle. DNA synthesis, a pivotal occurrence in chromosome duplication, transpires during the middle of interphase and serves as the foundation for segmenting interphase into three sub-phases.

G1 Phase: The initial sub-phase, labeled G1, designates the interval prior to the onset of DNA synthesis. G represents a gap, and G1 signifies the foremost gap between cell division and DNA synthesis. This phase denotes a pause amid cell activities. In G1 phase, the cell bolsters its protein reservoir, augments the quantity of several organelles like mitochondria and ribosomes, and enlarges in size. As G1 phase draws to a close, there is heightened enzymatic activity essential for DNA synthesis.

S Phase: Following the G1 Phase is the ensuing sub-phase, S, wherein DNA synthesis (replication) effectively transpires. At the commencement of the S phase, each chromosome remains singular. By the culmination of this phase, subsequent to DNA replication, the chromosomes exist in pairs, with each comprising two sister chromatids.

G2 Phase: The third sub-phase, termed G2 phase, spans the interval from the accomplishment of DNA synthesis to the instigation of cell division. During this phase, augmented protein synthesis takes place as the ultimate stride in readying the cell for division. The conclusion of the G2 phase heralds the commencement of mitosis. Consequently, the sequence of interphase sub-stages is as follows: G1 Phase → S Phase → G2 Phase.

M Phase: This phase encompasses the assembly of the microtubular apparatus, which attaches to the chromosomes and effectuates the separation of sister chromosomes. This phase is referred to as mitosis.

C Phase: In this phase, the cell undergoes division, engendering two daughter cells. This stage is termed cytokinesis.

Cell Cycle 

The duration of each phase is variable. For human cells, the average cell cycle extends to about 24 hours, with mitosis lasting for 30 minutes, G1 Phase spanning 9 hours, S Phase occupying 10 hours, and G2 Phase lasting 4 hours and 30 minutes. In yeast, the complete cycle is accomplished in just 90 minutes.