Interphase

Interphase is the period of the cell cycle between successive nuclear divisions, during which a cell grows, carries out its metabolic functions, and duplicates its DNA in preparation for mitosis. It is traditionally divided into three phases: G1 (gap 1), S (DNA synthesis), and G2 (gap 2). Some cells may enter a quiescent state called G0 from G1 if they no longer divide.

During G1, the cell increases in size, synthesizes RNA and proteins, and accumulates nutrients and energy reserves. Checkpoints monitor conditions and integrity, regulating progression to the S phase.

In the S phase, DNA replication occurs, producing identical sister chromatids held together at centromeres. Histones and other chromatin components are synthesized as chromosomal material doubles. The nuclear envelope remains intact, and chromatin remains relatively decondensed.

G2 is a second growth phase in which the cell continues to grow and produce components needed for mitosis, such as microtubules and other spindle apparatus components. The G2 checkpoint assesses DNA replication completeness and DNA integrity before mitosis begins.

Interphase is characterized by active metabolism, organelle biogenesis, and general cytoplasmic expansion. Chromosomes are replicated but not yet condensed into visible chromosomes; the nucleus remains intact with a distinct nucleolus in many cells.

The length of interphase varies by organism and cell type and typically accounts for about 90% of the cell cycle in many somatic cells, with M phase constituting the remainder. Entry into mitosis is tightly regulated by cell-cycle checkpoints that respond to DNA damage, cell size, and growth conditions.