Interphase accounts for approximately 95% of the cell cycle. Far from a resting period, it is the time when the cell grows, duplicates its entire genome, and prepares the molecular machinery needed for division.
The Three Stages of Interphase
G1 Phase
Growth & metabolic activity
S Phase
DNA replication
G2 Phase
Preparation for mitosis
G1 is the first and typically longest phase of interphase. During G1, the cell grows in physical size, synthesises new proteins and organelles, and carries out its specialised functions within the organism. The cell contains a diploid (2n) set of unreplicated chromosomes at this stage.
External growth factors and mitogens stimulate the cell to progress through G1. However, the cell does not commit to division immediately. Instead, it must pass a critical decision point known as the restriction point (R point), located in late G1.
The restriction point acts as a molecular switch. Before R, the cell depends on external mitogenic signals (growth factors) to continue through the cycle. Once it passes R, the cell becomes independent of these signals and is irreversibly committed to entering S phase and completing division. This concept was first described by Arthur Pardee in 1974 and remains one of the most important principles in cell biology.
Not all cells continuously cycle. Cells that have exited the active cell cycle enter a quiescent state called G0. These cells remain metabolically active — carrying out their normal functions — but do not replicate their DNA or divide. Many differentiated cells in the adult body, such as mature neurons and skeletal muscle fibres, reside permanently in G0. Other cells, like hepatocytes (liver cells), normally remain in G0 but can re-enter the cycle if stimulated, for example during tissue repair.
Quick Check
What is the restriction point (R point) in G1?
S phase is the period of DNA synthesis. During this phase, the cell replicates its entire genome so that each daughter cell will inherit a complete and accurate copy of the genetic information. In a human cell, this means duplicating approximately 6.4 billion base pairs of DNA.
DNA replication is semi-conservative: each new double helix consists of one original (template) strand and one newly synthesised strand. This mechanism, first demonstrated by Meselson and Stahl in 1958, ensures faithful transmission of genetic information from parent cell to daughter cells.
Replication begins at many origins of replication simultaneously and proceeds bidirectionally, forming replication forks. DNA polymerase reads the template strand and synthesises the complementary strand, while helicase unwinds the double helix ahead of the fork.
After replication, each chromosome consists of two identical copies called sister chromatids, joined together at a region called the centromere. The sister chromatids will remain attached until they are separated during anaphase of mitosis.
Alongside DNA replication, the cell also synthesises large quantities of histone proteins. Histones are needed to package the newly replicated DNA into chromatin — the complex of DNA and protein that makes up chromosomes. Without new histones, the duplicated DNA could not be properly organised.
Fill in the Blank
During S phase, each chromosome is replicated to form two identical________joined at the centromere.
G2 is the final phase of interphase and serves as a preparatory period for mitosis. The cell continues to grow and synthesises proteins that will be essential for cell division. Chief among these is tubulin, the protein subunit of microtubules that will form the mitotic spindle — the apparatus responsible for separating chromosomes during mitosis.
The cell also produces other components needed for division, including proteins involved in chromosome condensation and components of the centrosome, which organises the spindle poles.
Before the cell can enter mitosis, it must pass the G2/M checkpoint. This checkpoint verifies two critical conditions: that all DNA has been completely replicated, and that any DNA damage sustained during S phase or G2 has been repaired. If errors are detected, the checkpoint halts cell cycle progression, giving the cell time to fix the damage or — if the damage is irreparable — triggering apoptosis (programmed cell death).
“The G2 checkpoint prevents cells from entering mitosis when DNA is damaged, providing an opportunity for repair and stopping the proliferation of damaged cells.”
— Lodish et al., Molecular Cell Biology, 8th Edition (2016)
Quick Check
After S phase is complete, how many copies of each chromosome does the cell contain?
Fill in the Blank
Cells that have permanently exited the cell cycle (such as mature neurons) are said to be in the________phase.
Chapter Summary