Interphase begins immediately after cell division brings a new cell into being. It is the longest portion of the cell life cycle, lasting on average 20 hours in eukaryotes, which is about 90% of the life cycle of the cell.

Once thought to be largely a time of rest between division events, interphase is now known to be a period of intense metabolic activity. Cell maturation and growth occurs during this time, as do preparations for the next cell division.

At the end of interphase, cells are significantly larger than at the beginning, but otherwise display little outward sign of the significant biochemical changes they have undergone.

Interphase is broken into four sub-phases, G1, G0, S, and G2.

G1
G1, which stands for Growth or Gap 1, is the first step in interphase. It is a time of cell growth. During this phase the cell doubles in size, accumulates nutrients, and performs normal metabolic activities. Hormones stimulate rapid growth and vigorous protein synthesis.

Partway through G1, a cell reaches a restriction point. If it passes through the restriction point, it is committed to continuing on the cycle to cell division. If it doesn’t pass the restriction point, the cell turns off the cycle until it is stimulated by the growth factors it needs to move forward.

G0
Many cells take a detour away from the cell cycle during G1, and into a state called G0. The departure may be permanent – some, like blood and nerve cells, never return to undergo division, but instead carry out their functions until they die. Others resume the cell cycle when properly stimulated – lymphocytes are generally in G0, but when they encounter the proper antigen they begin the process of cell division.

The ability to enter or leave G0 is a reflection of the health of a cell. Aging or damaged cells are forced into G0 when they lose the ability to undergo division. Cancer cells are unable to go into G0 at all, and instead divide continuously and disastrously.

S
During S, which stands for synthesis, a cell’s DNA replicates. At the onset of S, each chromosome is a single strand of DNA. At the end of S phase, each chromosome has been copied. The two identical strands of DNA, called sister chromatids, are joined together at a point called the centromere. Although the amount of DNA in the cell has doubled, it is still in a very loosely coiled form and is not visible to the naked eye.

Structures known as centrosomes also duplicate during S. The centrosomes will play a pivotal role in splitting up the chromosomes during cell division.

G2
G2 is the last step of interphase, and includes the final preparations for cell division as well as additional cell growth. Significant protein synthesis takes place, most of it going toward the construction of microtubules. Microtubules are a prime component of the spindle apparatus, which will help distribute the cell’s contents during division.