What Happens During the S Phase of Interphase

DNA repli cation occurs in the S phase of the interphase. It is the second phase of the interphase, which follows the G₁ phase. During G₁ phase, the cells fulfill the requirements for DNA replication and then, the DNA replicates during the S phase. At the end of the S phase, each chromosome composes two sister chromatids. However, as the number of chromosomes remains unchanged, the ploidy of the cell also remains unchanged. After a successful DNA replication, the cell enters into the G₂ phase, which is the third and the final phase of the cell cycle.

Key Areas Covered

1. What is Interphase
– Definition, Stages, Function
2. What Happens During the S Phase of Interphase
– Role of S Phase in Interphase

Key Terms: Centrosome, DNA replication, G₁ Phase, G₂ Phase, Interphase, S Phase

What is Interphase

Interphase is the first stage of cell cycle. It is followed by mitotic division, which is the second stage of the cell cycle. The main function of interphase is to prepare the cell for mitotic division. Interphase is composed of three phases: G₁ phase, S phase, G₂ phase.

G₁ phase – The first stage of the interphase is the G₁ phase. It is the longest phase of the cell cycle. Cells are metabolically active during the G₁ phase. Protein synthesis occurs during the G₁ phase, synthesizing the associated proteins in DNA replication.
S phase – The second phase of the interphase is S phase. DNA replication occurs during the S phase.
G₂ phase – The third phase of the interphase is G₂ phase. During G₂ phase, organelles are replicated. Some of the proteins required for the manipulation of replicated chromosomes are also synthesized during the G₂ phase. The G₂ phase is followed by the mitotic division.

Figure 1: Mitosis

What Happens During the S Phase of Interphase

DNA replication occurs during the S phase of interphase. S phase is preceded by the G₁ phase. During the G₁ phase, the building blocks for DNA replication, its associated proteins, and energy are produced in the cell. After passing the G checkpoint, the cell enters into the S phase.

Throughout the interphase, DNA remains in the form of chromatin, a semi-condensed form of DNA. During the S phase, an identical copy of each chromosome is synthesized by DNA replication. The two DNA molecules of the same chromosome are then called the sister chromatids. They remain attached from the centromere.

The centrosome is also duplicated during the S phase. They move to the opposite poles of the cell and start the formation of the mitotic spindle. The mitotic spindle is responsible for the movement of chromosomes during mitosis.

Conclusion

DNA replication occurs during the S phase of interphase. The S phase is the second phase of the interphase, which follows the G₁ phase. The proteins required by DNA replication are synthesized during the G₁ phase.