What happens at the S phase of the cell cycle?

The S phase, also known as the Synthesis phase, is the second stage of interphase in the eukaryotic cell cycle. It comes after the G1 phase and before the G2 phase. The most important event of the S phase is DNA replication, which means the cell copies its entire genetic material so that each daughter cell can receive an exact copy of the genome after cell division.

In G1 phase, the cell has a normal diploid set of chromosomes. But during the S phase, each chromosome replicates to form two identical sister chromatids, which are attached at a central region called the centromere. However, the number of chromosomes does not change. For example, a human cell has 46 chromosomes before and after the S phase, but the DNA content doubles from 2C to 4C. This increase is crucial because without complete replication, cells cannot divide properly.

DNA replication begins at specific points on the DNA called origins of replication. Multiple origins are used simultaneously to speed up the process in eukaryotic cells. Several important enzymes and proteins are involved:
  • DNA helicase: unwinds the DNA helix
  • Single-stranded binding proteins (SSBPs): keep the DNA strands open
  • Primase: adds RNA primers
  • DNA polymerase: adds new complementary DNA bases
  • DNA ligase: joins Okazaki fragments on the lagging strand
Besides DNA, the centrosome also duplicates during this phase. The centrosome contains the centrioles and is essential for forming the spindle apparatus in mitosis or meiosis.

The S phase is highly regulated and multiple checkpoints monitor whether DNA replication is proceeding correctly. If any error or damage is detected, the cell can stop the cycle to repair the problem before continuing to the G2 phase.

In this way, the S phase ensures that each new cell receives a full and accurate set of genetic information. Mistakes in this phase can lead to mutations or chromosomal abnormalities, which may result in diseases like cancer.
The S phase, also known as the Synthesis phase, is the second stage of interphase in the eukaryotic cell cycle. It comes after the G1 phase and before the G2 phase. The most important event of the S phase is DNA replication, which means the cell copies its entire genetic material so that each daughter cell can receive an exact copy of the genome after cell division.








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