Decision of cell division occurs at the end of G1 phase.

G1 phase of the cell cycle is the first gap phase, which follows cell division and is followed by the S phase. During G1 phase, the cell grows in size and prepares itself for DNA replication. The decision of cell division, which is also known as the restriction point, occurs at the end of G1 phase.

Explanation:

The decision of cell division is a critical checkpoint in the cell cycle, which ensures that the cell has sufficient resources and the necessary signals to proceed with DNA replication and mitosis. The decision is made based on several factors, including the availability of nutrients, growth factors, and the integrity of the DNA.

At the end of G1 phase, the cell undergoes a series of checks to determine if it is ready to proceed with cell division. These checks are carried out by a complex network of proteins, including cyclins and cyclin-dependent kinases (CDKs).

If the cell receives a positive signal from these proteins, it proceeds with DNA replication and enters S phase. However, if the signal is negative, the cell enters a state of quiescence, also known as the G0 phase, in which it remains until the conditions for cell division improve.

In conclusion, the decision of cell division occurs at the end of G1 phase, where the cell undergoes a series of checks to determine if it is ready to proceed with DNA replication and mitosis. This checkpoint is critical for maintaining the integrity of the DNA and ensuring that cells divide only when necessary.