What phase of the cell cycle begins when DNA replication starts?

The phase of the cell cycle that begins when DNA replication starts is the S Phase, or synthesis phase. During this phase, each chromosome is duplicated, resulting in two sister chromatids for each chromosome. This process is critical for ensuring that when a cell divides, each daughter cell receives an identical set of chromosomes.

The S Phase follows the G1 Phase, which is focused on cell growth and the synthesis of proteins necessary for DNA replication. After the S Phase, the cell enters the G2 Phase, where it continues to grow and prepares for mitosis, while the M Phase is the stage where actual cell division occurs. Therefore, the correct identification of the S Phase is essential, as it marks the critical step in the cell cycle where genetic material is duplicated, setting the foundation for successful cell replication.