Explanation:

S phase

• The G1? phase is the gap between M phase and S phase and is marked by synthesis of RNA and protein (excluding histones) and duplication of all cell content except DNA. 
• S phase of DNA replication is followed by G2? phase during which cell double-checks the duplicated DNA content. 
• The M phase of cell cycle is marked by the division of the nucleus and cytokinesis to produce daughter cells. G0? phase is the quiescent stage.