S phase is when DNA is replicated.

• A. Protein synthesis
• B. Cell growth
• C. Cell division
• D. DNA is replicated

Answer: (D)

S phase is the part of the cell cycle where genetic material is copied. In this stage the cell duplicates its DNA so that, after division, each daughter cell will receive an identical set of chromosomes. The process creates two sister chromatids for each chromosome, held together at the centromere until mitosis. Enzymes such as helicase, DNA polymerase, primase, and ligase drive unwinding, copying, and rejoining the DNA strands. This replication is the defining event of S phase, while other phases focus on growth (G1), preparation for division (G2), or the actual separation of chromosomes (mitosis in the M phase). Protein synthesis and cell growth occur across multiple phases, but the specific hallmark of S phase is DNA replication.