DNA replication occurs in S phase

Steps:

• Recall cell cycle phases: G1 (growth), S (DNA synthesis), G2 (preparation), M (mitosis).
• S phase features semiconservative DNA replication, where each strand serves as template for new strand.
• Identify photomicrograph showing cells with replicating DNA, often appearing as thickened chromosomes.
• Match to option C, depicting S phase characteristics.

Why C is correct:

• S phase definition: DNA doubles semiconservatively via base-pairing, ensuring one parental and one new strand per helix.

Why the others are wrong:

• A: G1 phase lacks DNA replication; cells grow without chromosome duplication.
• B: G2 phase follows replication; DNA is already duplicated but not dividing.
• D: M phase involves mitosis; chromosomes separate, no new DNA synthesis.

Final answer: C