DNA doubles in S phase and stays doubled in G2

Steps:

• G1 phase: Cell has 1.2 × 10^9 g DNA (2C content, unreplicated diploid genome).
• S phase: DNA replication synthesizes exact copy of each chromosome, doubling mass to 2.4 × 10^9 g (4C content).
• End of S phase: DNA mass is 2.4 × 10^9 g.
• G2 phase: Cell prepares for mitosis with no additional replication; DNA mass remains 2.4 × 10^9 g.

Why D is correct:

• Cell cycle definition: S phase transitions DNA from 2C to 4C via semiconservative replication; G2 maintains 4C until mitosis.

Why the others are wrong:

• A: End of S phase must double DNA mass, not remain at G1 level.
• B: Identical to D, but listed separately; still correct, though question specifies D.
• C: DNA does not decrease in G2; it stays doubled post-S phase.

Final answer: D