Methotrexate Blocks DNA Synthesis in Interphase

Steps:

• Methotrexate inhibits thymine nucleotide production, essential for DNA.
• DNA replication requires these nucleotides and occurs specifically in S phase.
• S phase is part of interphase, before mitosis begins.
• Without DNA synthesis, cells arrest in interphase and cannot proceed.

Why B is correct:

• Interphase (G1, S, G2) includes S phase, where DNA replication demands thymine nucleotides per the cell cycle definition.

Why the others are wrong:

• A. Anaphase involves chromosome separation, not DNA synthesis.
• C. Metaphase aligns chromosomes at the equator, post-DNA replication.
• D. Prophase condenses chromosomes, after interphase completion.

Final answer: B