Genome duplication timing: In the eukaryotic cell cycle, replication of the entire genome occurs during which phase?

Introduction / Context:
Accurate duplication of DNA is a key prerequisite for successful cell division. The cell cycle divides this task into distinct phases to ensure high fidelity and enable repair before segregation.

Given Data / Assumptions:

• Cell cycle phases are G1, S, G2, and M.
• DNA synthesis must be temporally segregated from chromosome segregation.
• Question asks for the phase in which genome replication occurs.

Concept / Approach:
S phase (synthesis phase) is dedicated to replication of the entire genome. Replication origins fire in an organized program, replication forks progress, and checkpoints monitor integrity. G1 prepares for S, G2 prepares for mitosis, and M segregates the already duplicated chromosomes.

Step-by-Step Solution:

Verification / Alternative check:
DNA content measurements by flow cytometry show a doubling during S phase from 2C to 4C content.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing replication licensing (G1) with actual DNA synthesis (S).

Final Answer:
S phase.