DNA replication takes place in which phase of cell cycle –

**Core Concept**
DNA replication is a crucial process in cell cycle that ensures the duplication of genetic material before cell division. This process involves the unwinding of double-stranded DNA, the synthesis of new complementary strands, and the replication of genetic material.

**Why the Correct Answer is Right**
DNA replication occurs during the S phase of the cell cycle, which is the period of synthetic activity. During this phase, the cell replicates its DNA by unwinding the double helix, synthesizing new complementary strands, and assembling the replicated chromosomes. This process requires the activation of various enzymes, including DNA polymerases, helicases, and topoisomerases, which work together to facilitate DNA replication.

**Why Each Wrong Option is Incorrect**
**Option A:** The G1 phase is the first growth phase of the cell cycle, during which the cell prepares for DNA replication by growing and synthesizing proteins. While it is an essential phase, it is not the phase where DNA replication occurs.
**Option B:** The G2 phase is the second growth phase of the cell cycle, during which the cell prepares for cell division by replicating organelles and producing mitotic spindle fibers. While it is a critical phase, it is not the phase where DNA replication occurs.
**Option C:** The M phase is the mitotic phase of the cell cycle, during which the replicated chromosomes are separated into two daughter cells. While it is an essential phase, it is not the phase where DNA replication occurs.

**Clinical Pearl / High-Yield Fact**
The S phase is a critical phase of the cell cycle, and any disruption to this phase can lead to genetic mutations, chromosomal abnormalities, and cancer. Understanding the cell cycle and DNA replication is essential for medical professionals to diagnose and treat various diseases.

**Correct Answer:** S. The S phase is the period of synthetic activity where DNA replication occurs.