Question

In eukaryotic organisms, replication of DNA occurs in \rule{2cm{0.15mm}}

• A. anaphase
• B. $\text{G}_1$ phase
• C. $\text{G}_2$ phase
• D. S phase

Hint:

Remember the shortcut: G$_1$ = Growth, S = Synthesis (DNA replication), G$_2$ = Preparation. Most biology questions directly test this sequence.

Answer 1

The Correct Option is D

Concept: The cell cycle in eukaryotic organisms is divided into two major phases: interphase and M phase. Interphase is further subdivided into three stages: G$_1$, S, and G$_2$. Each phase has a specific role in cell growth and division.
• G$_1$ phase (Gap 1): The cell grows in size, synthesizes proteins, and prepares the necessary enzymes for DNA replication.
• S phase (Synthesis phase): DNA replication takes place, resulting in the duplication of chromosomes.
• G$_2$ phase (Gap 2): The cell prepares for mitosis by synthesizing proteins required for cell division.
• M phase: Includes mitosis and cytokinesis, where the cell divides into two daughter cells.

Step 1: Understand what DNA replication means.
DNA replication refers to the process of copying the genetic material so that each daughter cell receives an identical set of chromosomes. This must occur before cell division.

Step 2: Identify the phase where duplication of DNA occurs.
During the S phase, each chromosome replicates to form two identical sister chromatids joined at the centromere. This ensures genetic continuity.

Step 3: Analyze each option.
• (A) Anaphase: This is a stage of mitosis where sister chromatids separate, not replicate.
• (B) G$_1$ phase: Only growth and preparation occur; DNA replication does not take place.
• (C) G$_2$ phase: Post-replication stage; the cell prepares for mitosis.
• (D) S phase: The only phase where DNA synthesis (replication) occurs.

Step 4: Conclusion.
Thus, DNA replication occurs exclusively during the S phase of the cell cycle. Final Answer: Option (D)