During the S phase (synthesis phase) of the eukaryotic cell cycle, which key event occurs inside the nucleus?

Introduction / Context:
The eukaryotic cell cycle is divided into several distinct phases, each with specific events. Understanding what happens in G1, S, G2, and M phase is fundamental for cell biology, genetics, and cancer biology. Many exam questions focus on the S phase because it is where the genetic material is copied, which is crucial for accurate cell division. This question asks you to identify the main process that occurs during the S phase of the cell cycle.

Given Data / Assumptions:

• The question specifically refers to the S phase, also called synthesis phase.
• Options include cell division, general growth, DNA replication, and protein synthesis.
• We assume familiarity with the basic structure of the cell cycle: G1, S, G2, and M phases.

Concept / Approach:
In eukaryotic cells, interphase consists of G1, S, and G2 phases. During G1, the cell grows and carries out normal metabolic activities. The S phase is dedicated to replication of DNA. Each chromosome is duplicated so that it consists of two sister chromatids connected at a centromere. This ensures that when the cell later divides in mitosis, each daughter cell receives a complete set of genetic information. G2 involves further growth and preparation for mitosis, including synthesis of some proteins and enzymes. The M phase is where mitosis and cytokinesis actually occur, leading to division of the nucleus and cytoplasm.

Step-by-Step Solution:
Step 1: Recall the sequence of cell cycle phases: G1, S, G2, and M. Step 2: Identify that the S phase stands for synthesis phase, emphasising the synthesis of DNA. Step 3: Understand that during the S phase, each chromosome is replicated to form two identical sister chromatids. Step 4: Recognise that cell division (mitosis) itself occurs later in the M phase, not during S phase. Step 5: Note that protein and enzyme synthesis occurs throughout the cell cycle, but DNA replication is the defining event of S phase. Step 6: Select DNA replication as the key event occurring during S phase.

Verification / Alternative check:
Cell biology textbooks describe experiments involving DNA labelling with radioactive thymidine or other markers. These experiments show that incorporation of labelled nucleotides into DNA occurs specifically during the S phase. Flow cytometry studies also demonstrate that the DNA content of cells doubles during S phase, confirming that this phase is dedicated to DNA replication. Mitosis is always shown after S and G2 in diagrams, emphasising that chromosome duplication must be completed beforehand.

Why Other Options Are Wrong:

• Cell division through mitosis and cytokinesis: This occurs mainly in the M phase, not during S phase.
• Cell growth with increase in cytoplasm and organelles only: General growth is characteristic of G1 and to some extent G2, rather than being specific to S phase.
• Protein and enzyme synthesis without copying of DNA: Protein synthesis happens throughout the cycle, but the hallmark of S phase is DNA replication.

Common Pitfalls:
Learners sometimes confuse S phase with M phase because both are important for cell division. A useful memory aid is that S stands for synthesis of DNA, while M stands for mitosis. Another common mistake is thinking that most growth happens in S phase. In reality, S phase is focused mainly on copying genetic material, so it is more about preparing the chromosome content than about cytoplasmic growth.

Final Answer:
During the S phase of the cell cycle, the key event is DNA replication, resulting in duplication of the genetic material.