Question

The Phase in which two identical copies of DNA are formed is:

• A. \( \text{M phase} \)
• B. \( \text{G2 phase} \)
• C. \( \text{G1 phase} \)
• D. \( \text{S phase} \)

Hint:

The \( S \) phase is critical for genetic continuity during cell division. Errors in DNA replication during this phase can lead to mutations or chromosomal abnormalities.

Answer 1

The Correct Option is D

Step 1: Understanding the phases of the cell cycle. The cell cycle consists of the following phases: 
1. G1 phase: The cell grows and prepares for DNA replication. 
2. S phase: DNA replication occurs, resulting in the formation of two identical copies of DNA. 
3. G2 phase: The cell prepares for mitosis by synthesizing necessary proteins and organelles. 
4. M phase: The cell undergoes mitosis and cytokinesis, resulting in two daughter cells. 

Step 2: Explanation of DNA replication. During the \( S \) phase (synthesis phase), the DNA in the cell is duplicated, ensuring that each daughter cell receives an identical copy during cell division. 

Step 3: Comparison with other options. - Option \( (A) \): \( M \) phase is the mitotic phase where cell division occurs, not DNA replication. 
- Option \( (B) \): \( G2 \) phase is for cell growth and preparation for mitosis, not DNA synthesis. 
- Option \( (C) \): \( G1 \) phase is the initial growth phase before DNA replication. 

Conclusion: DNA replication occurs in the \( S \) phase, making \( (D) \) the correct answer.

Answer 2

This question asks in which phase of the cell cycle a cell ends up with two identical copies of its DNA. The cell cycle runs through four main stages, each with a distinct job, so matching "DNA copied" to the right stage is the key step.

1. M phase: This is the mitosis stage, where the cell that already has duplicated DNA physically divides into two daughter cells. Copying does not happen here, only splitting of what was already copied earlier.
2. G2 phase: This stage comes right after DNA copying is finished. The cell checks for errors and builds proteins and structures needed for mitosis, but it does not make new DNA copies.
3. G1 phase: This is the growth stage before copying starts. The cell increases in size and produces the enzymes and building blocks it will need, but the DNA itself is still a single set at this point.
4. S phase: The S stands for synthesis. This is the stage where the cell's entire DNA content is copied, so that each strand of the double helix serves as a template for a new matching strand, producing two identical sister chromatids for every chromosome.

Only one of the four stages is defined by the actual copying of DNA, while the others involve either preparation before copying or division after it.

Therefore, the correct answer is S phase.