Question

During which phase of the cell cycle are chromosomes most condensed and visible under a microscope?

• A. G1 phase
• B. S phase
• C. G2 phase
• D. M phase (mitosis)

Hint:

To perform karyotyping or prepare chromosome spreads, researchers treat dividing cells with colchicine.
This chemical disrupts spindle fibers, arresting mitosis at metaphase (within the M phase) when chromosomes are at their maximum level of condensation.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
During the cell cycle, DNA is packaged differently depending on the phase.
During interphase (\(G_1\), \(S\), and \(G_2\) phases), DNA exists as loosely wound chromatin to allow transcription and replication.
During cellular division (mitosis), DNA must condense into highly structured packages to ensure accurate segregation.

Step 3: Detailed Explanation:
Let us analyze chromosome condensation across the phases of the cell cycle:
- Interphase (\(G_1\), \(S\), and \(G_2\) phases): Chromosomes are unwound and dispersed throughout the nucleus.
Under a standard light microscope, individual chromosomes cannot be distinguished.
- M phase (Mitosis): As the cell enters mitosis, chromosomes undergo high condensation:
1. Prophase: Chromatin fibers begin to condense, and individual chromosomes become visible.
2. Metaphase: Condensation reaches its maximum level.
The chromosomes align at the equatorial plate of the cell, and each duplicated chromosome is held together at its centromere as sister chromatids.
At this stage of the M phase, chromosomes are highly condensed and visible, making them ideal for counting, karyotyping, and diagnosing numerical chromosomal abnormalities.
Therefore, the most condensed and visible chromosome states are observed during the M phase.

Step 4: Final Answer:
Chromosomes are most condensed and visible during the M phase, matching Option (D).