Considering the main stages of the eukaryotic cell cycle (G1, S, G2, and M), which phase is generally the shortest in duration under normal conditions?

Introduction / Context:
The eukaryotic cell cycle is divided into several distinct phases that together coordinate cell growth, DNA replication, and cell division. These phases are G1 (first gap), S (DNA synthesis), G2 (second gap), and M (mitotic phase). Each phase has a characteristic duration, and knowing which phases are long or short helps in understanding how cells spend most of their life cycle. This question asks which of these phases is generally the shortest in duration.

Given Data / Assumptions:

• The phases under consideration are G1, S, G2, and M.
• We are looking for the phase that is usually shortest, not an exact time for every cell type.
• We assume typical actively dividing eukaryotic cells, such as those in tissue culture or growing tissues.
• We also assume textbook descriptions that compare relative phase lengths.

Concept / Approach:
Interphase, which includes G1, S, and G2, is the part of the cell cycle where the cell grows, carries out normal metabolism, replicates DNA, and prepares for division. These phases tend to be relatively long. The M phase, which includes mitosis and cytokinesis, is the stage where the cell actually divides into two daughter cells. In many cell types, mitosis is comparatively brief, often lasting about an hour or less, while interphase can last many hours. Therefore, the M phase is generally the shortest phase in duration compared with G1, S, and G2 in a typical cell cycle.

Step-by-Step Solution:
Step 1: Recall that G1 phase is a period of cell growth and preparation for DNA replication and can occupy a large portion of the cell cycle. Step 2: Remember that S phase is when DNA replication takes place and requires sufficient time to accurately duplicate the entire genome. Step 3: Recognise that G2 phase involves further growth and preparation for mitosis, including synthesis of proteins needed for division. Step 4: Understand that M phase includes mitosis and cytokinesis, during which the cell divides into two daughter cells, and usually occupies a relatively short period. Step 5: Compare the typical durations: interphase (G1 + S + G2) is usually much longer than M phase. Step 6: Conclude that M phase is generally the shortest phase in actively dividing eukaryotic cells.

Verification / Alternative check:
Cell cycle diagrams in textbooks often show that interphase takes up about 90 percent of the entire cycle, while mitosis occupies the remaining 10 percent or less. For example, a cell cycle of 24 hours may have about 11 hours in G1, 8 hours in S, 4 hours in G2, and 1 hour in M. While exact times vary between cell types, the pattern that M phase is the shortest is consistent. Experimental observations using time lapse microscopy and labelled DNA also support these proportions, confirming that M phase is shorter than G1, S, or G2.

Why Other Options Are Wrong:
Option B: G1 phase is often the longest phase, as the cell grows and decides whether to commit to another round of division. Option C: S phase, where DNA replication occurs, is a substantial part of the cell cycle and not typically the shortest phase. Option D: G2 phase, while shorter than G1 and S in some cells, is still usually longer than the mitotic phase. Option E: All phases are not equal in length; interphase phases collectively occupy far more time than M phase.

Common Pitfalls:
Some students misinterpret diagrams and think that because each phase is drawn as a separate slice of a circle, they must be equal in duration. Others may focus on the dramatic events of mitosis and assume it is long, when in reality it is fast compared with the long period of growth and preparation. To avoid confusion, remember that cells spend most of their time in interphase and only a relatively short time undergoing visible mitosis, making M phase the shortest in most typical cell cycles.

Final Answer:
In most actively dividing eukaryotic cells, the M phase (mitotic phase) is generally the shortest phase of the cell cycle.