Question

The cells which do not divide enter \(\quad\) phase from \(\mathrm{G}_{1}\) phase. (a) S-phase (b) Directly \(\mathrm{G}_{2}\) -phase (c) \(\mathrm{G}_{0}\) -phase (d) Any one of these

Short answer

The cells which do not divide enter the G0 phase from the G1 phase. So, the correct answer is (c) G0-phase.

Step-by-step solution

Understanding the Cell Cycle

In biology, the cell cycle is a series of phases that cells go through to divide and multiply. The main phases in the cell cycle are: G1 phase, S phase, G2 phase, and M phase. After M phase, cells either continue back to G1 or enter a resting phase known as G0.

Grasping the G1 Phase

The G1 phase is the first phase in the cell cycle. In this phase, cells increase in size and prepare to duplicate their DNA.

Phase of Non-dividing cells

Cells that do not or cannot divide anymore exit the cell cycle from G1 phase and enter what is known as the G0 phase. This is not a part of active cell cycle, but rather a phase of dormancy or quiescence.

Key concepts

G1 phase

The G1 phase, often dubbed the "first gap" phase, is a vital part of the cell cycle. Positioned right after the M phase, it is the first step in getting ready for cell division. During the G1 phase, cells focus on growing larger. They accumulate proteins and develop organelles, getting ready for the next big step: DNA replication. This preparation is crucial, as the cell must ensure that it is adequately equipped before it starts to duplicate its DNA in the subsequent S phase.
This phase is where the cell decides whether to continue in the cycle or exit to spend time in a non-dividing state. The decision is significant because it determines the cell's future path, namely, to divide or to enter a quiescent state known as G0.

G0 phase

The G0 phase is an intriguing aspect of the cell cycle. Cells that enter this phase are essentially in a resting state, termed as "quiescence." This state is entered from the G1 phase when a cell has chosen to stop dividing. Unlike the active cell cycle phases where preparations or divisions occur, the G0 phase serves as a pause.
Cells might stay in G0 for diverse reasons:

• Lack of nutrients or signals needed for division
• The need to specialize into a specific function or differentiate
• Being part of an adult organism where certain cells do not need to divide

It is important to note that while the G0 phase can be a reversible state—with some cells re-entering the cycle when conditions become favorable—some differentiated cells enter G0 permanently, committing to their roles in the body.

Non-dividing cells

Cells that choose the path of non-division are those that exit the active cell cycle, primarily during the G1 phase, to enter the G0 phase. Non-dividing cells are usually highly specialized, dedicated to performing specific functions within the body. For instance, neurons and muscle cells are classic examples, spending their lives working away in G0 without returning to the cycle to divide.
Such cells play critical roles in maintaining the body's daily functions without proliferating further. Here are some reasons why cells opt for non-division:

• They have reached a stage of full differentiation where division is no longer needed.
• The surrounding environment or their internal state signals that division is unnecessary or should be halted.
• Some cells remain in G0 as they age and cannot support further division.

Understanding non-dividing cells accentuates the cell's ability to decide its lifecycle path based on its surroundings and needs, an essential process for balance and functionality in the body.