Question

Nerve and skeletal muscle cells in the adult, which do not divide, remain in the a. ${\mathrm{G}}_1$ phase. b. $S$ phase. c. G2 phase. d. M phase.

Short answer

Nerve and skeletal muscle cells in the adult, which do not divide, remain in the $G_1$ phase. These non-dividing cells stay in the $G_1$ phase and enter the resting G0 phase.

Step-by-step solution

G1 Phase

The G1 phase is the first phase of the cell cycle in which cells prepare for DNA replication and cell division. In this phase, cells grow in size and synthesize proteins required for cell division. However, if a cell decides not to divide, it stays at the G1 phase and enters a resting state called the G0 phase.

S Phase

The S phase is the stage of the cell cycle where DNA replication takes place. In this phase, cells synthesize new DNA strands, resulting in the duplication of the cell's genetic material.

G2 Phase

The G2 phase is the stage where the cell prepares for mitosis. During this phase, cells grow in size, synthesize proteins, and perform other activities necessary for cell division.

M Phase

The M phase refers to mitosis, the process of cell division where one cell divides into two daughter cells. This phase ensures that both daughter cells inherit an equal distribution of genetic material from the parent cell. From the above analysis, adult nerve and skeletal muscle cells, which do not divide, remain in:

Answer

a. G1 phase. These cells stay in the G1 phase and enter the resting G0 phase, where they do not undergo cell division.

Key concepts

G1 Phase

The G1 phase is a critical part of the cell cycle, acting as the very first step before cells commit to dividing. In this phase, cells are busy growing and producing the proteins required for the next stages of the cycle. However, not all cells proceed to divide. Some cells, especially fully matured ones, pause in this stage. When a cell decides that it doesn’t need to divide any further, it transitions into a phase known as G0.

In the G0 phase:

• Cells perform their regular functions.
• They do not prepare for DNA replication or cell division.
• This phase is often considered a resting phase, but cells are still active in their specialized roles.

Nerve and skeletal muscle cells, which do not continually divide, generally reside in this resting state for an extended period or even permanently.

Skeletal Muscle Cells

Skeletal muscle cells are specialized cells essential for movement and strength in our bodies. They are unique because:

• They have more than one nucleus and are hence multinucleated.
• Their structure is long and cylindrical, allowing for flexibility and contraction.

Once fully developed, these cells generally do not divide. Instead, during development, they grow larger and increase in strength and functionality. When they reach maturity, they stay in the G1 phase, entering the G0 phase, which means they halt the cycle and stop preparations for division.

This specialized resting state allows skeletal muscle cells to focus on contracting and supporting the body's movement rather than dividing.

Nerve Cells

Nerve cells, also known as neurons, are the core components of the nervous system. They play a crucial role in transmitting signals throughout the body. Similar to skeletal muscle cells, neurons are:

• Highly specialized and do not typically divide after reaching maturity.
• Designed to carry electrical impulses, which are crucial for brain function and communication in the body.

These cells also find themselves in the G0 phase after the G1 phase. Instead of preparing for division, they focus entirely on their role in signal transmission.

Their inability to divide post-maturation underscores the unique nature of their function, making nerve cell damage particularly impactful, as the body struggles to replace or repair them once they are lost.