Question

why do most cells in the human body divide infrequently?
they have defective microtubules.
they have exited the cell cycle and entered g₀.
they no longer have active nuclei.
they no longer carry receptors for signal molecules.

Explanation:

• Option 1: Microtubules are involved in cell division (e.g., forming the spindle fibers). But defective microtubules would likely disrupt cell division rather than be the common reason for infrequent division in most cells.
• Option 2: The cell cycle has phases like \(G_1\), \(S\), \(G_2\), and \(M\). Cells that enter \(G_0\) (a quiescent phase) are not actively preparing for or undergoing division. Many cells in the human body (e.g., neurons in a non - regenerating state) are in \(G_0\), which explains infrequent division.
• Option 3: Most cells in the human body (except for some highly specialized cells like mature red blood cells) have active nuclei. The nucleus is crucial for cell function and regulation, and having an active nucleus does not prevent a cell from being in a non - dividing state.
• Option 4: Cells can still respond to other types of signals (e.g., mechanical signals) and not having receptors for a particular set of signal molecules is not the overarching reason for infrequent cell division.

Answer:

They have exited the cell cycle and entered \(G_0\).