Problem 60 - Objective Biology for NEET

Question

The cellular structure which always disappears during mitosis or meiosis is (a) Plastids (b) Plasma membrane (c) Nucleolus and nuclear envelope. (d) None of these

Step-by-Step Solution

Answer

The correct answer is (c) Nucleolus and nuclear envelope.

Step 1: Understand Cellular Structures

First, analyze each of the given cellular structures - Plastids, Plasma membrane, Nucleolus and nuclear envelope. Understand their functions and roles in the cell.

Step 2: Recalling the Processes

Then, recall the processes of mitosis and meiosis, focusing on what happens to these structures during each phase. During mitosis and meiosis, certain cellular structures have to disappear for the cell to divide properly.

Step 3: Matching the Structures to the Process

Plastids and Plasma membrane do not disappear during these processes. The nucleolus and the nuclear envelope, however, disappear during the prophase to allow chromosomes, carrying genetic information, to segregate into the two new cells formed.

Key Concepts

Cellular Structures

In biology, understanding cellular structures is crucial. Every cell consists of various components, each having distinct functions. Key structures include:

• Plastids: Found mostly in plant cells, these are involved in processes like photosynthesis and storage of energy-rich chemicals.
• Plasma Membrane: The cell's outer boundary that controls what enters and exits the cell.
• Nucleolus: The site of ribosome production within the nucleus.
• Nuclear Envelope: Encases the nucleus, ensuring protection and controlled substance traffic.

Each of these structures plays a part in the cell's operation, but their roles differ when it comes to cell division processes like mitosis and meiosis. Recognizing these differences is vital in understanding how cells function and replicate.

Nucleolus

The nucleolus is a small, dense region within the nucleus. It has a crucial role in producing and assembling ribosomes, which are essential for protein synthesis.
Ribosomes themselves are complexes that interpret genetic code to form proteins, making the nucleolus indispensable for cell operation.
However, during the early stages of cell division, namely prophase in both mitosis and meiosis, the nucleolus disappears. This disappearance allows the cell to reorganize the nucleus and prepare for division, enabling genetic material to align correctly and separate efficiently. Though it vanishes momentarily, the nucleolus reforms after division to resume its critical functions.

Nuclear Envelope

The nuclear envelope, or nuclear membrane, envelops the nucleus of eukaryotic cells. It serves as a barrier that protects the genetic material inside and regulates the movement of substances in and out of the nucleus.
During prophase of cell division, the nuclear envelope breaks down. This breakdown is essential because it allows for the release of chromosomes into the cytoplasm, facilitating their alignment and separation. The dissolution of the nuclear envelope is a temporary measure allowing the mitotic spindle to access chromosomes and pull them apart correctly.
After division, the nuclear envelope reforms around each set of separated chromosomes in the daughter cells, safeguarding the newly replicated genetic material.

Prophase

Prophase marks the first stage of cell division, in both mitosis and meiosis. During this phase, cells prepare for the actual division that will follow.
Several things happen:

• The chromatin condenses into visible chromosomes.
• The nucleolus and the nuclear envelope disassemble, making way for the mitotic machinery.
• Centrosomes, the structures responsible for organizing microtubules, start moving to opposite poles of the cell.
• Spindle fibers, which will later help segregate chromosomes, begin to form.

This phase is crucial as it sets the stage for aligning genetic material and eventually distributing it equally between the soon-to-be daughter cells. Each event in prophase ensures that cells divide correctly with minimal errors.