Question

Nerve cells do not divide because they do not have (a) nucleus (b) centrosome (c) Golgi body (d) mitochondria.

Short answer

Nerve cells do not divide because they do not have a centrosome, which is essential for the process of mitosis.

Step-by-step solution

Understanding Cell Division

Cell division is an essential process that allows organisms to grow, replace damaged tissue, and reproduce. In eukaryotic cells, cell division is known as mitosis and involves a series of steps where the nucleus divides, followed by the division of the cell itself (cytokinesis).

Recognizing the Role of Cell Organelles in Division

For a cell to successfully divide, it requires various organelles to perform specific functions. The nucleus contains DNA and is responsible for passing genetic information to the daughter cells. The centrosome is critical in forming the spindle apparatus during mitosis, which is essential for the equal distribution of chromosomes. The Golgi body modifies and packages proteins for transport, and the mitochondria produce the energy required for the cell's activities, but neither is directly involved in cell division.

Identifying the Correct Option

Since the centrosome plays a critical role in cell division by organizing the microtubules that segregate chromosomes during mitosis, the absence of a centrosome would directly prevent a nerve cell from dividing. Nerve cells, or neurons, are known to be post-mitotic, meaning they generally do not divide after differentiation, partly because they lack centrosomes. Therefore, the correct option that prevents nerve cells from dividing is (b) centrosome.

Key concepts

Mitosis

Mitosis is a fundamental process of cell division that ensures genetic material is duplicated and evenly distributed between two daughter cells. During mitosis, a cell's DNA is precisely copied, and then the cell splits into two. This occurs through distinct stages known as prophase, metaphase, anaphase, and telophase. During prophase, DNA condenses into visible chromosomes, and the mitotic spindle begins to form. In metaphase, chromosomes align in the center of the cell. Anaphase witnesses the separation of sister chromatids to opposite poles, mediated by the spindle fibers. Finally, telophase sees the formation of new nuclear membranes around each set of chromosomes, ultimately leading to cytokinesis where the cell cytoplasm splits and two new cells are formed.

Mitosis is crucial for growth, tissue repair, and asexual reproduction in single-celled organisms. However, in the context of nerve cells, or neurons, they are an exception as they do not typically undergo mitosis once they have matured and specialized, which is intricately related to their role in the nervous system.

Centrosome

The centrosome, often called the 'microtubule organizing center,' is a key cellular structure that not only coordinates the build-up and breakdown of microtubules but also plays a critical role during mitosis. Centrosomes are composed of two centrioles and surrounding matrix known as pericentriolar material. They duplicate just before the cell undergoes division. When a cell enters mitosis, the centrosomes help in the formation of the mitotic spindle, which is necessary for pulling apart the cell's chromosomes into the two daughter cells.

Neurons in the mature nervous system lack centrosomes, which is one of the reasons why they are unable to undergo cell division. The absence of this crucial organelle implies that once a neuron is fully differentiated, it remains in a 'post-mitotic' state, meaning it doesn't divide anymore. The absence of a centrosome in nerve cells underscores their uniqueness, and it is interesting to note that this is a cellular adaptation that reflects the neurons' specialized functions.

Neuron Differentiation

Neuron differentiation is the process by which neural stem cells develop into mature, specialized nerve cells. This is a critical phase in the development of the nervous system. During differentiation, neural stem cells undergo a series of changes in shape, size, and function. They generate the specific properties necessary for neurons to carry out their role—transmitting signals throughout the body via electrical and chemical means.

Once this differentiation process is complete, neurons become highly specialized cells designed for communication. They form complex networks with other neurons through structures such as dendrites and axons. Due to the specialized nature of their function, post-mitotic neurons are intensely focused on maintaining synaptic connections rather than engaging in cell division, which explains their inability to regenerate or multiply through mitosis.

Post-mitotic Cells

Post-mitotic cells are cells that have exited the cell cycle and no longer divide. Neurons are classic examples of such cells; once they have finished the process of differentiation, they typically do not undergo further division. This is integral to their function, as the stability of neural connections is key to the reliable transmission of information across the nervous system. With a few exceptions, most neurons do not regenerate, and damage to nerve cells can result in permanent loss of function.

The 'post-mitotic' status of neurons also explains the importance of early neural development and why protecting the brain and spinal cord from injury is of such high significance. Since mature neurons do not divide, they are effectively irreplaceable, making injury to these cells particularly consequential for organismal health and well-being.