Question

Find out the incorrect statement: (a) Centrioles forms basal bodies of cilia or flagella. (b) Centriole gives rise to spindle fibres at the time of cell division in animal cell. (c) Centrosome is an organelle usually containing two cylindrical structures called centrioles. (d) Peripheral fibril of centriole is doublet.

Short answer

The incorrect statement is (d). The peripheral fibril of centriole is not a doublet but a triplet.

Step-by-step solution

Evaluation of Statement (a)

We confirm that Centrioles indeed form the basal bodies of cilia or flagella. This statement is therefore correct.

Evaluation of Statement (b)

It is known that Centriole is indeed responsible for giving rise to spindle fibres at the time of cell division in an animal cell. This statement is therefore correct.

Evaluation of Statement (c)

The centrosome is an organelle that often contains two cylindrical structures called centrioles. This statement is therefore correct.

Evaluation of Statement (d)

The peripheral fibril of the centriole is not a doublet but a triplet. This makes statement (d) incorrect.

Key concepts

Centrioles

Centrioles are essential cylindrical structures that have crucial roles during cell division in animal cells. A typical centriole is composed of a set of nine microtubule triplets arranged in a cylindrical fashion. These triplets are made of a protein called tubulin.

During cell division, centrioles work by organizing the microtubules that form the spindle apparatus, which is vital in separating chromosomes into the two new cells. The centrioles replicate themselves before cell division, ensuring that each new cell receives a pair. Many students find it fascinating to discover that although centrioles are not present in most plant cells, these cells can still undergo cell division, utilizing different mechanisms for organizing their microtubules.

Spindle Fibres

Spindle fibres are the skeletal structures that form during the process of cell division, specifically during mitosis and meiosis. Composed mostly of microtubules, these fibres are responsible for separating chromosomes into the daughter cells.

To facilitate understanding, imagine the spindle fibres as a team of workers who ensure that each new cell inherits the correct number of chromosomes. They attach to the chromosomes via structures called kinetochores and help pull the chromosomes apart during cell division. Without spindle fibres, cells would have difficulty distributing chromosomes evenly, leading to potential genetic disorders.

Centrosome

The centrosome is sometimes called the 'cell division control center.' Located near the nucleus in animal cells, it houses a pair of centrioles and is surrounded by a mass of protein called the pericentriolar material. This organelle regulates the formation of the spindle fibres during cell division.

The centrosome duplicates itself before a cell divides, and each new centrosome moves to opposite ends of the cell. This movement helps in the formation of the spindle apparatus. For students who struggle with visualizing cellular components, there are excellent online resources including images and animations that depict the dynamic role of centrosomes in cell division.

Cilia and Flagella

Cilia and flagella are hair-like projections on the cell surface that aid in cell movement or the movement of substances around the cell. Despite having different lengths and numbers (cilia are typically shorter and more numerous, while flagella are longer and fewer), they have a similar basic structure.

At the base of every cilium and flagellum lies a centriole-derived structure called a basal body, which is critical for anchoring and providing a template for the growth of these cellular appendages. Both cilia and flagella consist of a '9+2' arrangement of microtubules – nine pairs forming a ring around two central microtubules. Importantly, cilia can also function like a conveyor belt, moving materials across the cell surface, while flagella are more focused on propelling the cell through liquid. These functions are hugely significant for organism survival, and a breakdown in their operation can lead to various diseases.