A centriole is a small, cylindrical organelle found within the cytoplasm of most eukaryotic cells. Composed primarily of tubulin protein, centrioles are not enclosed by a membrane and do not contain DNA or RNA.
Centriole Structure
A centriole is cylindrical, typically measuring 0.2 to 0.25 micrometers (µm) in diameter and 0.3 to 0.7 µm in length. Its walls are constructed from nine sets of microtubule triplets, arranged in a circular pattern. Each triplet consists of three microtubules. This “nine triplet” configuration is conserved across species.
Centrioles are commonly found in pairs near the cell’s nucleus, oriented perpendicularly. This pair, along with a surrounding cloud of dense protein material called the pericentriolar material (PCM), forms the centrosome. The PCM serves as an anchoring site for microtubules, which are tube-shaped molecules that contribute to the cell’s skeletal system.
Role in Cell Division
Centrioles are important for cell division in animal cells, primarily through their association with the centrosome. The centrosome functions as the main microtubule-organizing center (MTOC) in these cells, organizing microtubules throughout the cytoplasm. During the cell cycle, centrioles duplicate.
As the cell prepares for division (mitosis or meiosis), duplicated centrosomes separate and migrate to opposite poles. From these poles, they organize the formation of mitotic spindle fibers, composed of microtubules. These spindle fibers attach to chromosomes and facilitate their accurate segregation into daughter cells. While centrioles are involved, cell division can occur without them, though it may sometimes lead to irregularities in spindle development and chromosome separation.
Role in Cilia and Flagella Formation
Centrioles also play a role in the formation of motile cellular appendages such as cilia and flagella. A centriole can migrate to the cell membrane and transform into a basal body. The basal body then serves as a template for the assembly of the axoneme, the microtubule core of cilia and flagella.
Cilia are short, hair-like structures that move fluids or act as sensory receptors, found in various cell types. For example, cilia in the respiratory tract help move mucus and trapped particles away from the lungs. Flagella are typically longer and fewer in number, primarily facilitating cell locomotion, such as the tail of a sperm cell. Their formation and function are important for cellular movement and signaling.
Centriole Presence in Cells
Centrioles are found in the cells of animals and some lower plant forms. They are present in diverse organisms including mosses, ferns, charophytes, bryophytes, seedless vascular plants, cycads, and Ginkgo. However, centrioles are generally absent in higher plants, such as conifers and flowering plants, and in most fungi.
These organisms utilize different mechanisms for organizing their microtubules and forming their mitotic spindles during cell division. For instance, plants rely on other microtubule-organizing centers that do not involve centrioles to ensure proper chromosome segregation. The presence or absence of centrioles thus reflects evolutionary adaptations in cellular organization across different biological kingdoms.