Role of Centrioles in Microbiology

In microbiology, centrioles are cylindrical cell structures that are composed of groupings of microtubules, which are tube-shaped molecules or strands of protein. Without centrioles, chromosomes would not be able to move during the formation of new cells. 

Centrioles help to organize the assembly of microtubules during cell division. To put it simply, chromosomes use the centriole's microtubules as a highway during the cell division process.

Where Centrioles Are Found

Centrioles are found in all animal cells and only a few species of lower plant cells. Two centrioles—a mother centriole and a daughter centriole—are found within the cell in a structure called a centrosome. 

Composition

Most centrioles are made up of nine sets of microtubule triplets, with the exception of some species, such as crabs which have nine sets of microtubule doublets. There are a few other species that deviate from the standard centriole structure. Microtubules are composed of a single type of globular protein called tubulin.

Two Main Functions

During mitosis or cell division, the centrosome and centrioles replicate and migrate to opposite ends of the cell. Centrioles help to arrange the microtubules that move chromosomes during cell division to ensure each daughter cell receives the appropriate number of chromosomes. 

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Centrioles are also important for the formation of cell structures known as cilia and flagella. Cilia and flagella, found on the outside surface of cells, aid in cellular movement. A centriole combined with several additional protein structures is modified to become a basal body. Basal bodies are the anchoring sites for moving cilia and flagella.

Important Role in Cell Division

Centrioles are located outside of, but near the cell nucleus. In cell division, there are several phases: in order of occurrence they are interphase, prophase, metaphase, anaphase, and telophase. Centrioles have a very important role to play in all phases of cell division. The end goal is in moving replicated chromosomes into a newly created cell.

Interphase and Replication

In the first phase of mitosis, called interphase, centrioles replicate. This is the phase immediately prior to cell division, which marks the start of mitosis and meiosis in the cell cycle.

Prophase and Asters and the Mitotic Spindle

In prophase, each centrosome with centrioles migrates toward opposite ends of the cell. A single pair of centrioles is positioned at each cell pole. The mitotic spindle initially appears as structures called asters which surround each centriole pair. Microtubules form spindle fibers that extend from each centrosome, thereby separating centriole pairs and elongating the cell.

You can think of these fibers as a newly paved highway for the replicated chromosomes to move into the newly formed cell. In this analogy, the replicated chromosomes are a car along the highway.

 Metaphase and Positioning of Polar Fibers

In metaphase, centrioles help to position polar fibers as they extend from the centrosome and position chromosomes along the metaphase plate. In keeping with the highway analogy, this keeps the lane straight.

Anaphase and the Sister Chromatids

In anaphase, polar fibers connected to chromosomes shorten and separate the sister chromatids (replicated chromosomes). The separated chromosomes are pulled toward opposite ends of the cell by polar fibers extending from the centrosome.

At this point in the highway analogy, it is as if one car on the highway has replicated a second copy and the two cars begin moving away from each other, in opposite directions, on the same highway.

Telophase and Two Genetically Identical Daughter Cells

In telophase, the spindle fibers disperse as the chromosomes are cordoned off into distinct new nuclei. After cytokinesis, which is the division of the cell's cytoplasm, two genetically identical daughter cells are produced each containing one centrosome with one centriole pair.

In this final phase, using the car and highway analogy, the two cars look exactly the same, but are now completely separate and have gone their separate ways.