Centrioles form the microtubule skeleton of the cell during interphase and duplicate during S phase of interphase, along with the DNA. Interphase consists of the G1, S, and G2 phases. Centrioles come in pairs, like two cylinders connected together at one end, similar to nunchakus. A centriole pair is surrounded by a cloud of proteins called the centrosome. Centrioles duplicate semi-conservatively, just like DNA, meaning new daughter centrioles grow on the side of each parent centriole. After the daughter centrioles finish growing in G2 phase, each of the two new parent-daughter pairs will separate during mitosis, or cell division. In G1, the older centriole of a parent-daughter pair has the ability to attach to the cell membrane and form a cilium. A cilium is a hair-like structure that sticks out from a cell’s membrane.
The building blocks of centrioles differ between species, but the core components are the same. A enzyme called PLK4 controls the stability of centrioles and the formation of new centrioles. A centriole is like a cylinder. The walls of the cylinder are nine parallel panels that tethered together. Each panel is made of three microtubules attached lengthwise, side-to-side. Microtubules are made of the proteins alpha and beta tubulin. The middle of the cylinder is a ring structure, like the spokes of a wheel. The protein SAS-6 forms a nine-spoke structure, like a nine-armed starfish, that holds the walls of the cylinder together. Lastly, one of the two centrioles has crown-like structures at its tip called distal and subdistal appendages. These appendages contain the protein CEP164.
During the G1 phase of interphase, the older centriole of the pair forms the microtubule skeleton of the cell. Microtubules are hollow tubes that can grow and shrink inside a cell. Microtubules support the shape of a cell. They also serve as railways on which proteins and small membrane pouches are pulled by motor proteins. The subdistal appendages on the older centriole of the pair are what allow the microtubule network to form. The subdistal appendages are like the command center of the microtubule network during interphase. Microtubules start here and extend throughout the cell.
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Hand in Hand
How do the centrioles in a pair stay next to each other throughout interphase? Centriole pairs are held together at one end. Proteins connect one end of each centriole. The free end of one of the centrioles -- the parent, or older of the two -- has the crown-like appendages. From G1 phase until the end of G2 phase, proteins connect centrioles in a pair by what is called the “G1-G2 tether.” Starting during S phase, when each centriole in a pair grows its own daughter centriole, each parent centriole holds on to its daughter centriole by what is called the “S-M linker.” During mitosis, the G1-G2 tether will break to allow each of the two new parent-daughter pairs to separate into different cells. As the parent-daughter pairs mature after mitosis, the S-M linker is replaced by a new G1-G2 tether.
Building a Cilium
Some cells have what is called a cilium, a hair-like structure that sticks out of a cell’s membrane. Some cells have many cilia, such as the cells that line your airway and brush mucus up into your throat. A cilium grows from a centriole. The crown-like distal appendage anchors a centriole to the cell's membrane. Once anchored, the centriole serves as the base from which the cilium grows out of the cell.