Long, hollow, unbranched tubes composed of subunits of the protein tubulin.
Its walls consist of globular proteins arranged in longitudinal rows called protofilaments aligned parallel to the tubule.
They also contain microtubule associated proteins which bind to the microtubules to increase stability and allow for assembly.
These proteins vary across different microtubules. The proteins determine the strength and stability of the microtubule relative to each other.
They are stiff enough to resist bending and compressing, helping them maintain the cell’s shape and internal organization.
Microtubules facilitate intracellular motility by facilitating the transport of materials from between membranes via motor proteins that move these materials.
Kinesins have a globular head that binds to a microtubule and hydrolyses ATP.
They consist of a head, a rod-like stalk and a fan-shaped tail to bind to the cargo.
They move step-wise (i.e., hand over hand mechanism or a processive manner) on a single protofilament of a microtubule. This is similar to how a person would walk, except without bilateral symmetry.
The speed of their motion is proportional to the ATP concentration.
Dyneins are huge proteins with two heavy chains and intermediate and light chains.
They are associated with the movement of the cilia and flagella.
They tend to move in the opposite direction of most kinases (towards the negative end of the microtubule)
They act as force-generating agents in positioning chromosomes and spindles during mitosis.
They position organelles around towards the minus-end of the microtubule.
Dynactin regulates dynein activity by increasing its processivity.
Microtubule-Organizing Centers are areas within the cell where the nucleation (i.e., initial formation) of the microtubules takes place
They control the number of microtubules, their polarity, the number of protofilaments that make up their walls, and the time and location of their assembly.
All MTOCs share a type of tubulin, -tubulin, a critical component in microtubule nucleation which determines the number of protofilaments around the microtubule.
The centrosome consists of two barrel-shaped centrioles surrounded by an amorphous, electron dense pericentriolar material (PCM)
Each centriole contains nine evenly spaced blades containing three microtubules (A, B, C).
Only the A tubule is the complete tubule and they are connected to the center of the centriole.
Centrioles typically form at right angles. Centriole duplication means that in a pair of centrioles, one is older than the other
The PCM forms a new centrosome. It is the material that initiates microtubule formation.
This is probably because the PCM contains fibers that serve as attachment sites for the structures that will become the microtubules.
Basal Bodies are MTOCs which generate cilia or flagella.
The microtubules of the cytoskeleton are normally subject to depolymerization and repolymerization as the requirements of the cell change from one time to another. This is inherent in their dynamic instability.
One prominent polymerization pathway involves GTP hydrolysis which operates on GTP, a molecule important for the assembly of tubulin dimers.
The hydrolysis produces GDP, which is replaced by another GTP molecule, which “recharges” the tubulin dimers so that it can be used again for assembly.
During times of rapid microtubule growth, tubulin dimers are added more rapidly than their GTP can be hydrolyzed. The protofilaments also have structures that promote adding more GTP on the end, and which contributes to the microtubule’s dynamic instability.
The absence of MAPs means that the microtubule on its own is unstable.
The cilia move like oars and moves the cell perpendicular to its motion.
They tend to occur in large numbers.
They are coordinated in their motion.
They can also act as receptors and channels
The flagella occur singly or in pairs and move in wavelike motions
The core is called the axoneme, which contains microtubules that run longitudinally through the entire.
These microtubules are arranged in a 9 + 2 array (9 peripheral doublets, 2 central).
The peripheral doublets contain a complete A tubule and an incomplete B tubule.
A central sheath connects to the A tubules via radial spokes , and encloses the central tubules.
The doublets are connected to each other via a nexin link, an elastic protein based linkage.
The nexin is part of the Nexin-Dynein Regulatory Complex which controls the motion of the flagella. Part of this complex includes dynein arms connecting to the cytoplasm.
Conformational change in dynein causes them to do a power stroke.
The nexin links resist the sliding motion caused by dynein doing this.
By having one side of the cilium have dynein actively moving and another side which is inhibited, the overall effect is moving the cilium in one direction.
Deuterosomes are amorphous structures assembled for secondary cilia which arise from primary cilia.
Flagella and Cilia grow from their tips. This is done via movement of particles through the flagella via intraflagellar transport.
Proteins form particles which form trains that are moved out to the tip of the cilium by a kinesin motor.