Question

a) Explain the polarity of monomers of microtubules? What it results from?
b) Although it is not possible to distinguish the polarity of intermediate filaments. Explain why

Answer 1

Q1)a)

Microtubules are the main components of cytoskeleton, they are made up of rigid hollow rods of 25nm diameter, which help them in maintaining the structure of the cell and they also helps in various cell functions. They are made up of a single type of globular protein tubulin. Tubulin is a dimer of two closely related 55-kd polypeptides, -tubulin and -tubulin, also a third type of tubulin called -tubulin found in the centrosome, which helps in initiating microtubule assembly.

Tubulin dimers combine to form microtubules, it consist of 13 linear proto filaments arranged around a hollow core. The proto filaments, which are made up of head to tail tubulin dimers, are arranged parallelly. Microtubules are polar structures having two distinct ends, a fast-growing plus end and a slow-growing minus end. The polarity helps in determining the direction of movement along the microtubules.

Centrosome is the initiation site for the assembly of microtubules, it grows outward from the centrosome to the periphery of the cell, the initiation of microtubule’s growth at the centrosome forms the polarity of microtubules in a cell. Usually microtubules grow by the addition of tubulin to their plus ends, and they grow outward from the centrosome toward the cell periphery.

The major protein in the centrosome which nucleates the assembly of microtubules is -tubulin,

Microtubules have a distinct polarity, which helps them in their biological function. Tubulin polymerizes end to end, with the -subunits of one tubulin contacting with the -subunits of the next dimer. So in a proto filament at one end an -subunit is exposed and at the other end a -subunit is exposed. They are called the (?) and (+) ends. The proto filaments bundle parallel to one another with the same polarity, so in a microtubule, at one end there is a (+) end, with -subunits exposed, at the other end, the (?) end, have the subunits exposed. Elongation of microtubule takes place at both (+) and (-) ends, usually it is more fast at the (+) end.

Q1) b)

Intermediate filaments are made up of different types of proteins. They play an important role in maintaining the structure and in giving strength to the cell. They are made of 45 nm long and 2–3 nm wider rod-like dimers. The intermediate filament units, long axis remains parallel to the filament axis.

The Intermediate filaments do not show polarity and are more stable and their constituent subunits do not bind to nucleotides, they do not have polarity because, the monomers are found in both directions along the axis of the filament. In Intermediate filaments the reversible association and dissociation of dimers can take place throughout the length of the filament.

Or two monomers form a parallel dimer. Two dimers remain side by side which is called anti-parallel alignment, they form a tetramer, so polarity is lost during the antiparallel alignment.

In Intermediate filaments, the tight association between Proto filaments gives them a high tensile strength, by which they become the most stable component of the cytoskeleton. As a result Intermediate filaments are usually found in strong structures like hair, scales and fingernails.