Final answer:
Intermediate filaments are made of intertwined fibrous proteins like keratin and form a non-polar structural network in cells. They are generated from the alignment of monomers into coiled-coil dimers, which assemble into tetramers and then larger filament bundles, providing tensile strength without polarity.
Step-by-step explanation:
Intermediate filaments are a key component of the cytoskeleton and are involved in maintaining the structural integrity of cells. These filaments are composed of several intertwined strands of fibrous proteins, such as keratin, vimentin, or neurofilaments, depending on the cell type. The fibrous subunits are long protein chains that have a central rod domain flanked by globular domains at both the N- and C-terminal ends. The monomers of intermediate filaments are inherently non-polar, meaning they lack inherent 'plus' or 'minus' ends, contrasting the inherent polarity found in microfilaments and microtubules. This quality explains why there is no polarity associated with intermediate filaments.
The assembly of intermediate filaments begins with the formation of coiled-coil dimers, which then pair up to form staggered tetramers in an antiparallel fashion. These tetramers further associate end-to-end to form longer filaments. They can span across the interior of the cell and contribute to tissue resilience by distributing forces throughout a tissue. A notable feature of intermediate filaments is their capacity to stretch due to the coiled rod domains, providing cells with tensile strength.
Intermediate filaments do not play a direct role in cell movement but instead provide structural support, for instance, aiding in resisting mechanical stress in epithelial cells. They are connected to cell junctions like desmosomes, help anchor organelles such as the nucleus, and are involved in maintaining intestinal microvilli and making up the nuclear lamina with specific intermediate filaments known as lamins.