Final answer:
Intermediate filament subunits are incorporated by forming lateral associations between coiled-coil dimers, not at the plus or minus ends as they are non-polar structures. These filaments are essential for maintaining cell shape and resisting tension.
Step-by-step explanation:
In intermediate filament assembly, subunits are not incorporated at the plus or minus ends because intermediate filament monomers are non-polar, which means, unlike microtubules and actin filaments (microfilaments), they do not have 'plus' and 'minus' ends. Therefore, the correct answer is that the subunits do not incorporate at either the plus or minus ends because these ends do not exist in intermediate filaments.
Instead, intermediate filaments assemble by forming lateral associations between coiled-coil dimers, where two monomers wrap around each other in a coiled fashion. These dimers then pair in an antiparallel, staggered manner to form tetramers, which can bundle together to form the filament structure. This assembly process contributes to the filament's ability to resist tensile forces, aiding the structural integrity of cells.
Intermediate filaments are crucial for maintaining cell shape, and unlike microfilaments and microtubules, intermediate filaments resist tension. They can also disassemble and reassemble as required when cells change shape.