Final answer:
The incorrect definition among the choices given is option (a), which incorrectly states that a filament is made of eight tetramers twisted together. In fact, intermediate filaments are composed of multiple tetramers, not a specific number like eight, and they pack together side-by-side.
Step-by-step explanation:
Intermediate filaments are critical components of the cellular cytoskeleton, characterized by their fibrous nature and role in providing mechanical strength to cells. The correct definitions of the subunit structures of intermediate filaments are as follows:
- Dimer: a pair of monomers that form a coiled coil structure.
- Tetramer: two dimers that align in a staggered arrangement.
- Filament: multiple tetramers twisted together to form a filament.
The option defined incorrectly in the provided choices is (a) filament: eight tetramers twisted together. Intermediate filaments are composed of multiple tetramers, not specifically eight, and these tetramers pack side-by-side to form the structure. Additionally, intermediate filaments do not have polar 'plus' and 'minus' ends like microtubules and actin filaments, as they are non-polar structures, which allows them to resist tension and provide durability to cellular structures such as the nuclear lamina and desmosomes.