Final answer:
Keratin intermediate filaments grow by adding monomers primarily at their (+) end, which is a true statement. This growth process is essential for keratinization in the skin and forming the structure of hair and nails.
Step-by-step explanation:
The statement that keratin intermediate filaments (IFs) grow by the addition of keratin monomers primarily at their (+) end is true. This process is similar to other cytoskeletal filaments, like microtubules, which also exhibit polar growth with a faster-growing (+) end and a slower-growing (-) end. In the context of keratinocytes in the stratum spinosum of the skin, these cells begin synthesizing keratin by putting together monomers. This synthesized keratin accumulates and becomes an integral part of the cell's structure.
As keratinocytes mature and move through the epidermal layers towards the stratum corneum, they fill up with more keratin, losing their nucleus and other organelles in the process, until they become part of the protective, outermost layer of the skin. This transformation from living cells in the lower layers to dead, keratin-rich cells in the outermost layer is critical for forming the barrier properties of the skin.
Also, hair growth starts with stem cells in the follicle dividing to produce new keratinocytes, which will eventually become part of the hair's structure. The keratin fibers present in hair, nails, and the epidermis of the skin are made up of protein monomers that have been joined together. Unlike some other biological polymerization processes which might involve hydrolysis, the assembly of keratin IFs is a process of adding on monomers.