Final answer:
The statement about ubiquitin marking proteins for degradation is true. Ubiquitin acts as a signal for a protein's end of life, leading to its degradation by the proteasome, which is crucial for cellular regulatory processes.
Step-by-step explanation:
The statement that ubiquitin is a small protein that is linked covalently to other proteins, marking them for internalization or degradation, is true. Ubiquitin acts as a signaling molecule indicating that the protein's lifecycle has come to an end. This signal initiates a series of events where the protein is moved to a proteasome, an organelle responsible for protein degradation.
The degradation path begins with the activation of the ubiquitin molecule, which is then transferred to an ubiquitin-conjugating enzyme, and finally, the protein marked for destruction is linked to the ubiquitin. As more ubiquitins bind to this complex, the polyubiquinated protein is delivered to the proteasome where it is digested to short peptides. The fragments are then broken down into free amino acids in the cytoplasm, effectively recycling the components of the protein for new syntheses or processes within the cell. This degradation pathway is essential for regulatory processes in the cell such as the cell cycle, gene expression, and responses to stress, thereby maintaining cellular homeostasis and proteostasis.