What describes protein turnover in cells?

• A. Protein synthesis only occurs during cell division
• B. Protein synthesis and degradation occur simultaneously at all times
• C. Proteins are never degraded once formed
• D. Protein synthesis stops after maturation

Answer: (B)

Protein turnover describes the ongoing balance between making new proteins and breaking down existing ones. The cell continually adjusts which proteins are present and in what amounts, both to carry out current needs and to remove damaged or obsolete proteins. Two major degradation systems—ubiquitin-proteasome for individual proteins and autophagy-lysosome pathways for larger structures—work alongside ribosomes that keep protein synthesis running. Because different proteins have different lifespans, synthesis and degradation happen at the same time, at all times, to maintain the protein complement the cell needs. This is why the description that synthesis and degradation occur simultaneously at all times best captures turnover.