How do new cyclin proteins appear in the cytoplasm?
They are made through protein synthesis.
They are recycled.
They are imported from outside the cell.
All of these choices are correct.
The correct answer and explanation is :
Correct Answer:
They are made through protein synthesis.
Explanation:
Cyclin proteins are essential regulators of the cell cycle, controlling transitions between different phases of cell division. Their appearance in the cytoplasm occurs primarily through protein synthesis, a process in which the cell constructs new proteins based on genetic instructions.
Protein Synthesis of Cyclins
- Transcription: The genetic instructions for cyclins are stored in the DNA within the nucleus. When a cell needs to produce a specific cyclin protein, an enzyme called RNA polymerase transcribes the cyclin gene into messenger RNA (mRNA).
- mRNA Processing and Transport: The mRNA undergoes modifications such as splicing and capping before exiting the nucleus and entering the cytoplasm.
- Translation: In the cytoplasm, ribosomes bind to the mRNA and translate it into a polypeptide chain, assembling amino acids in the correct order based on the mRNA sequence. Transfer RNA (tRNA) molecules assist in bringing the correct amino acids.
- Protein Folding and Modifications: After translation, the newly synthesized cyclin protein undergoes folding and possible post-translational modifications to become functional.
Why Other Choices Are Incorrect
- They are recycled: While some proteins can be recycled through degradation and re-synthesis, cyclins are typically degraded by proteasomes after fulfilling their role. They are not directly “recycled” into new cyclin molecules.
- They are imported from outside the cell: Cyclins are not taken in from external sources; they must be synthesized within the cell based on genetic instructions.
- All of these choices are correct: Since cyclins are primarily produced via protein synthesis, this option is incorrect.
Thus, the correct answer remains: Cyclin proteins appear in the cytoplasm through protein synthesis.