Question

Progression through the cell cycle is regulated by oscillations in the concentration of which type of molecule? a. \(\mathrm{p} 53, \mathrm{Rb}\), and other tumor suppressors b. receptor tyrosine kinases c. cyclins d. cyclin-dependent kinases

Short answer

The type of molecule that regulates the progression of the cell cycle by oscillating in concentration is c. Cyclins. These proteins control the cell cycle by activating cyclin-dependent kinases (CDKs) and their concentration fluctuates during different stages, ensuring a timely transition from one phase to another.

Step-by-step solution

Examine p53, Rb, and other tumor suppressors

p53, Rb, and other tumor suppressors are proteins that help to control the cell cycle and prevent the formation of tumors. However, these molecules do not oscillate in concentration during the cell cycle, meaning they cannot be the correct answer.

Examine receptor tyrosine kinases

Receptor tyrosine kinases are enzymes that play a role in signal transduction pathways, which are important for cellular communication and growth. While they are important for cell regulation, they also do not exhibit oscillations in concentration during the cell cycle and thus cannot be the correct answer.

Examine cyclins

Cyclins are a group of proteins that control the progression of the cell cycle by activating cyclin-dependent kinases (CDKs). Their concentration oscillates during the different stages of the cell cycle, contributing to the timely transition from one phase to another. This makes cyclins a suitable answer for regulating the progression of the cell cycle.

Examine cyclin-dependent kinases

Cyclin-dependent kinases (CDKs) are a family of proteins that regulate the cell cycle in cooperation with cyclins. CDKs depend on cyclins for their activation, but their concentration does not oscillate during the cell cycle. Therefore, CDKs are not the correct answer.

Conclusion

Considering all the answer choices, the correct answer for the type of molecule that regulates the progression of the cell cycle by oscillating in concentration is: c. Cyclins

Key concepts

Cyclins

Cyclins are one of the principal molecules responsible for the orderly transition through the cell cycle. Their role is to ensure that cell division occurs accurately and at the correct time.

The concentration of cyclins in a cell rises and falls with the stages of the cell cycle, an oscillation vital for cell cycle regulation. For instance, cyclin D is associated with the transition from G₁ to S phase, while cyclin B is crucial for the G₂ to M phase transition.

• Cyclins activate cyclin-dependent kinases (CDKs).
• Their regulated degradation helps control the cycle's progression.
• Abnormal cyclin levels can lead to unchecked cell division and cancer.

Misregulation of cyclin levels can result in significant consequences, emphasizing their importance in cell biology.

Cyclin-dependent Kinases

Cyclin-dependent kinases (CDKs) serve as the regulators that drive the cell cycle, but only when paired with their cyclin partners. With no cyclins, CDKs are inactive and unable to affect cell cycle progression.

CDKs are serine/threonine kinases—enzymes that modify other proteins by chemically adding phosphate groups. This phosphorylation alters the activity and function of the target proteins. Here's how CDKs operate:

• CDK activity is dictated by associating with specific cyclins.
• Together, the cyclin-CDK complexes phosphorylate target substrates needed for cell cycle progression.
• An important CDK complex example is CDK2-cyclin E, critical for leading cells from G₁ to S phase.

While the concentration of CDKs does not oscillate, their activity cyclically changes due to their dependence on cyclins, illustrating the harmonious interplay between these two molecules.

Tumor Suppressors

Tumor suppressors, including p53 and Rb (retinoblastoma protein), act as the cell cycle's guardians, ensuring cells don't divide in a detrimental manner. They are the checkpoints that prevent potential errors or damage from being passed on during cell division.

Here's what makes them essential:

• Tumor suppressors can pause cell cycle progression, repairing DNA damage or ensuring essential processes are complete before moving forward.
• The protein p53 can activate DNA repair mechanisms or trigger apoptosis if the damage is irreparable.
• Rb prevents cells from entering S phase under unfavorable conditions.

While pivotal in halting the cell cycle under specific circumstances, tumor suppressors do not oscillate like cyclins. Instead, their levels and activities are modulated by other regulatory mechanisms, reinforcing their role as 'brakes' rather than 'pedals' within the cell cycle.