Question

The cell ______ of multicellular eukaryotes is regulated by Cdks and external signals.

Answer 1

The cell cycle of multicellular eukaryotes is regulated by Cdks and external signals, with cyclins and Cdk enzymes playing crucial roles at various checkpoints to ensure proper cell division and prevent uncontrolled cell growth, which can lead to cancer.

Step-by-step explanation:

The cell cycle of multicellular eukaryotes is regulated by Cdks (cyclin-dependent kinases) and external signals. The progression of the cell through the cell cycle is managed by checkpoints. These checkpoints, found at various stages of the cycle such as the end of G₁, the G₂/M transition, and during metaphase, ensure the cell is properly prepared for the next phase, including DNA replication and mitosis. The roles of cyclins and Cdks are central to this regulation as they form complexes that must be activated by phosphorylation to be functional. Cyclin concentrations vary throughout the cycle, thus determining when these crucial Cdk/cyclin complexes form and become active, moving the cell cycle forward at the appropriate times.

Specifically, cyclins are regulatory proteins whose levels fluctuate, and they need to bind to Cdks to exercise control over the cell cycle. Once bound and phosphorylated, these complexes activate proteins that are crucial for progressing to the next cell cycle phase. Deregulation of this cycle, such as mutations in the genes coding for cyclins and Cdks, can lead to uncontrolled cell proliferation, which is a hallmark of cancer.

Answer 2

The cell cycle in multicellular eukaryotes is regulated by cyclin-dependent kinases (Cdks) and cyclins which are influenced by both external and internal signals. Cyclin levels fluctuate and when bound to stable Cdks, control the cycle's checkpoints. Phosphorylation of these complexes is a key activation step necessary for the progression through the cycle.

Step-by-step explanation:

The cell cycle of multicellular eukaryotes is regulated by Cdks (cyclin-dependent kinases) and external signals. These regulatory mechanisms ensure that cells only progress through the cell cycle's stages when appropriate conditions are met. Cyclins are proteins whose levels fluctuate during the cell cycle and only regulate the cycle when bound to Cdks. For the complex to be active, phosphorylation is required, which is carried out by the enzyme activity of the Cdks. As such, each checkpoint in the cell cycle is controlled by the presence of cyclins that are specific to, and fluctuate at, that particular phase of the cell cycle, ensuring a precise and timed progression through cell division.

Cyclin/Cdk complexes form at specific points in the cycle and are responsible for regulating different checkpoints. For example, growth factors can trigger a cell to begin dividing by increasing cyclin concentration, which then bind to stable levels of Cdks. These complexes are then phosphorylated to activate their function, which is to phosphorylate other proteins that advance the cell to the next phase of the cycle.

Errors in the regulation can lead to uncontrolled cell division, potentially causing cancer. Therefore, cyclins and Cdks are crucial for maintaining the integrity of the cell cycle and preventing disease.