Final answer:
Maturation-Promoting Factor (MPF) is a protein complex essential for the regulation of the cell cycle, specifically the entry into mitosis. It is formed by two subunits: cyclin and cyclin-dependent kinase (cdk). MPF activates other proteins through phosphorylation, leading to critical events such as chromatin condensation and spindle formation necessary for cell division.
Step-by-step explanation:
Maturation-Promoting Factor (MPF) and Its Role in Cell Cycle
During the cell cycle, specific regulatory proteins are responsible for signaling the cell when to divide, replicate DNA, or enter other phases, such as mitosis. One critical regulatory protein complex is the Maturation-Promoting Factor (MPF), also known as Mitosis Promoting Factor. This complex plays a pivotal role in advancing cells through the cell cycle, particularly into mitosis. The MPF is comprised of two subunits, cyclin and cyclin-dependent kinase (cdk). Cyclin is a regulatory protein whose levels fluctuate in the cell—rising after cytokinesis, peaking at mitosis, and then falling. The cdk is an enzyme with a kinase activity that remains at relatively constant levels.
The activation of MPF is tightly linked to the accumulation of cyclin, which begins in the G1 phase. As the cell progresses towards mitosis, cyclin levels increase and bind to cdk subunits. Once a threshold concentration is achieved in G2, the cell is triggered to enter mitosis. MPF activates by phosphorylating other proteins, leading to chromatin condensation, nuclear envelope breakdown, and spindle fiber formation, essential for chromosome segregation and cell division. In addition to the M checkpoint, cell cycle control is also exerted at other checkpoints, including G1 and G2, helping to ensure that cells only divide when they are ready. The discovery of these crucial cell cycle regulators, including MPF, was awarded the Nobel Prize in Physiology or Medicine in 2001. Furthermore, cell signaling pathways are also vital for cell division, often regulated by growth factors that initiate a cascade of events through receptor tyrosine kinases (RTKs), RAS proteins, and the MAP kinase pathway, culminating in gene expression changes that promote cell division.