Answer:
a) Cyclins
Explanation:
The internal regulators of the cell cycle are cyclins and CDK (cyclin-dependent kinase). These are proteins that regulate the cell cycle in different checkpoints.
The cyclin proteins stimulate another kind of protein i.e. CDK, which helps in the entry and exit of cells in different checkpoints. The checkpoints are G1, S, and G2 phases. Without this protein regulation the cell cycle can't run properly, nondisjunction of chromosomes, and cancer may result.
The growth factors help in the migration of cells in different parts of the body. This causes cellular growth. These include hormones and enzymes.
Mitotic spindle/spindle fibers separate the double number of chromosomes and put them equally in 2 daughter cells. During the cell cycle, the spindle fibers are not active. It is active in cell division.
Cancer cells are uncontrolled growth of cells. This results due to the failure of cell cycle regulators. Thus cells divide rapidly.
