Which negative regulatory molecule can trigger cell suicide (apoptosis) if vital cell cycle events do not occur?

a. p53

b. p21

c. retinoblastoma protein (Rb)

d. cyclin-dependent kinase (Cdk)

Negative regulators are the second kind of cell-cycle regulator molecules whose primary work is to halt the cell.

p53 is a type of multifunctional protein that plays an important role in a cell's commitment to division when damaged DNA is present in cells in the G1 phase. To combat genotoxicity or cell stress, tumour protein p53 acts as an essential nuclear transcription factor to control the expression of several genes involved in cell death or apoptosis, growth inhibition, and the process of ageing.

Thus, the correct option is - (a).

p21 is a negative regulatory molecule but cannot trigger cell suicide (apoptosis). Instead, p21 silenced promoted apoptosis.

Thus, option (b) is incorrect.

Retinoblastoma protein (Rb) is a negative regulatory molecule but cannot trigger cell suicide (apoptosis).

Thus, option (c) is incorrect.

Cyclin-dependent kinase (Cdk) is not a negative regulatory molecule, instead, it is a positive regulatory molecule.

Thus, option (d) is incorrect.