In the diagram, the speed bumps (look like short walls) represent __________. A) The only times at which genes may be actively expressed B) Points in time when the cell divides C) One-hour timepoints D) Checkpoints in the cell cycle . CDK inhibitorsCDK inhibitorsCDK inhibitorsP15 P18p21p27 p57CDK inhibitorsP19Cyclinp21p27p57P16CDK2CyclinCyclinTITCDK4CDK6CyclinCDK2P21RBRBCyclinP27CDK1G2MCyclinCDK1TTTCDK inhibitors

Introduction :- Cell cycle is defined as the series of steps that a cell passes through to get divide into two genetically identical cells . Cell cycle involves two main phases :- interphase and Mitotic phase . Interphase is also known as the growth phase of the cell cycle and a cell spent almost 95% of cell cycle time in this phase. Interphase is divided into the G1 , S , G2 phase . In G1 and G2 , growth of different types of proteins , macromolecules takes place . In the S phase ( Synthesis) phase of the cell cycle , DNA synthesis ( replication) takes place . Mitotic phase is the phase in which actual division of the cell takes place . It is divided into four sub-phases :- Prophase , metaphase , Anaphase and telophase .Answer :- Option D) Checkpoints in the cell cycle is the correct option. So , in the diagram, the speed bumps (look like short walls) represent checkpoints in the cell cycle . Explanation for correct option D) :- Cell cycle checkpoints acts the regulators of the cell cycle and allows the cell to move from phase to another at right time . Cell cycle checkpoints include :- cyclins , cdk ( cyclin dependent kinases) , cdk inhibitors and other inhibitiory Proteins. Cell cycle checkpoints makes sure that cell cycle is running smoothly , for example :- cell remains in the S phase until the DNA replication ( synthesis) completed . And if any damages to DNA molecule occur in any phase of the interphase then the cell cycle gets halted due to the action of cell cycle checkpoints because damaged DNA molecule can not be allowed to be distribute in daughter cells . Cell cycle checkpoints also controls and regulates the movement of dividing cell from one phase to another .Explanation for incorrect options :- option a) The only times at which genes may be actively expressed is incorrect because activities of gene expression are controlled by the modification at the chromosomal and DNA level . Option b) Points in time when the cell divides is incorrect because the point in time when the cell divides is called mitosis ( karyo-kinesis and Cytokinesis) . Option c) One-hour timepoints is incorrect because cell cycle checkpoints does not acts as timepoints but as regulator of cell cycle , so that two genetically identical daughter cells are formed .