AP BIOLOGY

Detailed explanation-1: -The cell cycle has checkpoints (also called restriction points), which allow certain genes to check for problems and halt the cycle for repairs if something goes wrong. If a cell has an error in its DNA that cannot be repaired, it may undergo self-destruction (apoptosis ).

Detailed explanation-2: -Genetic analysis of human cancers has revealed that proteins involved in the G1/S checkpoint are inactivated in the majority of cases, and that alterations of the DNA damage checkpoint are likely to be responsible for resistance of tumour cells to chemotherapic agents or irradiation.

Detailed explanation-3: -Overall, the cell cycle has many regulatory steps that are crucial to maintaining homeostasis inside of the cells during the replication of DNA and the division of cells. Mutation inside specific genes can lead to the loss of balance inside the cells, thus resulting in cancer growth.

Detailed explanation-4: -In normal proliferating cells, initiation of these processes is controlled by genetically-defined pathways known as checkpoints. Tumors often acquire mutations that disable checkpoints and cancer cells can therefore progress unimpeded into S-phase, through G2 and into mitosis with chromosomal DNA damage.