
Concept introduction: Cell proliferation is a process of increasing the cell number to create a balance between the cell divisions and cell loss. Abnormal proliferation of cells leads to the formation of tumors and cancers. Proliferation process is controlled using positive and negative regulators present in the cell.

Answer to Problem 1SQ
Correct answer:
Transforming growth factor beta (TGF-β), which blocks the cell cycle when there is DNA damage, is an example of a negative growth regulator.
Therefore, option d. is the correct answer.
Explanation of Solution
Reasons for the correct statement:
Positive regulators of the cell initiate the cell to divide, proliferate, and survive. However, the negative regulators are required for a cell to stop the cell division and activate the abnormal cell to undergo programmed cell death.
Option d. is given as “Transforming growth factor beta (TGF-β), which blocks the cell cycle when there is DNA damage”. TGF- β is a cytokine molecule which helps the cells to proliferate, differentiate, adhesion, and apoptosis. They act as mediator molecules to stop the cell cycle when DNA undergoes damage.
Hence, option d. is the correct answer.
Reasons for the incorrect statements:
Option a. is given as “human growth hormone, which stimulates cell division in bone”. This is incorrect because it does not have a negative growth regulator activity. Therefore, option a. is incorrect.
Option b. is given as “human growth hormone receptor, which is part of the signal transduction pathway for the hormone”. This receptor can only activate the signal transduction pathway, but it cannot inhibit the growth of cell. Therefore, option b. is incorrect.
Option c. is given as “prostate-specific antigen, which is a tumor maker found on the surface of prostate cancer cells.” This is not a negative regulator of cell, but it induces the cancer development. Therefore, option c. is incorrect.
Hence, options a., b., and c. are incorrect.
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