Question

The ________ checkpoint is also known as a restriction point.
A. G1 checkpoint
B. G2 checkpoint
C. M checkpoint
D. None of these

Answer 1

Hint:
The process or sequence of events by which a cell replicates its genome and synthesizes the other constituents of the cell and eventually divides into two daughter cells is known as the cell cycle. The interphase and M phase are the two basic phases of the cell cycle.

Complete answer:
The interphase is further subdivided into the G1 phase (during which the cell is metabolically active, continuously grows, and does not replicate its DNA), S phase (during which the DNA replicates), and G2 phase (important proteins are required that are essential for mitosis). The mitosis phase (M phase) is the equational division, at the end of which two daughter cells are produced with the same number of chromosomes as the parent cell.
During this cell cycle, there are three major checkpoints:
G1 checkpoint: It is the point at which the cell becomes committed to entering the cell cycle. It is also known as a restriction checkpoint in mammalian cells and a starting point in yeast cells. This checkpoint can either delay the G1 phase and enter an inactive phase called the G₀ phase or it can proceed past the restriction phase. The main indication for the cell to 'restrict' and not enter the cell cycle.
G2 checkpoint: In the G2 phase, as important proteins are produced for the M phase, the cell is subjected to ensure that the proteins are made properly and whether the cell is ready to enter the cell cycle or not. Multiple checkpoints are present that regulate the transition from G2 to the M phase. The cell once again checks for any damage in the DNA as well.
M checkpoint: This checkpoint occurs at a point in metaphase where all the chromosomes are aligned at the mitotic plate in the centre of the cell. This point is known as the mitotic spindle checkpoint. At the checkpoint, the cell examines whether all the spindle fibres are correctly attached to spindle microtubules. It ensures that the cell is ready to enter the anaphase.

Thus, the correct answer is:- G1 checkpoint.

Option ‘A’ is correct

Note:
Dysfunctioning of the G1 checkpoint can be seen in cancer cases where the cells continually and inappropriately re-enter the cell cycle and do not enter the quiescent stage (G₀ phase).