Explain `G_1`phase of cell cycle.

Explanation

The first gap phase-2C amount of DNA in cells of `G_1`. The cells become metabolically active and grows by producing proteins, lipids, carbohydrates and cell organelles including mitochondria and endoplasmic reticulum. Many checkpoints control the cell cycle. The checkpoint called the restriction point at the end of `G_1` determines a cells fate whether it will continue in the cell cycle and divide or enter a stage called `G_0` as a quiescent stage and probably as specified cell or die. Cells are arrested in `G_1` due to:
(i) Nutrient deprivation.
(ii) Lack of growth factors or density dependent inhibition.
(iii) Undergo metabolic changes and enter into `G_0` state.
Biochemicals inside cells activates the cell division. The proteins called kinases and cyclins activate genes and their proteins to perform cell division. Cyclins act as major checkpoint which operates in `G_1` to determine whether or not a cell divides.