The checkpoint called the restriction point at the end of G₁, it determines a cells fate whether it will continue in the cell cycle and divide or enter a stage called G₀  as a quiescent stage and probably as specified cell or die.

1. Control activities of the cell.
2. Genetic information copied from cell to cell while the cell divides.
3. Hereditary characters are passed onto new individuals when gametic cellsfuse together in sexual reproduction.

The replication of chromosomes in the absence of nuclear division and cytoplasmic division resultirig in numerous copies within each cell is called endomitosis. Chromonema do not separate to form chromosomes, but remain closely associated with each other. Nuclear membrane does not rupture. So no spindle formation. It occurs notably in the salivary glands of Drosophila and other flies. Cells in these tissues contain giant chromosomes (polyteny), each consisting of over thousands of intimately associated, or synapsed, chromatids. eg Polytene chromosome.

A Culture of animal cells in which the cell cycles were asynchronous was incubated with 3H-Thymidine for 10 minutes. Autoradiography showed that 50% of the cells were labelled. If the cell cycle time (generation time) was 16 hrs how long was the S period?
Length of the S period = Fraction of cells in DNA replication x generation time
Length of the S period = 0.5 x 16 hours = 8 hours.

The first gap phase - 2C amount of DNA in cells of G₁. 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 G₁, determines a cells fate whether it will continue in the cell cycle and divide or enter a stage called Go as a quiescent stage and probably as specified cell or die. Cells are arrested in G₁ due to:
(i) Nutrient deprivation.
(ii) Lack of growth factors or density dependent inhibition.
(iii) Undergo metabolic changes and enter into G_o 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 G₁ to determine whether or not a cell divides.

S phase

(i) Some cells exit G₁ and enters a quiescent stage called G₀, where the cell remains metabolically active without proliferation.
(ii) Cells can exist for long periods in G₀ phase. In G₀ cells cease growth with reduced rate of RNA and protein synthesis.
(iii) The G₀ phase is not permanent. Mature neuron and skeletal muscle cell remain permanently in G₀.
(iv) Many cells in animals remains in G₀ unless called on to proliferate by appropriate growth factors or other extracellular signals. G₀ cells are not dormant.

Difference between Mitosis and Meiosis: