Apoptosis is programmed cell death, and it usually occurs when the DNA of the cell is damaged beyond repair.

Photosynthesis and glycolysis are normal metabolic processes of the cell, and would not result from irreversible damage. Endocytosis and exocytosis are also normal cell processes or taking up substances into the cell (endocytosis) or expelling them (exocytosis) in the form of vesicles.

The M phase is also known as mitosis, and is the time where the cell is ready to divide. At this time, the cell has synthesized enough proteins and has successfully replicated its DNA, so growth and synthesis are not priorities.

Remember that G1, S, and G2 are all divisions of interphase. In interphase, the cell is preparing to divide by synthesizing proteins and replicating DNA, so these three phases place a heavy emphasis on growth and protein synthesis.

During G₁, the cell undergoes growth as it increases in size and produces organelles. This is followed by DNA replication is S phase, further growth in G₂, and mitosis in M phase.

Interphase in the cell cycle encompasses the G₁, S, and G₂ phases, as it shows the period of growth and DNA replication that a cell must go through to prepare for mitosis. Cell division, which occurs during the M phase, is the only portion of the cell cycle that is not included in interphase.

Because they do not divide, central nervous system nerve cells do not need to experience growth (G₁ and G₂ phases), DNA replication (S phase), or mitosis (M phase). As a result, they spend most of their lives arrested in G₀, a resting phase.

Cancer can often be the result of a problem with the checkpoint at the end of the G₂ phase, as this is the last stop for regulation before the cell undergoes division.

If this checkpoint is not functioning effectively, cells can undergo rapid and unregulated division, resulting in cancer. p53, a cancer suppressor gene, plays a key role in this checkpoint, and is commonly found to be mutated in cancer patients.

The S phase is when the cell replicates its DNA, resulting in chromatid pairs that will split apart during mitosis. The G phases are mainly dedicated to protein synthesis and cell growth. The M phase is the act of mitosis.

The G1 phase of the cell cycle serves the important function of giving the cell time to grow and replicate organelles. Formation of the mitotic spindle does not occur until prophase of mitosis, and DNA synthesis takes place during the S phase of the cell cycle. The G1 phase ensures that both daughter cells have adequate cellular machinery and organelles to survive after mitosis.

The key to this question is to remember the difference between chromosomes and sister chromatids. During the G2 phase the cell has already undergone DNA replication (in the S phase). Each of the chromosomes will therefore have identical sister chromatids. There will be 92 sister chromatids, but only 46 chromosomes. A chromosome is one entity that consists of the two sister chromatids bound together at the centromere.

During the G1 phase, DNA has not yet been replicated and there is only one copy of genetic information. The cell, however, is still diploid and contains information for 46 chromosomes, but only 46 chromatids. The S phase accounts for the duplication of chromatids, but does not change the total number of chromosomes.

A cell cycle has three crucial checkpoints. The checkpoints function to verify cellular processes and materials that are required for subsequent steps in the cell cycle.

The first checkpoint occurs between the G1 and S phase. The purpose of this checkpoint is to check if the cell is ready for DNA replication. The second checkpoint occurs between the G2 phase and prophase of mitosis (beginning of the M phase). The purpose of this checkpoint is to ensure the completion of replication and the integrity of DNA. The third checkpoint occurs between metaphase and anaphase. This checks if the chromosomes are properly attached at the metaphase plate. All three checkpoints are essential and are irreversible.

For this question we must identify processes that immediately follow a checkpoint. Option I is not correct because it is not preceded by a checkpoint. Option II is correct because sister chromatid separation occurs during anaphase, and anaphase is preceded by the third checkpoint. Similarly, option III is also correct because DNA replication occurs during the S phase, which is preceded by the first checkpoint.