During mitosis, the metaphase checkpoint ensures that each duplicated chromatid is attached to the spindle apparatus. If the metaphase checkpoint is not satisfied, the cell will not enter anaphase. This checkpoint is essential for preventing aneuploidy, a condition in which there are an abnormal number of chromosomes in a cell.

The G1 checkpoint determines if the cell will enter the S phase to replicate DNA and prepare for cell division. Quiescent cells, cells that do not frequently divide, are often stopped at the G1 checkpoint for long periods. The G2 checkpoint is used to prevent the cell from entering mitosis if there were errors in the replication of DNA during the S phase. Failure to pass the G2 checkpoint can result in apoptosis. There is no G0 checkpoint.

Active S cyclin-CDK phosphorylation in the cell cycle is primarily intended to ensure that each portion of the cell's genome is copied once and only once. Daughter cells that do not copy a complete genome will likely die; however, carrying extra copies of certain genes will also negatively affect daughter cells, and phosphorylation of proteins that make up pre-replication complexes safeguards against this.

Cell division, or mitosis, is a rather small portion of a cell's life and includes prophase, metaphase, anaphase and telophase.  The majority of a cell's life is spent in interphase.

Most cells spend about 90% of their time in interphase. Note that mitosis and meiosis comprise only about 10% of the cell cycle.

Chromosomes and Chromatid are both incorrect because eukaryotic DNA is condensed into these tightly packed chromosomal structures during M phase of mitosis. Plasmids are not found in eukaryotes and an unfolded continuous strand of DNA would be too long to fit within a nucleus. Thus, Chromatin is the correct answer choice; chromatin is a protein-DNA complex in a loosely packed form which allows for gene transcription which is necessary during the G1 phase of the cell cycle. 

The G₁ Checkpoint is the correct answer, because if a cell gets a signal at this checkpoint then the cell goes on to complete the S, G₂, and M phases and will end up dividing. If this signal is not received at the G₁ checkpoint then the cell enters the non-dividing G₀ phase.

Once the cell passes the G1 checkpoint, the cell becomes committed to the cell cycle and enters the S phase where DNA is replicated. The checkpoint is to ensure the cell has grown enough and has enough resources to begin DNA replication. The next checkpoint is the G2 checkpoint, where the cell checks and makes sure the DNA replicated correctly before beginning mitosis. If the cell does not pass this checkpoint, it commences apoptosis and dies.