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.

Remember that DNA is replicated during the S phase of the cell cycle. Immediately after mitosis, the cell contains a single chromatid for each chromosome, for a total of 46 chromatids. This period corresponds to the G1 phase, during which the cell produces proteins and grows.

The S phase follows the G1 phase and doubles the amount of DNA to 92 chromatids, composing 46 complete chromosomes with two chromatids each. The S phase is followed by the G2 phase, which is followed by the M phase (or mitosis). Through this remaining period, until the conclusion of mitosis, the cell contains 92 chromatids.

Only G1 phase precedes the S phase and has half the amount of DNA as the rest of the cell cycle; therefore, cells in the G1 phase have the least amount of DNA.

Histones are nuclear proteins that organize and compact DNA. Histones act as core proteins around which DNA molecules can wrap and form nucleosomes. Since histones are proteins, they are mostly made up of amino acids.

Recall that amino acids are the building blocks of proteins, nucleotides are the monomers of nucleic acids, and monosaccharides are the monomers of carbohydrates. Fatty acids are a common component in lipids.

The checkpoints in the cell cycle are necessary to make sure the cell is ready to divide. Certain checkpoints will check the size of the cell, the integrity of the DNA, and the proper attachment of spindle fibers. If a cell fails to use these checkpoints, it can continue to divide even when it is not ready.

This unregulated division is a hallmark of cancer, in which a cell continuously divides without the necessary checkpoints. Cancerous cells often display genetic defects due to bypassing checkpoints.

Nucleosomes are complexes of DNA wrapped around eight histone molecules, often resembling "beads on a string" during interphase. The tight wrapping of DNA around the histones prevents it from being transcribed, but also condenses it. Nucleosomes can be loosened to regulate the transcription of associate DNA and genes.

Looped domains are higher order structures in which the DNA is more tightly packed. Heterochromatin refers to clumps of tightly packed chromatin. Histone tails are extensions of the histone molecules. Nucleoids are the region in prokaryotic cells that houses the cell's DNA.

Interphase alternates with the mitotic phase and accounts for approximately 90% of the cell cycle. During interphase, the cell grows (G1 phase), replicates/synthesizes its chromosomes (S phase), and completes preparation for cell division (G2 phase).

The M phase occurs after the G2 phase and corresponds with mitosis, which is not considered part of interphase.

Cyclin proteins bind to cyclin-dependent kinases and drive the cell division cycle. One mechanism to control this process is through the amount of cyclin protein present and available to bind and activate cyclin-dependent kinases (CDKs). The cyclin-CDK complexes provide signals for the cell to progress through cell cycle checkpoints. After completion of a stage of the cell cycle, the cyclin protein is rapidly degraded to prevent inappropriate signaling. Overproduction or accumulation of cyclin protein can cause inappropriate continuation through the cell cycle and unregulated cell division.

The stages of the cell cycle begin with G1, during which the cell grows and prepares for chromosome replication by synthesizing proteins and cellular structures. The chromosomes do not actually duplicate themselves until the next step, the S phase. After the S phase comes G2, another resting/growth period, and if the cell is properly developed it can then finally enter mitosis. If the cell stops at the G1 checkpoint, it has been arrested before it can enter the S phase and its chromosomes will be unable to replicate.

The three key checkpoints in the cell cycle occur after the G1 phase, before chromosome replication, after the G2 phase, before mitosis, and after metaphase, before chromatids are separated.

The restriction point is a decision point in G1 of the cell cycle. If the cell passes this point, the cell cycle will progress to the S phase. If the cell does not pass this point, there is likely a lack of stimulation from nutrients or growth factors, or some type of internal damage that must be corrected before the cell cycle can progress. Prolonged lack of progression past the restriction point can result in a quiescent cell, which enters the G0 phase and does not readily divide.

The other two cell cycle checkpoints occur after the G2 phase (before mitosis) and after metaphase (before anaphase).

The DNA will appear uncondensed and restricted to the nucleus. This is because a majority of the cells found in the sample will be in interphase, the longest stage of the cell cycle. Since it is the longest phase in the cell cycle, a majority of cells will be undergoing this phase at any given sample time. It is a point when the cell is undergoing normal cell processes and preparing for replication. During this phase, the DNA is uncondensed, allowing for transcription and giving DNA replication enzymes easy access to the nucleotide sequence.

DNA is condensed into 46 discrete chromosomes and restricted to the nucleus during early prophase, which makes up only a short period of the cell cycle. DNA is never condensed into 23 discrete chromosomes in somatic cells (such as epithelium), since 23 chromosomes would indicate a haploid cell. In humans, haploid cells only exist in the gametes, or sex cells. While prokaryotes have a single circular chromosome floating in the cytoplasm, such structure for a eukaryotic genome is not possible.