Understanding the Cell Cycle - AP Biology

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.

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.

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.

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.

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.

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 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.

Interphase is the entirety of the cell cycle with the exception of mitosis. During the G1 period of interphase, the cell grows and produces many protein products to replicate organelles. G1 is followed by the S phase, during which DNA replication occurs. This is followed by the G2 phase, which prepares the cell for division.

The cell then enters prophase of mitosis, the M phase, during which chromosomes condense. Later in mitosis the sister chromatids are separated during anaphase. The G1 phase, S phase, and G2 phase are all portions of interphase, while the M phase corresponds to mitotic division.

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 "S" phase, known as the synthesis phase, is the portion of the cell cycle where DNA is replicated. The other stages listed do not contain DNA replication. G0 is a cell cycle arrest phase, where a cell remains dormant, awaiting signals to re-enter the cell cycle.

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.