12.2: Cell cycle

Note

Checkpoints are the most critical, and the full summary of mitosis is for background.

Cells on the path to cell division proceed through a series of precisely timed and carefully regulated stages of growth, DNA replication, and division that produce two genetically identical cells. The cell cycle has two major phases: interphase and the mitotic phase. During interphase, the cell grows, and DNA is replicated. During the mitotic phase, the replicated DNA and cytoplasmic contents are separated, and the cell divides.

The cycle is divided into four distinct phases, G₁, S, G₂, and M (mitosis), and for most mammalian cells in culture this process takes about twenty-four hours to complete. The majority of differentiated cells in the body are not dividing, retained in a resting state or G_o (figure 12.7).

Figure 12.7: Overview of the cell cycle.

Interphase

During interphase, the cell undergoes normal processes while also preparing for cell division. For a cell to move from interphase to the mitotic phase, many internal and external conditions must be met. The three stages of interphase are called G₁, S, and G₂.

G₁ phase

The first stage of interphase is called the G₁ phase, or first gap, because little change is visible. However, during the G₁ stage, the cell is quite active at the biochemical level. The cell is accumulating the building blocks of chromosomal DNA and the associated proteins, as well as accumulating enough energy reserves to complete the task of replicating each chromosome in the nucleus.

S phase

Throughout interphase, nuclear DNA remains in a semicondensed chromatin configuration. In the S phase (synthesis phase), DNA replication results in the formation of two identical copies of each chromosome (sister chromatids) that are firmly attached at the centromere region. At this stage, each chromosome is made of two sister chromatids and is a duplicated chromosome. The centrosome is duplicated during the S phase. The two centrosomes will give rise to the mitotic spindle, the apparatus that orchestrates the movement of chromosomes during mitosis. The centrosome consists of a pair of rod-like centrioles at right angles to each other. Centrioles help organize cell division.

G₂ phase

In the G₂ phase, or second gap, the cell replenishes its energy stores and synthesizes the proteins necessary for chromosome manipulation. Some cell organelles are duplicated, and the cytoskeleton is dismantled to provide resources for the mitotic spindle. The final preparations for the mitotic phase must be completed before the cell is able to enter the first stage of mitosis.

The mitotic phase

To make two daughter cells, the contents of the nucleus and the cytoplasm must be divided. The mitotic phase is a multistep process during which the duplicated chromosomes are aligned, separated, and moved to opposite poles of the cell, and then the cell is divided into two new identical daughter cells. The first portion of the mitotic phase, mitosis, is composed of five stages, which accomplish nuclear division. The second portion of the mitotic phase, called cytokinesis, is the physical separation of the cytoplasmic components into two daughter cells (figure 12.8).

Mitosis

Mitosis is divided into a series of phases — prophase, prometaphase, metaphase, anaphase, and telophase — that result in the division of the cell nucleus (figure 12.8).

Figure 12.8: Summary of the mitotic phase.

During prophase, the “first phase,” several events must occur to provide access to the chromosomes in the nucleus. The nuclear envelope starts to break into small vesicles, and the Golgi apparatus and endoplasmic reticulum fragment and disperse to the periphery of the cell. The nucleolus disappears. The centrosomes begin to move to opposite poles of the cell. The microtubules that form the basis of the mitotic spindle extend between the centrosomes, pushing them farther apart as the microtubule fibers lengthen. The sister chromatids begin to coil more tightly and become visible under a light microscope.

During prometaphase, many processes that were begun in prophase continue to advance and culminate in the formation of a connection between the chromosomes and cytoskeleton. The remnants of the nuclear envelope disappear. The mitotic spindle continues to develop as more microtubules assemble and stretch across the length of the former nuclear area. Chromosomes become more condensed and visually discrete. Each sister chromatid attaches to spindle microtubules at the centromere via a protein complex called the kinetochore.

During metaphase, all the chromosomes are aligned in a plane called the metaphase plate, or the equatorial plane, midway between the two poles of the cell. The sister chromatids are still tightly attached to each other. At this time, the chromosomes are maximally condensed.

During anaphase, the sister chromatids at the equatorial plane are split apart at the centromere. Each chromatid, now called a chromosome, is pulled rapidly toward the centrosome to which its microtubule was attached. The cell becomes visibly elongated as the nonkinetochore microtubules slide against each other at the metaphase plate where they overlap.

During telophase, all the events that set up the duplicated chromosomes for mitosis during the first three phases are reversed. The chromosomes reach the opposite poles and begin to decondense (unravel). The mitotic spindles are broken down into monomers that will be used to assemble cytoskeleton components for each daughter cell. Nuclear envelopes form around chromosomes.

Control of the cell cycle

There are three key checkpoints in the cell cycle that provide regulation oversight:

• G₁ checkpoint,
• G₂/M checkpoint, and
• Metaphase checkpoint.

Progress through these checkpoints is regulated by a family of cyclin dependent kinases (CDKs). These proteins are constitutive (always present) and inactive. CDKs bind specific cyclin activators, which are required for activity of the kinase. CDKs are present throughout the cell cycle, but expression of the cyclins is restricted to certain times in the cycle, and they are rapidly degraded as the cells progress through the checkpoints. Through binding of cyclins and negative regulation by phosphorylation by CDK inhibitors (CKIs), the cycle is tightly regulated in a restricted manner.

The cyclin and CDK complex can be produced from a combination of different cyclins (A‒D) and different CDKs (1‒6).

Rb-protein

Rb-protein (pRb, retinoblastoma protein) is an important substrate of the G₁/S‒CDK complexes. During the G₀ and G₁ phases, Rb is present in an unphosphorylated (hypophosphorylationed) form, which binds to the transcription factor E2F and thereby blocks it from initiating transcription. When the cycle moves into the S1 phase, pRb becomes phosphorylated (by the CyclinD/CDK4/6 active complex), which allows for the release of E2F.

DNA damage

During the process of DNA replication, DNA damage will halt the process until it can be repaired. Likewise, extrinsic damaging factors can trigger a DNA repair process. Protein p53 is commonly known for its role in DNA repair mechanisms. Under nonstressful conditions it is bound to mdm2 within the cytosol. In response to stress and DNA damage, it is activated, through ATM- or ATR-mediated phosphorylation. Once active, it functions as a transcription factor and induces the synthesis of protein p21.

p21 will then act as a CDK inhibitor (Cip/Kip family) and blocks the action of the G₁‒CDK complex. This will halt the cell cycle at the transition to the S1 phase, and the DNA can be repaired at leisure (figure 12.9). When this has been successfully completed, p53 is dephosphorylated, ubiquitinylated, and passed on to the proteasome. Thus, p53 does not accumulate in normal cells.

Figure 12.9: Summary of cell cycle checkpoints and the role of CDK inhibitors in halting cell cycle progress.

If the DNA repair systems do not succeed in eliminating the DNA damage, a steady increase in the concentration of phosphorylated p53 finally drives the cell into apoptosis. Proteins pRb and p53 are products of tumor suppressor genes. Complete absence of them, due to mutations, leads to accelerated cell division, a typical feature of tumors. In fact, somatic mutations in pRb and p53 have been found in more than half of all human tumors (figure 12.9).

References and resources

Text

Clark, M. A. Biology, 2nd ed. Houston, TX: OpenStax College, Rice University, 2018, Chapter 10: Cell Reproduction, Chapter 11: Meiosis and Sexual Reproduction, Chapter 16: Gene Expression.

Karp, G., and J. G. Patton. Cell and Molecular Biology: Concepts and Experiments, 7th ed. Hoboken, NJ: John Wiley, 2013, Chapter 11: Gene Expression: From Transcription to Translation, Chapter 12: The Cell Nucleus and the Control of Gene Expression, Chapter 13: DNA Replication and Repair, Chapter 14: Cellular Reproduction.

Le, T., and V. Bhushan. First Aid for the USMLE Step 1, 29th ed. New York: McGraw Hill Education, 2018, 41–43, 46.

Nussbaum, R. L., R. R. McInnes, H. F. Willard, A. Hamosh, and M. W. Thompson. Thompson & Thompson Genetics  in Medicine, 8th ed. Philadelphia: Saunders/Elsevier, 2016, Chapter 3: The Human Genome: Gene Structure and Function.

Figures

Grey, Kindred, Figure 12.7 Overview of the cell cycle. 2021. https://archive.org/details/12.7_20210926. CC BY 4.0.

Grey, Kindred, Figure 12.8 Summary of the mitotic phase. 2021. https://archive.org/details/12.8_20210926. CC BY 4.0. Added Mitosis cells sequence by LadyofHats. Public domain. From Wikimedia Commons. And Figure 2. CC BY 4.0. From Lumen.

Grey, Kindred, Figure 12.9 Summary of cell cycle checkpoints and role of CDK inhibitors in halting cell cycle progress. 2021. https://archive.org/details/12.9_20210926. CC BY 4.0.