The Main Phases Of The Cell Cycle

Reproduction is among the most fundamental characteristics of a living cell. The understanding of the intricate workings of cellular reproduction and the startling similarities between the various processes across a wide variety of organisms from yeast to humans, helps to unambiguously assert the fact that all organisms evolved out of a single common ancestor a few billion years ago. This fact makes for a very vital starting point to crucial studies involving human applications, especially towards combating cancer, which is the result of uncontrolled cell division.

The cell cycle is the chain of events that leads to reproduction through cell division. A majority of cells complete four tasks during the cell cycle: growth, replication of DNA, segregation of chromosomes into two identical sets and division.

In view of the above statement, the cell cycle is divided into two phases:

Interphase – Which is further subdivided into G1, S and G2 phases (in the same sequence as mentioned), in addition to continuous growth and replication of various organelles in the cytoplasm and the accumulation of necessary material, to divide without reducing in size.

Mitotic phase – Where the cell actually divides.

Further the duration of each of these phases depends on the organism and the type of cell concerned, for instance:

• The embryonic cell cycle consists of cell divisions in a short constant duration.
• Growing cell cycle of various organisms which divide in longer and variable durations.
• Some cells do not divide and go through a phase called G0, and some cells take a long time before the initiation of division.

An ordinary microscope is sufficient to differentiate between the Mitotic phase, which is characterized by the presence of the short, thick chromosomes, and the Interphase. But this process is not sufficient to detect the more subtle G1, S and G2 phases. The S phase is where the DNA duplication takes place, hence it can be detected by the ability of the cells in this phase to incorporate labelled DNA precursors, which are detected thereafter by killing the cells. The disadvantage of this method though, is that it is very time consuming.

The best method to detect all the phases of the cell cycle is through the use of FACS (Fluorescence Activated Cell Cytometry) which can detect the fluorescence from DNA binding dyes, by using highly sensitive spectrophotometers, which in turn can be plotted as a distribution of relative amount of DNA against the number of cells.

The logical sequence of the cell cycle is revealed by synchronizing cells and creating multinucleated cells called heterokaryons. In this process:

G1, S or G2 + M (Mitotic) phase cells- Results in the Interphase nuclei breaking down and chromosomes condensing. This reveals that the M phase is a dominating one and has factors to induce other nuclei to divide.

S + G1- Results in nuclei in S phase. Also revealed here was that the heterokaryon did not proceed to M phase until the G1 nucleus finished DNA replication, showing that the process involves feedback control (here, arresting the nucleus in S phase till the G1 nucleus completed its S phase).

S + G2- Results in no change in G2 but S phase continued as it does. This demonstrates that G2 phase has factors that block replication.

The entire cycle is controlled by several enzymes that act as biochemical switches. The majority of this activity is controlled by protein kinases which are present in proliferating cells throughout the cell cycle.

It was discovered that kinase activity rises and falls cyclically during the process. Another group of complexes called cyclins, named so as their concentration changes in a cyclic fashion during the cell cycle, is responsible for controlling the kinase activity, hence the kinases are called cyclin dependent kinases (cdk). The function of these protein complexes were understood by their conspicuous cyclic variation in concentration and the fact that their presence increased dramatically during the cell cycle.

Also independent genetic evidence was found from fact that mutants with defective cell cycles were found to have mutations in genes encoding cyclin and cdk proteins.

The complex cdc6 increases during G1 phase and attaches to the Origin of Replication complex (ORC) attached to the DNA at sites called origins of replication and forms the pre-replicative complex. S-cdk attaches to the pre-replicative complex in late G1 and triggers DNA replication. It also phosphorylates cdc6 causing it and other proteins in the pre-replicative complex to detach from ORC. This helps prevent replication from recurring. Also the phosphorylated cdc6 is led to degradation to prevent reinitiation.

DNA damage can prevent cells from entering the S phase by the increased concentration of p53, a transcription regulator, by activating cdk inhibitor p21, which binds on G1/S-cdk and prevents entry to S phase. M-cdk is prevented from activation by inhibition of the phosphatase that removes the inhibitory phosphate group of M-cdk.

M-cdk triggers the production of condensin complex, which locks and coils the chromatids to form chromosomes. The M phase is characterized by the following sub-processes:

Prophase: Wherein microtubule organizing centres, called centrosome in animal cells and spindle pole body in yeast etc., which divide in the interphase forms the mitotic spindle, which is formed as a result of dynamic instability (random projection of the microtubules in all directions leading to a stable configuration as the spindle fibres). Complexes called kinetochores in the chromosome attach them to the spindle fibres. Also at the end of this phase the nuclear envelope breaks down.

Metaphase: The chromosomes align at the equator region of the cell.

Anaphase: Link between the sister chromatids break and the chromatids move to opposite poles due to tension forces. Onset of this phase marks the end of cell cycle as it is easy to observe and there is a change in machinery that regulates it.

Telophase: Sister chromatids reach the opposite poles, the nuclear envelope starts to re-form.

Cytokinesis: The actual cell division takes place here due to the contractile ring made of actin and myosin in animal cells by the pinching of cytoplasm and cell wall formation in plants demarcating the two daughter cells.

References:

• Essential Cell Biology; Alberts et al.
• The Cell; Cooper, G.M and Hausman, R.E
• Cell Cycle; Morgan, A and Hunt, T
• https://www.mechanobio.info/cytoskeleton-dynamics/what-is-the-cytoskeleton/what-are-microtubules/what-microtubule-dynamic-instability/
• www.yourgenome.org
• www.lehigh.edu