Cell Cycle

Cell division cycle or cell cycle is sequential process when cell duplicate or divides. Prokaryotic cell, cells without nucleus cell divides by binary fission process. However in eukaryotic cells cell divides in two stages namely interphase and mitosis. In simple terms, interphase is a stage where the cells accumulate nutrients for its growth and duplicates DNA to utilize during mitosis. During mitosis stage, each cell divides in to two cells and produces their offspring’s. It plays an important role when fertilized egg results in organism maturity, thereby resulting in the growth of internal organs, hair, blood cells and skin.

Phases of cell cycle

Cell division occurs in four stages as described below.

• G1 phase
• S phase or synthesis phase
• G2 phase or interphase
• M phase

M phase is of two types namely mitosis and cytokinesis. When chromosomes divide in to two daughter cells, it is referred as mitosis. When cytoplasm divide itself in to two half cells then it is referred as cytokinesis. Only when the previous process completes the full phase cycle, only then it can enter into next phase of cell cycle. When a cell stop dividing and leaves the cell cycle during the phase of cell cycle, they are referred as quiescent cells indicated as G_O phase.

Resting phase

Quiescent and senescent cells are referred as post mitotic phase. Certain cells like non proliferative cells of eukaryotic organism directly enter in to quiescent stage. Some cells can remain in resting period for longer period of time as in neurons. This is common in completely differentiated cells. Senescence of cells is due to degradation or damage in DNA such as apoptosis.

Interphase

This phase helps in proper utilization of nutrients just before entering in to cell division cell cycle. This is also referred as preparation stage. It is classified in to three stages such as G1, S and G2 phase. As it name indicates it is cyclic in nature. Hence interphase is following phase of mitosis and cytokinesis.

G1 phase

It is also referred as growth phase. During this G1 phase biosynthesis of cells proceeds at a higher rate. It also produces more number of enzymes for replication of DNA which are required in S phase. It can last for any number of time periods.

S phase

DNA synthesis occurs in this phase. After which chromosome replication commence. Number of DNA cells consequently doubles whereas the cell ploidy is same. Synthesis of protein and RNA transcription takes place at slower rate. Exception of this phase is histone production.

G2 phase

Biosynthesis occurrence in specific microtubule production is prevalent. Microtubules are essential for mitosis process. When protein synthesis is inhibited prevents cells from mitosis entering.

Mitosis

Chromosomes are separated in to two daughter cells. Cytokinesis consequently results in division of nuclei, cell membrane, cytoplasm and cell organelles. Mitosis is prevalent in eukaryotic cells and it is complex and is regulated in sequence. The process is different for different organism. Following are mitotic phase subdivision.

• Prophase
• Metaphase
• Anaphase
• Telophase
• Prophase: Genetic material which is bound loosely starts condensing collectively to form a chromosome. Chromosome is already duplicated in S phase. It has centromere where two sister chromatids adheres by cohesion complex. During cell division single centrosomes which are present near nucleus are inherited. Spindle fibers are formed when two centrosomes nucleate to form microtubule. Centrosomes with microtubules are pushed to the other end of cell by molecular motor proteins.

• Metaphase: Microtubules are attached to kinetochores during prometaphase stage. Chromosomal centromeres assemble all along the equatorial plane or metaphase plate. Every kinetochore should attach to spindle fibers. If all the chromosomes are not aligned then kinetochores produce mitotic spindle checkpoint as signal resulting in premature progression of anaphase.

• Anaphase: When kinetochores are attached to microtubule cluster during metaphase stage, cell enters in to anaphase. After which two sister chromatids get separated and nonkinetochore microtubules also elongates. Separation of sister chromatids are called as early anaphase and elongation of nonkinetochore microtubules are called as late anaphase. In both the phases of anaphase chromosomes are pulled apart. As a result genetic material identical copies are found in offspring.

• Telophase: Reverse process of prophase and prometaphase is called as telophase. Elongation of nonkinetochore microtubules continues so that sister chromatids get attached to the other end of the cell. Nuclear envelope is formed around sister chromosomes. Chromosomes are then surrounded by nuclei and spread out to form chromatin. Formation of chromatin indicates the completion of mitotic cell division.

• Cytokinesis: It is not continuation phase of telophase. Cytokinesis takes place in parallel with telophase. Microtubules move towards the central part of cell. Cell division results in the formation of vesicles like Golgi apparatus. Each offspring has complete identical copy of genome. Cytokinesis end is remarked as the mitotic phase completion.