Cell cycle and Mitosis

Cell cycle and Mitosis

All cells are produced by divisions of pre-existing cell.Continuity of life depends on cell division.A cell born after a division,proceeds to grow by macromolecular synthesis,reaches a species determined division size and divides.The cycle acts as a unit biological time and defines life history of a cell.Cell cycle can be defined as the entire sequence of events happening from the end of one nuclear division to the beginning of the next.The cell cycle involves the following three cycles.

1. Chromosome cycle. In it, DNA synthesis alternates with mitosis.During DNA synthesis,each double helical mitosis the duplicated copies of the genome are ultimately separated.
2. Cytoplasmic cycle. In it, cell growth alternates with cytokinesis.During cell growth many other components of the cell become double in quantity and during cytokinesis cell as a whole divides into two.Usually the karyokinesis is followed by the cytokinesis but sometimes the cytokinesis does not follow the karyokinesis and results into the multinucleate cell,e.g.,cleavage of egg in Drosophila.
3. Centrosome cycle.Both of the above require that the centrosome is inherited reliably and duplicated precisely in order to form the two poles of the mitotic spindle;thus ,centrosome cycle forms the third component of the cell cycle.

Howard and Pelc have divided cell cycle into four phases or stages :G_1,S,G_2, and M phase.The G₁ phase ,S phase, and G₂ phase are combined to form the classical interphase.

1. G₁ After the M phase of previous cell cycle,the daughter cells begin G₁ of interphase of new cell cycle.G₁ is a resting phase.It is called first gap phase,since no DNA synthesis takes place during this stage :currently ,G₁ is also called first growth phase,since it involves synthesis of RNA,proteins and membranes that leads to the growth of nucleus and cytoplasm of each daughter cell towards their mature size.

G₁ phase is most variable as to duration;it either occupies 30 to 50 percent of the total time of the cell cycle or lacks entirely in rapidly dividing cells(e.g.,blastomeres of early embryo of frog and mammals).Terminally differentiated somatic cells that no longer divide,are arrested usually in the G₁ stage;such a type of G₁ phase is called G₀ phase.

2. S phase.During the S phase or synthetic phase of interphase,replication of DNA and synthesis of histone proteins occur.New histones are required in massive immediately at the beginning of the S period of DNA synthesis to provide the new DNA with nucleosomes.So at the end of S phase each chromosome has two DNA molecules and a duplicate set of genes.S phase occupies roughly 35 to 45 percent of the cell cycle.
3. G₂It is a second gap or growth phase or resting phase of interphase.During G₂ phase,synthesis of RNA and proteins continue which is required for cell growth.It occupies 10 to 20 percent time of cell cycle.As the G₂ phase draws to a close,the cell enters the M phase.

General Events Of Interphase

The interphase is characterized by the following features:

The nuclear envelope remains intact.The chromosomes occur in the form of diffused,long,coiled and indistinctly visible chromatin fibres. The DNA amount becomes double.Due to accumulation of ribosomal RNA and ribosomal proteins in the nucleolus the size of the latter is greatly increased.

4.M phase or Mitotic phase. The mitosis occurs in the somatic cells and it is meant for the multiplication of cell number during embryogenesis and blastogenesis of plants and animals.Mitosis starts at the culmination point of interphase,It is a short period of chromosomes condensation,segregation and cytoplasmic division.As a process,mitosis is remarkably similar in all animals and plants.It is a smoothly continous process and is divided arbitrarily into following stages or phases for convenient reference:

1. The appearance of thin thread like condesing chromosomes marks the first phase of mitosis,called prophase.The cell becomes spheroid,more refractile and viscous.The prophase chromosome is composed of two coiled filaments,the chromatids,which are the result of the replication of DNA during the S phase.As prophase progresses,the chromatids become shorter and thicker and two sister chromatids of each chromose are held together by a special DNA containing region called the centromere or primary constriction.During prophase,proteins of the trilaminar kinetochores start depositing or organizing on the centromeres of each chromosomes.In the cytoplasm,the most conspicuous change is the formation of the spindle or mitotic apaartus.In the early prophase,there are two pairsof centrioles,each one surrounded by the so called aster which is composed of microtubules radiating in all directions.The two pairs of centrioles migrate to opposite poles of the cell along with the asters and become situated in antipodal positions.Between the separating centrioles forms a spindle.The microtubules of the spindle are arranged like two cones to base,broad at the cente or equator of the cell and narrowing to a point at either end or pole.Mitotic spindle contains three main types of fibres:1.polar fibres,which exerted from the two poles of the spindle toward the equator; kinetochore fibres,which attach to the kinetochores of centromeres of each mitotic chromosomes and extend toward the poles;and 3.astral fibreswhich radiate outward from the poles toward the periphery or cortex of cell.In cells of most higher plants,however,spindle forms without the aid of centioles and lacks asters.There are various available with respect to the dissolution of nuclear envelope and the nucleolus.In a number of primitive classes of plants and animals the nuclear envelope does not dissolve during mitosis.

Prometaphase. The breakdown of nuclear envelope signals the commencement of prometaphase and enables the mitotic spindle to interact with the chromosomes. This stage is characterized by a period of frantic activity during which the spindle appears to be trying to contain and align the chromosomes at the metaphase plate.Infact at this stage the chromosomes are violently rotated and oscillated back and forth between the spindle poles because their kinetochores are capturing the plus ends of microtubules growing from one or other spindle pole and are being pulled by the captured microtubules.

2.Metaphase. During metaphase the chromosomes are shortest and thickest.Their centromere occupy the plane of the equator of the mitotic aapartus,although the chromosomal arms may extend in any direction. Metaphase occupies a substantial portion of the mitotic phase,as if the cell pause until all their chromosomes are lined up appropriately on the metaphase plate.At metaphase subunits are added to the plus end of a microtubule at the kinetochore and are removed from the minus end at the spindle pole.Thus,a poleward flux of tubulin subunits occurs,with the microtubules remaining stationary and under tension.

3.Anaphase. The anaphase begins abruptly with the synchronus splitting of each chromosome into its sister chromatids,called daughter chromosomes,each with one kinetochore.Synchronus splitting of each centromere during prophase is evidently caused by an increase in cytosolic Ca²⁺.In fact Ca²⁺-containing membrane vesicles accumulate at spindle poles and releases calcium ions to initate anaphase.Anaphase involves the following two steps:

1. Anaphase A. During it,there is poleward movement of chromatids due to shortening of the kinetochore microtubules.During their poleward migration,the centromeres remain foremost so that the chromosomes characteristically appear U,V or J-shaped.
2. Anaphase B. It involves separation of poles themselves accompanied by the elongation of the polar microtubules.The astral microtubules also help in anaphase B by their attractive interaction with cell cortex.

References

Dhami, P S, and J K Dhami. A textbook of zoology Vol. II and Vol.III. Latest edition. New Delhi: Pradeep publication, n.d.

Kotpal, R L. Modern textbook of Zoology. Meerut, India: Rastogi Publication, n.d.

Rastogi, S C. Cell, and Molecular biology. New Delhi: New Age International (P) Limited, 2001.

Verma, P S, and V K Agrawal. cell biology,Genetics,Molecular Biology,Evolution, and Ecology. New Dehli, India: S. Chand and company Ltd., 2012.