Mitosis: Introduction

Cell cycle is the sequential process taking place to regulate the growth of organism; cell divides to produce a genetic replica and enters the stage of cell growth.

Cell growth involves the synthesis of organic material and integrates information across its counter parts for synchronous development of the whole body.

The cell synthesis phase lasts till a cell reaches its maturity; on initiation the cell again divides to produce new cell and the process continues.

Cell cycle is a sequential development of cell between two cell divisions. The cycle is genetically controlled and are programmed in every cell and are specific for each region.

Varied species has variable time length of cell cycle decided by physiological and influences pertaining to their niche.

Two Phases of cell cycle are: Interphase and Mitotic Phase.

Interphase involves G1, synthesis and G2 phase; chromosomal replication and development is regulated by the phase; determines the quality and quantity of chromosome entering the daughter cells and a balance is maintained by the phase.

Karyokinesis and Cytokinesis division, segregation of chromosome and cell takes place during Mitotic Phase.

The notable feature of cell cycle is eukaryotic organisms though diverse and distinct have a common type of cell division over the Kingdom of Eukaryotes is a scientific wonder and research have emphasized that timing of a cell entering the cell cycle is essential in cell cycle regulation.

Phases of Cell Cycle

The cell cycle is common for all eukaryotic organisms; travelling through 2 major phases based on the cell division: Interphase and Mitotic Phase.

Interphase consists of 3 phases Gap 1 phase, Synthesis Phase and Gap 2 Phase.

Similarly, mitosis has four phases Prophase, metaphase, anaphase, and telophase.

The development of cells through these phases are influenced and facilitated by heterodimeric protein kinases – Cyclin and Cyclin Dependent Kinases.

Mitosis Phase

The changes in above phases are minimal or not clearly visible in microscopes whereas the changes in M Phase are easily detectable.

The phase has 4 parts in which the division takes place systematically and continuously.

The cell stages are easily visible in plant parts as the specialized dividing region – Meristem is prevalent in roots and shoots are continuously dividing providing a mechanical support and functional integrity to the plants.

The 4 phases are: Prophase, Metaphase, Anaphase and Telophase. Each phase has a distinctive change to be identified and Eukaryotic cells replicates in the same order in most of the organisms.

I. Prophase

The prophase is marked by chromosomal condensation and disintegration of cellular components and assembly of cytoskeletons for cell division.

RNA synthesis is inhibited.

The nuclear envelope disintegrates.

The chromatin condensation during G2 phase completes forming Chromatids.

Chromatids are held together by centromere.

The formation of cytoskeleton is initiated along with microtubules.

Kinetochores are formed.

II. Metaphase

Nuclear membrane is eliminated completely chromosomes are completely condensed.

The cytoskeleton – spindle fibers develop and attach to the kinetochores.

This phase also signifies with equatorial alignment of chromosome.

The phase has a checkpoint where it makes sure all the chromatids are well connected to the kinetochores.

Feeble connection leads to unequal distribution of chromosomes in the daughter cells forming defective cell when not eliminated becomes lethal to the organism.

III. Anaphase

Chromosomal split forms daughter chromatids; travels to the opposite poles.

The chromosomes are V – Shaped as they are dragged to the opposite sites by the shortening of the spindle fibers.

So that daughter cells have an equal number of chromosomes to function regular cellular activities.

IV. Telophase

Microtubules disappear and chromosomes decondense to chromatin mass.

Nuclear envelope starts to form.

The disintegrated organelles form again with 2 nuclei and nucleoli.

This reappearance of the cell organelles and equal distribution of the cellular materials ensures each cell to function independently in the cell cycle.

These phases mark the Karyokinesis were the nucleus and other cell parts are newly formed.

V. Cytokinesis

Cytokinesis is the formation of daughter cells after mitosis; indicated by furrow which starts to differentiate two daughter cells grows gradually forming a cell plate while the organelles formed gets segregated.

Cell plate represents the lamella between 2 cell walls.

Mitosis Regulation

The cell cycle is completely regulated by Cyclins and Cyclin dependent protein kinases; belongs to serine – threonine sub classes; Phosphorylates and dephosphorylates the activating components to proceed through the cell cycle depending on the external cues.

The commitment to enter cell division is irreversible when a cell traverse from G1 phase to synthesis Phase is controlled by CYC – CDK complexes as all other phase to enter Mitotic phase.

Mitotic Phase requirements are dealt by the previous phases were DNA replication and chromosomal condensation along with mitotic transcriptional factors and other proteins are synthesized to ensure the cell division which is also supported by the phytohormones.

The main CYC – CDK complex in mitotic cycle is the CYCD-CDKA whose expression is essential for daughter cells to develop well depending on the nutrient provided.

The transition from G2 to M phase is mediated by CYCB – CDKB from G2 phase ensures a proper cell division; expressed from S phase by minimal accumulation gradually increases in quantity to transit from G2 phase to Mitotic phase.

Mitotic CYC – CDK complexes are synthesized during S phase but remains phosphorylated which induces inactivation of the compound.

Following DNA replication phosphatase cdc 25 is activated; dephosphorylation of threonine and tyrosine removes the inhibition from Wee 1 kinase promotes expression of Mitotic CDK – CYC inducing mitosis and forms 2 daughter cells.

Endoreduplication

In endoreduplication the cells undergo continuous division without the formation of cell wall (i.e.) CYTOKINESIS increasing the chromosomal number of the plant.

This is referred as ploidy and in plants; increases the yield and stability of the plant; is under debate.

Endoreduplication are predominant in endosperms. The protein expression producing the mitotic phase is inhibited by regulatory mechanism for the expression of endoreduplication.

The main feature in endoreduplication is that it initiates after a mitosis takes place.

Several factors are involved in the ubiquitination of the cyclin to ensure the cellular restriction to a particular function.