Cell Cycle, Cell Cycle Phases, and Cytokinesis

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.

History of Cell Cycle

The history of mitosis dates back to 18th and 19th century where the aid from microscope and visibility of cell division under the microscope supported the discovery of the cell division.

The discovery was earlier than the DNA discovery was a breakthrough in the scientific community as it answered the most intriguing question of Humans “How do we grow? Develop? Reproduce? What is the driving factor for the growth? How do we resemble our parents?” etc.,

Before the discovery of cell division there were many theories on how the cells are related to the overall development of an organism’s lifecycle.

One of which was Rudalph Virchow’s theory of cell: “Omnis cellula e cellula” which states that a cell originates from a pre-existing cell.

Walther Flemming discovered and published a detailed book on cell division in 1882 after discovering cell division in 1879.

He named it “Mitosis” after the Greek word Mito – “Wrapping thread” owing to the thread like appearances of the chromosomes.

He conducted a detailed study and deduced staining techniques to understand cell cycle and named the stages of each division as Prophase, metaphase, Anaphase and Telophase.

The cell division was identified in Salamander’s embryo. Flemming supported Virchow’s cell theory with precision and stated that “Omnis nucleus e nucleo” which states that a nucleus origin from a pre – existing nucleus and highlighting the chromosomal segregation and laid foundation for the theory of inheritance where the chromosomes play an important role carrying the genetic information from the parent to the offspring.

Cell Cycle Phases

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.

Mitotic 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.

Prophase: The prophase is marked by chromosomal condensation and disintegration of cellular components and assembly of cytoskeletons for cell division. RNA synthesis is inhibited.

Metaphase: Nuclear membrane is eliminated completely chromosomes are completely condensed. The cytoskeleton – spindle fibers attach to the kinetochores. The chromosomes are aligned in the equatorial plate.

Anaphase: Chromosomal split forms daughter chromatids; travels to the opposite poles. The chromosomes are V – Shaped as they are dragged to the opposite sites.

Telophase: Microtubules disappear and chromosomes decondense to chromatin mass. Nuclear envelope starts to form. The disintegrated organelles form again.

Cytokinesis

Next part of cell cycle is Cytokinesis where the duplicated sister chromosome is designated a separate functional unit by the formation of microtubules to form the rigid cell membrane.

Cytokinesis in Higher Plants

Cell division is complete when the daughter chromatids are segregated and are given the status of independent functioning.

This is achieved by cell cleavage and cell wall formation of the dividing cells; marked by cytokinesis.

Cell formation from the previous cells and the division of nucleus and chromosome segregation is known as karyokinesis.

Cytokinesis completion is required for a cell to attain maturity by entering the Interphase of the next cell cycle.

Predominant transfer of membrane bound organelles. But the initiation of the phase takes place in late anaphase and in Telophase with Preprophase Band (PPB). Contrast to animals; plants lack Microtubule Organizing centers (MTOC) – centrosomes which is supported by PPB formed at the median plate perpendicular to the equatorial plate.

Tubulins and Dynein’s segregates chromosomes; whereas Actin filaments guide the cell wall formation between the cells as cell division predominantly produces daughter cells in adjacent sides.

Cell Wall formation: Phragmoblast

Cell wall formation separates the daughter cells is the main event in CYTOKINESIS.

Cellulosic cell wall formation is semi – rigid and are guided by PHRAGMOPLASTS.

Phragmoplasts arise from the PPB after Telophase formed by the interzonal or interpolar microtubules; at the middle region of the cell will use microtubules and Golgi vesicles to form a cell plate.

The cell plate formation takes place centrifugally referred as Nascent Cell Plate.

The nascent cell plate forms the initial semi rigid cell wall extends and reach the adjacent cell wall from the center.

Cell communication becomes essential in a tissue where the availability of the nutrients and other information’s are passed through certain pores namely Plasmodesmata are formed by disintegrated Golgi vesicles carrying pectin.

Pectin of one vesicle when fuses with pectin of other vesicle they components mix and forms pores.

These early cells plate / nascent plate formation is supported by Golgi vesicles which contains glycoproteins and polysaccharides for the cell wall formation.

The microtubules forms semi – crystalline lattice on both side of the daughter cell.

Later they mature to form a rigid cell wall by G1 PHASE.

Segregation of Membrane Bound Organelles

By the time of cell division (i.e.) Cytokinesis the cell organelles are equally divided among the daughter cells.

The cell organelles which are bound by a membrane are transported to the cells by motor proteins to the poles.

Generally, mitochondria and chloroplasts are many in number which is sufficient for each cell is multiplied during the mitosis and transported before cytokinesis.

Endoplasmic reticulum which is a part of nucleus cuts off during cell plate formation to the daughter cell.

The chromatids completely condense and are bound by nuclear membrane.