Cell Cycle, Cell Cycle Phases, and Anaphase

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.

Types of Anaphase

Anaphase has 2 phases Anaphase A and Anaphase B were the segregation and spindle fiber withdrawal happen respectively.

Chromosomes separates to chromatids by breaking the wounding of COHESIN which holds the chromosome intact in the metaphase; is Anaphase A.

I. Anaphase A

Anaphase A involves the cell cycle regulator proteins of the M phase.

The anaphase elucidation is poorly studied for plants rather than animals; but the cell cycle and the mechanism is conserved over evolution of the living organism.

It can give a basic picture of the mechanism which is similar to all eukaryotes were the difference is the proteins and other components involved in them governed by the external cues and time.

The mechanism is a cascade of proteolysis, degradation and activation different protein molecules.

The first step is the proteolysis of cohesins mediated by APC/C along with Cdc ubiquitinates CYCB – CDKB complex also degrades the Securin.

Securin directly inhibits Separase in chromosomes cleaves Cohesin rings at Scc1 segregate the chromatids.

The separation induces respective poleward movement of the chromatids.

I. Anaphase B

Anaphase B is the self-detachment of microtubules from the KMT complex due to loss of MT protein components.

Dynein and Kinesin are the 2 proteins; these are motor proteins which pulls the KMT to opposite poles especially Dynein.

The removal of microtubules is mediated by depolymerization of the proteins components leads to separation of each tubulin component from the minus driven ends and Dynein at the control of the fibers near plasma membrane.

Removal of KMT complex is simultaneous with chromatid retraction to the poles.

Motor protein Kinesin undergoes a sliding elongation forming interpolar MT at the plus end.

The main function of the interpolar MTs are to separate the poles far apart from the chromosomal segregation.