Cell Cycle- Mitosis

 Cell Cycle & Cell Division

Hey guys, Welcome back to the bioeducation1, In this post you will learn about the Cell Cycle and its divison through which phases it has to pass. Let's go to the topic:- Cell Cycle.

Cell Cycle

Cell Cycle and Cell Division

Every cell arise from pre-existing cell as stated by cell theory given by Rudolf Virchow. This is achieved through the process of Cell Division.

What is Cell Cycle?

A series of events occur in the life of dividing cell, which is called Cell Cycle.

The Cell Cycle consists of two main phases:- Interphase and M (Mitotic & Meiotic) phase.

• Interphase

It was formerly called resting phase.
This phase occupies 95% portion of cell cycle.
The Interphase can be further divided into G1 phase, S phase and G2 phase.

1. G1 phase- It is the first growth phase also called the post-mitotic gap phase. As the name suggest the cell growth  take place in this phase.  It involves synthesis of proteins and molecules required for DNA replication. Depending on the cell this phase can be last for days, months or years. The cell may withdraw from cell cycle and can enter into the G0 or quiescent stage. The cell which has gone in G0 state may reenter the G1 phase or may get differentiated to become permanent cell.
2. Note: The loss of control of the cell in G0 state may lead to the uncontrolled division as in tumour.
3. S-phase- It is also called phase of Synthesis. Replication of DNA takes place during this stage. Genetic Material i.e. DNA duplicates. Nucleus becomes double in size. Chromosome replication takes place. Histone proteins are also produced. Once DNA replication is completed, the cell contains twice the total number of chromosomes and hence it is prepared for G2 phase.
4. G2 phase- It is second growth phase also called the pre-mitotic gap phase. It involves the Synthesis of tubulin, spindle protein and RNA. There is duplication of cell organelles such as centriole, chloroplasts and mitochondria. It is also phase which check the duplication of all cell organelles and proteins has taken place or not. This is the last chance for the organelles to duplicate. If everything is perfect the cell is ready to go for mitotic phase.

• Mitosis

Mitosis is a type of cell division in which cell divides to form two daughter cells both having the same chromosome number. 

Equal distribution of Genetic Material take place.

In this process, the nucleus and cytoplasm divides once.

It is somatic cell division (Formation of somatic cells).

It is an Equational Division (Maintain Chromosome Number in daughter cells equal to the mother cells)

The cell division involves simultaneous events, for the sake of your convenience, it is divided into two stages- Karyokinesis and Cytokinesis.

Karyokinesis- Karyokinesis (Nuclear Division) is the stage in which nucleus undergoes a series of events to form two daughter nuclei.

Karyokinesis is further distinguished into Prophase, Metaphase, Anaphase and Telophase.

1. Prophase

• Prophase is the longest phase.
• Prophase is in two forms- Early Prophase and Late Prophase. Sometimes Late Prophase is also called Prometaphase (under the separate phase)
• During Early Prophase, the chromatin network condenses and resolves into definite number of chromosomes.
• Initially every chromosomes appears as single-stranded thin and long.
• Chromosomes undergo dehydration and coiling to become thick and short.
• In late Prophase the Nuclear Membrane and nucleolus begin to disappear. The centrioles start move away from each other till they occupy polar position.

2. Metaphase

• In this phase, the condensation of chromosomes  get completed and the thick chromosomes get organised along the Equatorial plane of the cell.
• Protein formations called kinetochore around the centromere, prior to Metaphase.
• The kinetochore microtubules pull the sister chromatids back and forth until they align along the equator of the cell.
• There is formation of kinetic spindle and the spindle is made up of contractile proteinaceous chromosomal fibre and inter-polar fibres.

3. Anaphase

• It is the shortest phase of the Karyokinesis.
• There is separation of chromatids in which centromeres divide into two.
• The kinetic spindle has four types of fibres i.e. Astral Rays, Inter-polar fibres, Chromosomal fibres and Inter-chromosomal fibres (Inter-Zonal Fibres).
• The chromosomal fibres undergo contraction and pull the daughter chromosomes towards the respective poles.

4. Telophase

• In this phase, the Daughter chromosomes uncoil and undergo dehydration to form chromatin network.
• The nuclear Membrane and nucleolus begins to reappear around each poles of daughter chromosomes.
• Daughter nuclei are formed.

Cytokinesis- The division of the cytoplasm of the mother cell to form two daughter cells is called cytokinesis.

In Animal cells, the cytokinesis occurs by furrowing of plasma membrane that deepens to form two daughter cells. Thus method is called Centripetal Division.

In Plant cells, it takes place by formation and extension of cell plate from centre to the periphery. This method is called the Centrifugal Division.

Significance-

1. It plays an important role in growth, development and healing of multicellular organisms.
2. It forms the Somatic cells. It maintains the same number of chromosomes in daughter cells that of the mother cells.
3. It helps in repairing the damaged tissue of body parts.
4. It ensures equal distribution of Genetic Material and maintain characteristics of species.
5. It plays a vital role in asexual reproduction of unicellular organisms.

We have completed each and every topic with respect to Mitosis in cell cycle. That's all for now, the ret thing will be covered in the next post. We will soon cover the meiosis topic. Stay tuned.

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