Basic Cell Cycle

  Each human begins his or her life from a single cell and this single cell is known as the zygote. Zygote divides again and again to form an infant further. it is the cell division because of cell cycle, we developed from an infant to an adult.
    Besides growth cell division is also involved in repairing our damaged tissues. This role of cell division is responsible for the healing of wounds cuts and injuries.

Cell Division
   For these functions of growth and repair, it is important that the
• New cells formed should be identical to the parent cells.
• New cells should be identical in size shape and functions.
•  The number of chromosomes in each daughter cell should be same as that of the parent cell.

This means for the production of genetically identical daughter cells, Each parent cell must first undergo a growth phase in which,
- it increases in size
- it duplicates its chromosomes and other important components necessary for cells function.

Cell cycle
   In eukaryotes cells undergo a repeating pattern of growth followed by division.
This repeating sequence of cellular growth and division during the life of an organism is known as the cell cycle.
  Eukaryotic cell it undergoes a growth phase, then it divides the genetically identical daughter cells produced may further enter into this cell cycle to produce more cells.
  Chromosome duplication also takes place before cell division. There are 2n chromosomes in the original cell these chromosomes get duplicated so now there are 4n chromatids. Finally after cell division each daughter cell has 2n chromosomes.

The cell cycle can be divided into two major stages
   1). Interphase and
   2). M phase

1). Interphase
• Cells actually spend most of their time in the interphase.
• In interphase the chromosomes uncoil into extremely long and thin structures. They cannot be distinguished as individual threads in this stage.
• Also under light microscopy no dramatic change is visible inside the cell.
• Interphase is a not resting stage of cell. Interphase is an active time.
•  The cell is fulfilling its specialized or routine functions in the individual's body.
• In terms of the cell cycle interphase is the stage during which a cell prepares itself for cell division.
• Interphase is longer stage accounting for 90% of the cell cycle.

interphase is further divided into three phases
  - The G1 phase
  - The S phase and
  - The G2 phase
•  G stands for gap, but these phases are not just gaps because cell growth takes place during these phases.
•  S stands for synthesis, during this phase DNA synthesis or chromosome duplication takes place.

The G1 Phase
   New cells arise from a pre-existing cells. G1 phase is the duration between the birth of a new cell and the onset of its chromosome replication.
• During G1 phase the cell grows rapidly and carries out its normal routine functions.
• Cytoplasmic organelles such as mitochondria and endoplasmic reticulum usually duplicate during this phase.

The S phase
  S stands for synthesis, more specifically synthesis of DNA during this phase.
• The cell duplicates its chromosomes. these are the n chromosomes before duplication, when these chromosomes are duplicated during S phase we get two exact copies of each chromosome.
• These copies of each chromosome are attached to each other at a point called centromere and also along the arms.
• Although the chromosome is duplicated but remains attached to its exact copy at the centromere. As long as they are joined together these duplicated chromosomes are considered as a single chromosome.
• These duplicated chromosomes are not visible during interphase.

The G2 phase
G2 phase is the time period between the end of S phase and the beginning of M phase.
• During this time cell further grows and synthesises proteins that are required in the subsequent cell division.
• An important event that takes place during this phase is the assembly of microtubules
• These microtubules are visible outside the nucleus.

These were the three phases of interphase. so all the preparation for cell division is complete. The cell now enters the next stage which is known as M phase.

M Phase
M phase or mitotic phase is the stage during which actual cell division takes place.
The main events of the stage are
- Chromosome segregation and
- The division of cytoplasmic contents.
• M phase includes two main processes -
1). Mitosis and
2). Cytokinesis

Mitosis is the process in which duplicated chromosomes are separated into two nuclei. Each of these two nuclei contains same number and kind of chromosomes as the original cell.
On the other hand Cytokinesis is the process in which the entire cell divides into two daughter cells.
• M phase is the shortest part of the cell cycle