CBSE

Cell Cycle : The Life Cycle of a Cell


Cell Cycle : The Life Cycle of a Cell

Introduction:

Series of events which are passing through a cell from its birth to division into new daughter cells are collectively known as cell cycle. Newly formed daughter cells again enter into new cycle. Thus, the cycle can be thought of as the life cycle of a cell.

Before division of a cell, it must grow; copy its nucleic acid material (genetic material). The cell cycle is not a linear pathway; rather it’s a cyclical pathway.

When a cell does not enter into divisional phase for longer period or for throughout its life, but performs its normal metabolism, we can say that the cell is in G0 phase. Where G means gap or time lapse between two successive stages of the cycle.

Duration:
Formation of one cell to beginning of next cell cycle is called generation time. It varies from 30 minutes in bacteria to 90 minuets’ in yeast, 8-10 hours in mucous cells of intestine.

Stages of Cell Cycle

The cell cycle has two main phases

  1. Inter Phase
  2. Divisional Phase or M Phase
Inter Phase: The longest period of Cell Cycle

The period between two successive mitotic divisions is known as inter phase. This one is the longest period of cell cycle. In a human cell, one cycle may take 24 hrs. to complete.

The mitosis usually takes about 5 percent time among the entire cycle while inter phase takes the major duration of the cell cycle.

A lot of events occur during the inter phase. The inter phase is subdivided into 3 phases. These are as follows:

  • G1 Phase
  • S Phase
  • G2 Phase

G1 Phase (first growth phase/ post mitotic – pre synthetic phase):

At this stage, the cell is highly metabolically active and it is a continuous growing phase.

Characteristics
1. Synthesis of cellular components and necessary nutrients except DNA.
2. Growth of cell by cytoplasmic increase.
3. The stage produces cell organelles such as mitochondria, chloroplasts, ER, Golgi bodies, lysosomes.
4. Synthesis of new t RNA, r RNA, m RNA, and ribosome occurs.
5. The metabolic rate of the cell becomes very high.
6. This stage also produces structural and functional proteins which are required to get into the next phase, called S phase.

S phase : The invisible M Phase

This phase indicates the synthesis of DNA.

Characteristics:
1. Proteins associated with the chromosomes, namely histones are synthesized.
2. DNA replication takes place.
3. No increase in chromosome number but due to increase in Chromatid number per chromosome, net quantity of nucleic acid in nucleus doubles.

Hence this stage is also known as invisible M phase.

G2 Phase

The G2 or Gap 2 is another intense cellular synthetic phase.

Characteristics:
1. This stage starts synthesizing proteins of spindle fibres namely the tubulin.
2. Polymerization of spindle fibre forming proteins takes place to form spindle fibre.
3. G2 phase also stores proteins and energy by active metabolism to conduct the next divisional phase.

M phase:

This is divisional phase. This phase passes through karyokinesis (nuclear division ) and cytokinesis ( cytoplasmic division).

Characteristics:
1. M phase activates Specific CDKC (Cyclin dependent kinase) proteins.
2. Chromosomes start to condensate.
3. Spindle fibres start to develop.
4. Chromosomes are aligned at equatorial plane of cell.
5. Spindle fibres are attached to the kinetochore of chromosomes. 

Cell Cycle Check Points

How a cell scan the events that they are going on proper track?
Obviously, something is there within the cell which examines whether or not everything is occurring in order. If it goes wrong, then the cycle check points arrest the cell at that point and does not give the permit to enter into the next phase of the cycle.

At this arrested stage, the cell has only two things to do:

1. Either the cell rectifies the wrong, damaged, lost, deficient or excessive things of the cell OR,
2. It goes for Apoptosis. ( if the cell fails to rectify)

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Published on September 3rd, 2019 | by Prakash Nandi


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