Cell cycle class 11th


You must know that cell is the unit of life, but when you think about it, you start to wonder that how exactly is this a unit of life and why is it so important ? Well, you don't have to think about it anymore as in this article I'm going to be telling you the process of cell division and what cell cycle is. 

What is cell cycle ?

Before we get into the cell cycle, let us first understand in short about what a cell is. A cell is a basic membrane bound unit which keeps building higher and higher by cooperating with other specialised cells. Hence, they are also know as building blocks of large multicellular organisms. 

These cells carry a lot of information which is required for fertilization or for the development of the baby. All the components which are needed for an individual to survive is composed in this one cell and there are many such cells in your body which have a particular function. 

Related articles: Fertilization in humans  

Now that we know a little bit about cell, let us now take this further to cell cycle. Cell are divided into daughter cells by the process of division of cells which already exist and this process is called the cell cycle. By this process the cell duplicates its genome and gives rise to other components as well. 

A human cell requires about 24 hours to complete just one cell cycle. The continuity of life depends on the process of cell division which is basically the last phase of cell cycle. This division of cells starts when the cell has grown to a certain size. 

The division process of cell can be compared to the reproduction process in humans. Like to reproduce you need to have parents the same way to divide a cell you will need a mother cell which grows to a certain size and forms a daughter cell. The mother cell passes all the materials to the daughter cell or it can also be said that its duplicated and the cell cycle is said to be complete. 

Now that this one cell cycle has been completed, there is a pause or a break before another one starts. This break or time in between is called generation time

There are phases of cell cycle which are - Interphase and Mitotic phase about which we will be reading further. 



Phases of cell cycle:

The cell cycle has to go through three phases. If you go through your textbook it will be a little confusing so make sure you read this article from here on wards carefully. Cell cycle has three phases - Interphase, Mitotic phase and Meiotic phase.  

Interphase:

This phase is called the resting phase but actually this phase does most of the work. Total time consumed by this phase is 95%. Basically, the cell is preparing itself to undergo cell division. Interphase involves three stages - G1 phase, S phase and G2 phase. These stages carry a lot of importance in cell cycle.  

G1 phase:

This phase usually comes after the mitotic phase and is a resting phase. It is called the first gap phase or pre-synthetic or post-mitotic phase. There is no DNA synthesis taking place here.

It involves synthesis of RNA, proteins and membranes which helps in the growth of the daughter cells by growth of their nucleus and cytoplasm to reach a mature size. Chromatin (a substance which consist of DNA and proteins situated in chromosome) is fully extended in this phase.

Since it involves synthesis of RNA, there is transcription of three types of RNA - rRNA, tRNA and mRNA. This phase takes 30-50% of the time of cell cycle, but also this time which is consumed depends on the type of cell it is. 

The proteins which are synthesized during this phase are as follows:
  • Proteins needed to control the events in mitosis. 
  • Enzymes like DNA polymerase which are necessary for DNA synthesis but in the next stage. 
  • Tubulin and other apparatus proteins. 

S phase:

This phase is also known as synthetic phase. In this replication of DNA and synthesis of histones takes place which are required immediately when this S period starts so that it can provide the new DNA, that is the DNA which has been replicated with nucleosomes. 

The end product at this phase will be two DNA molecules and a duplicate set of genes in the chromosomes. Histone protein is reason for the formation of chromosomes in this phase. The total time consumed is 35-45%. 

G2 phase:

It is the pre-mitotic phase and is known as the second gap phase or growth phase or resting phase. In this phase DNA synthesis stops but synthesis of RNA and proteins (which are used for spindle formation) continues which is required for the growth of cell. 

ATP molecules are generated and stored so that it can be used as a form of energy in the process ahead. There is multiplication of mitochondria and plastids. Also, if there was any damage in RNA it is repaired in this stage. 

The total time consumed by this stage is 10-20%. After the complete of this phase the cell enters the Mitotic phase. 

After these stages, interphase is basically now come to an end. Now, to put it in short lets look at the events that have been achieved in this phase.
  • The nuclear envelope is intact. Chromosome are in the form of chromatin fibres which are diffused, long and coiled. 
  • The amount of DNA is not double and replication of DNA and synthesis of proteins like histones takes place. 
  • Due to increase in production in proteins the size of the nucleus is now increased.
  • Duplication of centrioles takes place as the daughter centrioles grow, which be at right angle. Hence, at the end of interphase we have two pairs of centrioles. 
  • The energy which is required for mitosis is synthesized along of protein synthesis. 

Mitotic phase:

In mitotic phase the cell divides in order to form two daughter cells which have the same chromosomes. It makes sure that there is equal distribution of genetic materials in two daughter cells. This cell division provides with healing and growth of the cells in multicellular organisms and multiplication in unicellular organisms. 

The division of nucleus and cytoplasm only takes place once and produces somatic cells. Hence, this phase is also called the somatic cell division. Once the cells are divided it maintains the same number of chromosomes in daughter cells as well as mother cells and it is also called equational division

Mitosis plays an important role in asexual reproduction in unicellular organsisms. The term mitosis was coined by a German Biologist Walther Flemming and he divided mitosis in two stages - Karyokinesis and Cytokinesis

Karyokinesis:

It is also called nuclear division where the nucleus has to undergo a series of changes so that it can form two daughter cells. Karyokinesis is further divided into four stages - Prophase, Metaphase, Anaphase and Telophase

Prophase:

Chromatin fibre networks become denser and settle into definite number of chromosomes. Now, the chromosomes are thick, long and single stranded. It keeps on revealing identical chromatids (which the centromere is holding on) as it keeps getting denser.

Once this phase starts the chromosomes undergo dehydration to become thick and short and when this process is coming to an end the nuclear membrane and nucleolus begins to disappear. Daughter centriols move away from each other and take positions at the polar ends. This is the longest phase

Metaphase:

Chromosomes are now positioned in equitorial plane and are now denser and thick. This is metaphase. It is characterised by the formation of kinetic spindle. This spindle is made up of contractile proteinaceous chromosomal fibres and inter-polar fibres. This is called mitotic apparatus

Anaphase:

It is the shortest phase in mitosis. The centromeres now divide into two which also separates the chromatids and this separated chromatids are called daughter chromosomes. Kinetic spindle are made up of microtubules which are made up of fibres which are as follows:
  • Astral rays - Pushing the centriolar pairs away from each other towards the end of the cell. 
  • Inter-polar fibres - The fibres which extend between two centrioles which are at opposite poles. 
  • Chromosomal fibres - This fibre connects centromeres to their respective poles. 
  • Inter chromosomal fibres - These are also called the inter-zonal fibres which are present between the centromeres of the daughter chromatids. 

Telophase:

The daughter chromosomes form chromatin network by uncoiling the undergoing hydration at poles. The nuclear membrane which had started to disappear in prophase now starts to reappear and spindle fibres disappear around each group of daughter chromosomes forming daughter nuclei. 

Cytokinesis:

Until now we only spoke about the division of daughter cells but the process in which there is a division of mother cell is called cytokinesis. In animal cells, daughter cells are formed by making a way into the plasma membrane of the mother cells. This way or gap is called the cleavage.




Significance of mitosis:

You can spot the significance or importance of mitosis in the explanation but to make it simple here is significance of mitos is at your glance:
  • Mitosis takes place in somatic cells. 
  • There is equal distribution of materials between the two daughter cells. 
  • The surface or volume ratio which was lost starts to restore. 
  • Helps in growth of multicellular organisms.
  • If there are any cells which must have been damaged in the previous phase is repaired during mitosis. 
  • Amount of content in DNA and RNA is maintained. 
  • Helps in development of organs and body of an organism. 
  • Helps unicellular organisms in asexual reproduction. 
  • Sex cells and gonads depend on mitosis for the increase in their number. 
  • Regeneration capacity in organisms is due to mitosis. 

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