Cell Cycle - Phases , Diagram , Types and Comparison

                                          

                                              Cell Cycle

In every organisms Cell continuously undergoes a series of events like growth , development & reproduction and completes its life cycle called the Cell Cycle . After the formation  of the Cell , it  first goes  under   resting period where they grows in size, its nuclear material increases, and prepare itself  for the next division.

• Duration of cell cycle - The duration of the  every cell cycle varies to  25 – 30 hr in different cells except  in case of  organisms like in Ecoli, the cell cycle is completed in 20 minutes,

• Stages of the cell cycle –

1. Interphase - Interphase is the interval period between two successive divisions when the cell does not show any division. But it prepares itself for it by synthesizing new proteins and nucleic acid. This phase is also known as the preparatory phase.
   
   
2. G1 Phase - It is the first gap phase where the cell grows physically larger, make copies cellular organelles, and makes the molecular building blocks that will need in later steps.
   
   
3. S phase -The S phase, or synthesizing phase is the phase where , the cell synthesis a complete copy of the DNA in its nucleus along with it also duplicates a microtubule-organizing structure called the centrosome. The centrosomes helps to  separate DNA during the M phase.
   
   
4. G2 Phase - In this phase the cell grows more, makes proteins and organelles, and begins to reorganise  its contents in preparation for mitosis. 
   
   
5. M Phase - M phase is the phase where a cell tends to divide and generating daughter cells. The stage is divided into two  stages -

Karyokinesis - The division of the nucleus is called karyokinesis. The term karyokinesis was proposed by Schneider in 1887. Karyokinesis has  following phase - prophase, metaphase, anaphase, and telophase.

Cytokinesis - Division of Cytoplasm in the cell is known as Cytokinesis proposed by Whiteman in 1887. 

• Types of Cell division -  Basically cell division is responsible for reproduction and growth in organisms . In both the processes chromosomes are properly distributed into the daughter cells. The process of cell division was firstly observed in 1882 by Fleming in a reptile Triturus masculosa but Beler studied it is in detail in 1920. 

There are three types of division - mitosis, meiosis, and amitosis.

                                                    Mitosis

The exact replication of the parent cell followed by its division into two daughter cells which are identical and contain the same number of chromosomes as found in the parent cell. This nuclear division was first observed by Strasburger in plant cells and by Fleming in animal cells. Fleming used the term mitosis fir this division. Mitosis takes place in somatic cells.

Process -   It is completed in four steps.

• Prophase
• Metaphase
• Anaphase
• Telophase. 

Prophase –

• · In the nucleus, the genetic material is loosely bundled in a coil called chromatin.
• · At the onset of prophase, the chromatin fibers become tightly coiled and condense into discrete chromosomes.
• · Inside the nucleus, the nucleolus also disappears from view.
• · The centrioles begin to move to opposite ends of the cell and the spindle fibers extend from the centromere.
• · Some fibers cross the cell to form the mitotic spindle fibers.

Metaphase

• · The term metaphase is derived from the Greek word 'meta' which means 'after'.
• · In the prometaphase after the microtubules are attached to the prometaphase, the chromosomes start pulling the chromosomes towards the ends of the cell.
• · The centromeres of the chromosomes assemble along with the metaphase plate also known as the equatorial plane.
• · It is an imaginary line that is in between the centrosome poles and is called the spindle equator.
• · This helps to ensure that when the chromosomes are separated the new nucleus will receive one copy of each chromosome. 

Anaphase

• · After the metaphase stage the chromosomes proceed to the anaphase stage.
• · The term anaphase is derived from the Greek word "ava" which means "up", or "against", or "back", or "re".
• · First the proteins that bind the sister chromatids are cleaved making the sister chromatids as separate daughter chromosomes and are pulled apart towards the respective centrosomes to which they are attached.
• · The microtubules at the poles pull the set of a chromosome that is attached to it the opposite ends of the cell. At the end of anaphase, the microtubules all degrade. 

Telophase

• · Telophase is derived from the Greek word "telos" meaning "end".
• · It is a reversal of prophase and prometaphase events. In the telophase stage, the polar microtubules continue to lengthen elongating the cell.
• · The daughter chromosomes attach at opposite site ends of the cell.
• · New membranes are formed around the daughter nuclei.
• · The chromosomes spread and are no longer visible under the light microscope.
• · The spindle fibers also disperse, cytokinesis may also begin during this stage. 

Cytokinesis

• · Cytokinesis is a separate process that begins at the same time as the telophase.
• · Cytokinesis is not a phase of mitosis, it is a separate process necessary for completing cell division.
• · In animal cells, a pinch like a cleavage furrow containing a contractile ring develops at the position of the metaphase plate separating the nuclei.
• · In the animal and plant cells the division of cells is driven by vesicles derived from the Golgi apparatus.
• · In plant cells, the rigid wall requires a cell plate to be synthesized between the two daughter cells.

Significance of Mitosis

• · Mitosis division is responsible for the growth and development of a single-celled zygote into a multicellular organism.
• · The chromosome number remains the same in the cells produced by this division.
• · The daughter cells have the same characters as those of the parent cell.
• · Mitosis division helps in maintaining the proper size.
• · Mitosis also helps in restoring wear and tear in body tissues, replacing damaged or lost parts, healing wounds, and regeneration of detached parts.
• · This method of multiplication is seen in unicellular organisms.
• · Mitotic division of cells is unchecked and it may result in uncontrolled growth of cells leading to cancer or tumor. 

                                                            Meiosis 

Meiosis is a special complicated type of cell division. Meiosis is also known as a reduction division. In this type of division four daughter cells are formed from a parent cell. J.B. Farmer in 1905 coined the term meiosis.

Meiosis is divided into meiosis I and meiosis II stages. It is further divided into Karyokinesis I and Cytokinesis I and Karyokinesis II and Cytokinesis II respectively.

Meiosis I

• · The pairs of homologous chromosomes, made up of two sister chromatids are split into two cells.
• · The resulting daughter cells contain one entire haploid set of chromosomes.
• · The first meiotic division reduces the ploidy of the original cell by a factor of two.
• · It produces two haploid cells (N chromosomes, 23 in humans).
• · Hence meiosis first is referred to as a reductional division.
• · A diploid human cell contains 46 chromosomes and is said to be 2N because it contains 23 pairs of homologous chromosomes. 

Prophase 1 – It is the longest phase of meiosis 1. It consists of the following steps –

Leptotene - At this stage condensation of chromatin starts and the fine chromatin fibers appear with granule like chromomere on them. The chromomere is the region where chromatin fibers are highly coiled.

Zygotene

• · Zygotene is also known as zygonema, it is derived from the Greek word that means 'paired threads.
• · The chromosomes in this line up with each other into homologous chromosome pairs.
• · This stage is known as the bouquet stage, due to the way the telomeres cluster at on end of the nucleus.
• · Synapsis of homologous chromosomes takes place in this stage.

Pachytene

• · The pachytene stage is also known as pachynema and is derived from Greek which means "thick threads".This is the stage where chromosomal crossing over occurs.
• · Nonsister chromatids of homologous chromosomes exchange segments over homologous regions.
• · Sex chromosomes are not identical and they exchange information over a small region of homology.
• · Chiasmata is formed where the exchange happens.

Diplotene

• · The diplotene stage is also known as diplonema, which is derived from the Greek word meaning "two threads".
• · During this stage there is the degradation of the synaptonemal complex and the homologous chromosomes separate a little from one another.
• · The chromosomes in this stage uncoil a little, this allows transcription of DNA.

Diakinesis

• · During the stage of diakinesis the chromosomes condense further.
• · The word diakinesis is derived from the Greek word which means "moving through".
• · This stage is the first part of meiosis where the four arms of the tetrads are visible.

Metaphase I
The homologous chromosomes align along the equatorial plane, this alignment happens due to the continuous counterbalancing forces exerted on the bivalents by the microtubules emanating from the kinetochores of the homologous chromosomes.

Anaphase I

As each chromosome has only one functional unit of a pair of kinetochores, the whole chromosomes are pulled towards the opposite poles which result in the formation of two haploid sets.

Telophase I

• The first phase of the meiotic division ends when the chromosomes arrive at the poles.
• The daughter cells now have half the number of chromosomes, the chromosomes consist of a pair of chromatids.
• The microtubules of the spindle network disappear and the nuclear membrane surrounds each haploid set.
  

Significance - 

• It plays an important role in sexual reproduction.
  
• The process of meiosis helps in the maintenance of chromosomes number in the species.
  
• It may lead to origin of new species.

                                                          Amitosis

In this type of cell division, the nucleus becomes elongated and constriction appears in the center or at the end due to which daughter nuclei produced are not in equal size. Nuclear division is not always followed by wall formation. This type of division takes place in some fungi and algae. 

Comparison between mitosis and meiosis -