CELL CYCLE

It is a series of processes by which cellular material is divided between daughter cells. It consists of a division and non-division phase of the life cycle of cell.

INTERPHASE

             The phase between the two division is called INTERPHASE. During this phase the nucleus and cytoplasm remain metabolically active and due to that an increase in the volume of the nuclear as well cytoplasmic substances takes place. It meludes the synthesis and organization of the substrate and enzyme necessary for DNA synthesis. It is the longest phase of cell cycle. Interphase is divided further into further three sub-stages. G1(gap), S(synthesis) and G2(gap two) phases.

G1 phase 
           During G1 phase, the synthesis and organiztion of  substrate (RNA) and the synthesis of enzyme for DNA  synthesis takes place. In the end of G1 phase the cell either may enter a resting phase or G0 (resting phase) or into S phase. The chromosomes occur in the form of long, diffused coiled and indistinctly visible chromati fibers. The size of the nucleus increased greatly due to accumulation of rRna and ribosomal proteins.


S pahse:
          During S phase of DNA occurs to replicate the genetic material A daughter pair of centrioles is originated during S period hear the already existing pair of centrioles.

G2 phase
           During the G2 sub stage, the ribosomes and spindle formation occurs

CELL DIVISIONS
      There are following three kinds of cells divisions takes place in animals and plants cells. Amitosis, Mitosis, and Meiosis.

AMITOSIS

        It is also called as DIRECT CELL DIVISION because  there are no spindle formation as well as other nuclear and cytoplasmic events. Simply, the nucleus elongates. a constriction appears which gradually deepens. Finally , the nucleus divides it into two halves, each of which contains a daughter nucleus. In this way two daughter cells are formed.
                Sometimes, the nucleus divides into two unequal parts. this is called as NUCLEAR BUDDING. But sometimes, nucleus divides into numerous equal parts this is termed as NUNLEAR FRAGMENTATION. in either case , each daughter nuclei gets equal amount of the parent cytoplasm and forms a daughter cell.
                 Amitosis takes place in unicellular organisms like protozoa,  bacteria. abnormal diseased tissues such as tumor, cancer and also tissues.


Cell Death (Necrosis and Apoptosis)
The death of the cells in higher organisms is due to either two processes, necrosis and apoptosis.
NECROSIS: Due to injuries, the death of the cells is called necrosis. As a result. the damaged cell ruptures and releases the toxins on side, which causes inflammation of the neighboring cells
APOPTOSIS: It is  also called "programmed cell death". It is proposed that the dying out cell gives some biochemical signal to phagocytes for autophagy. Thus the neighboring cells are least affected in this way. It is commonly observed in ebbryonic development of multicellular organisms where autophagy of many cells is required. It is also seen in the cells which have potential to become tumor.. 

MITOSIS

          It is also called as INDIRECT CELL DIVISION. It takes place in similar way both in animals and plants. As a result of mitosis. a parent cell gives rise two daughter cells having the same number of chromosomes  as the parent cell.
           It is a process but for the convenience it is usually divided into following four stages or phases; prophase, metaphase, anaphase and telophase.

PROPHASE (Gr. Pro - before; phasis - appearance):  It is the longest phase of mitosis. The beginning of the prophase is marked by a condensation of the very thin chromatin threads to form rod-like structure which are the duplicated chromosomes. Each prophase chromosomes is composed of two coiled filaments, the "SISTER CHROMATIDS". In each chromosomes there is a constriction called as CENTROMERE. The sister chromatids becomes thick and short. The nuclear membrane and nucleolus start disintegrating. The centrioles self-duplicate. separate and move to opposite sides of the cell. There also appear in the cytoplasm some translucent fibers that eventually stretch between and connect with the two sets of centrioles, forming a structure known as MITOTIC SPINDLE. The spindle consists of three kind of fibers, continuous fibers running from one pole to another, discontinuous or half fibers which from pole to the equator and astral fibers which arise from the centrioles. In higher plants, the spindle appears directly at the poles.

METAPHASE:

           It is the shortest phase. Formation of spindle gives the cell an orientation: the centrioles forms the poles and "a plane biseeting the spindle in the equator" which marks the plane of division or cleavage. The chromosomes become attached to discontinuous spindle fibers by their kinetochore of centromeres fron the EQUATORIAL PLATE or METAPHASE PLATE. 

ANAPHASE:

           It begins with the separation of two centromeres of each and every chromosome. The centromeres move a part and chromatids separate and begin their migration towards poles. Because they are how single threads with their centromeres. the chromatids are termed as CHROMOSOMES. Since each chromosome contains one chromatid, it is called MONAD. The end of the anaphase is marked by the reaching of the monads at the respective poles.

TELOPHASE:

          During  this phase, there are two clusters of chromosomes, each cluster containing exactly the same chromosomes as the other. The chromosomes starts to uncoil and becomes less and less condensed, gather into masses of chromatin which becomes surrounded by discontinuous segmentsof nuclear envelope. In this way two daughter nuclei are formed. At the finalstage nucleolus reappears. The high viscosity of the cytoplasm,characteristic of the metaphase and anaphase decreases.

CYTOKINESIS:

           By the process of CYTOKINSIS (division of cytoplasm) two daughter cells are formed. In case of animal cell, a constriction appears in the equatorial plane which gradually deepens to divide the cytoplasm into two equal halves. In case of plant cells, a cell plate is formed in the cytoplasm which divides the cytoplasm. 

Significance Of Mitosis: 

1. ) It provides the opportunity increasing the number of cells during development and growth of organs and body of an organism.
2. ) Mitosis help the cell in maintaining its proper size.
3. ) Due to mitosis old decaying and dead cells are replaced by new cells.
4. ) Through mitosis and equilibrium is maintained in the amount of DNA and RNA contents.