Basic character of life 
 The most basic characteristic of life is reproduction i.e. the production of new structures and individuals similar to the existing structures and individuals. Reproduction occurs at different levels of organization. Parts of the ceil such as the chromosomes produce new chromosomes.
 Cells then produce new cells; and individuals produce offspring like themselves. Rudolf Virchow proposed an important biological principle i.e. all cells come from cells. This principle tells us that the continuation of life, including all aspects of reproduction, is based on the reproduction of cells. 
We commonly refer cellular reproduction as cell division and it is a part of the whole life of a cell i.e. cell cycle. Cell cycle The cell cycle is the series of events from the time a cell is produced until it completes mitosis and produces new cells. The cell cycle consists of two major phases i.e. interphase and mitotic phase (M phase). The mitotic phase is a relatively short period of the cell cycle.
 It alternates with the much longer interphase, where the cell prepares itself for division. Typically interphase lasts for at least 90% of the total time required for the cell cycle. Interphase Interphase is the time when a cell's metabolic activity is very high, as it performs its various functions.
 It is divided into three phases, 
G1 (first gap), 
S (synthesis),
 and G2 (second gap). 
G1 phase: 
After its production, a cell starts its cell cycle in G1 phase. During this phase, the cell increases its supply of proteins, increases the number of many of its organelles (such as mitochondria, ribosomes), and grows in size. This phase is also marked by the synthesis of various enzymes that are required in the next phase i.e. S phase for DNA replication. 
 A German biologist, Walther Flemming in the 1880's, gave the detailed account of the stages of cell division. He observed that in a dividing cell the nucleus passes through a series of changes which he called mitosis. Inhibition of protein synthesis during G2 phase prevents the cell from undergoing mitosis. 
The eukaryotic cell cycle Somatic cells are those which are forming the body of organisms while germ line cells are those which give rise to gametes. Somatic cells undergo mitosis while germ line cells undergo meiosis. In common mitosis, nuclear envelope disappears and spindles are formed inside the cytoplasm. It is called “open k, mitosis”. 
Fungi and some protists undergo a variation called A. Karyokinesis: The division of the nucleus is “closed mitosis” where the further divided into four phases i.e. prophase, spindle forms inside the nucleus. metaphase, anaphase and telophase. .i. Prophase: Normally, the genetic material in the nucleus is in a loose thread-like form called chromatin. 
At the onset of prophase, chromatin condenses into highly ordered structures called chromosomes. Since the genetic material has already been duplicated earlier in S phase, each chromosome is made of two sister chromatids, bound together at the centromere. Each chromosome has kinetochore at the centromere.
 A kinetochore ls a complex protein structure that is the point where spindle fibers attach. close to the nucleus are two centrioles collectively called a centrosome. Each centriole replicates and thus two daughter antrosomes are formed.
 Each daughter centrosome acts as a coordinating center for e cell's microtubules. The two centrosomes give rise to microtubules by polymerizing vdning monomers to form polymers) the tubulin proteins present in the cytoplasm. The Cells that have temporarily or permanently stopped dividing are said to have entered a state of quiescence called GO phase. 
 S phase: In this phase the cell duplicates its chromosomes. The DNA molecule of each chromosome is copied, and new protein molecules are attached. The result is that each chromosome consists of two sister chromatids, which contain identical genes. G2 phase: In the G2 phase the cell prepares proteins that are essential for mitosis, mainly forthe production of spindle fibres.
 After the G2 phase of interphase, the cell enters the division phase i.e. M phase. It is characterized by mitosis, in which the cell divides into the two daughter cells. GO phase: In multicellular eukaryotes, cells enter the GO state from G1 and may remain quiescent for long periods of time, possibly indefinitely (as is often the case for neurons). 
Some cell types in mature organisms, such as some cells of the liver and kidney, enter the GO phase semi-permanently and can only be induced to begin dividing again under very specific circumstances. Other cells, such as epithelial cells, do not enter GO and continue to divide throughout an organism's life. The events of cell cycle are ordered and directional i.e each event occurs in a sequential fashion and it is impossible to "reverse" the cycle.
 Our body consists of about 200 trillion cells, all of them derived from a single cell at the start of our life as a fertilized egg. Many millions of successive cell divisions occurred while our body was reaching its present form. In each of these divisions, the genetic material was equally partitioned between the daughter cells and it happened through mitosis. 
Mitosis is the type of cell division in which a cell divides into two daughter cells, each with the genetic equivalent of the parent cell i.e. same number of chromosomes as were present in the parent cell. Because each resultant daughter cell should be genetically identical to the parent cell, the parent cell must make a copy of each chromosome before mitosis. 
This occurs during S phase of interphase. Mitosis occurs exclusively in eukaryotic cells. In multicellular organisms, the somatic cells undergo mitosis. Prokaryotic cells undergo a process similar to mitosis called binary fission. However, prokaryotes cannot be properly said to undergo mitosis because they lack a nucleus and only have a single chromosome with no centromere. membrane-bounded disc called the cell plate or phragmoplast. 
The plate grows outward and more vesicle fuse with it. Finally, the membranes of the cell plate fuse with the plasma membrane and its contents join the parental cell wall.
 The result is two daughter cells, each bounded by its own plasma membrane and cell wall . Nuclear membrane breaks during cell division so there is no distinct nucleus.
 In interphase nuclear material is in the form of fine chromatin which condenses during prophase to get the shape of chromosome.There are several features unique to meiosis, most importantly the pairing and genetic recombination between homologous chromosomes.

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