CLASSIFICATION,NOMENCULTUREAND GENERAL SURVEY OF ANIMALS

cell division.

One of the most important charaters of the living cells is their power to grog and divide. Thus new cells always arise by the division of the pre existing cells.this fact was not fully understood until a long period after the discovery of the cellular nature of the organism. in the year 1885,this idea was expressed by virchow in his saying;'when a cell reaches a limit of its size peculiar to it , it divides two parts.'these parts grow to their  full size and then divide again and again , increasing the number of cells.with the increase in the number of cells,the torn cells and tissues are repaired and animals grow to their size.The cells of the body of the vertibrates like frogs are of two kinds;the vegitative and the reproductive cells.The vegitative cells form the somatoplasm or soma the reproductive cells collectively form the germplasm.The somatoplasm is meant to carry on all activities  of the body other than reproduction,while the germplasm,as it s name suggests,is exclusively devoted to the formation of the reproductive cells or gametes.The rough idea of cell division  was first given by hofmeister in 1840.Thereafter,vonmohl[1844]described the cell division in animalscells and schleicher[1878]described the division of nucleous for which he coined the term karyokinesis.quite soon,strasburger[1880]described cell division in plant cell and Flemming[1882] coined the term mitosis. in 1885 weismann and bended emphasized that the reproductive cells and somatic cell divide in different ways.Sutton[1902] highlighted importance of the cell division in heredity and farmer andMoore[1905]coined the term meisis.                                                                                                                                                                   BASIS OF CELL DIVISION                                                                                                       In 1908 hertwig proposed his kernplasm theory according to which,during the life period of a cell a constant ratio always exists between cytoplasmic and nuclear masses and this maintain eqilibrium between the cytoplasm and the nucleos. when a growing cell attains its maximum size the eqilibrium is disiurbed. therefore, in order to regain a balanced proportionality between cytoplasmic andnucleoplasmic masses, the cell divides. A cell, during its lifetime, under goes asequence of events which constitutes a cell cycle.                                                                                                                                                                                                                                                                                                                                                                                                                           CELLS CYCLE                                                                                                         Cells undergo asequence of the cyclic events during their life period and this sequence itself is called the cell cycle.This cycle begins with the interphase and ends with the cell division. it consists of 4 sequential phases which include G1phase,S phase,G2 phase and M phase.The G1,S,G2 phases are parts of the interphase. TheG1 phase is the initial growth phase during which proteins and rna are syntesized. this phase lasts for about 8-10 hours and is followed by the Sphase which takes 6-8hours to be completed. during Sphase,the dna synthesis takes place; its amount becomes double and its replication is completed along with  doubling of the histone proteins. then occurs the G2phase during which proteins are synthesized again and replication of organelles such as mitochondria occurs. about 4-5hours are normally required for this phase,passing through prophase,metaphse,anaphase and telophase, the cell divides into two daughter cells by cytokinesis. the Mphase needs normally4-5 hours to be completed.                                                                                                                                                                                                                                                                                         TYPES OF DIVISION                                                                                    The organisms cells divide consequently, they increase in size, affecting the growth and eventually causing reproduction. they exhibit different patterns of division of growth and reproduction. however,in animals, the following three types of the division have been recognised;                                                                                               1.  Amitosis or direct division                                                                                                                         2.  Mitosis or equational division                                                                                                                     3.  Meiosis or meiotic division or reductional division.                                                                                                                                                                                                                                                                       1. AMITOSIS                                                                                                               This process of the cell division is brought  about  by the lengthening of the  nucleous, its constriction and final division into two, followed by the cell division. this process is very simple and takes place either in very much specialized cells as in mammalian cartilage or in those undergoing degenerative changes .                                                                                                                                                                                                        2.MITOSIS OR KAKYOKINESIS                                                                            This process of nuclear division is very common and characterized by a series of complicated changes which leads to the nuclear division. It is a remarkably precise,orderly and continuos process but for the sake of convenience it can be divided into the following clerly defined phases or stages;   1.prophase 2. metaphase, 3. anaphase ,4. telophase.  before mitosis begins the non dividing or resulting cells are said to be in inter phase. it  is characterized by the  presence of an  in active  centrosome aad indistinct chromosomes in the nucleous.                                                                                                                                                                                                         3.MEIOSIS                                                                                                                 The meiosis occurs in the formation of gametes in organisms which reproduce sexually. like mitosis, in meiosis also, nuclear  division is followed by cytopasmic cleavage, the cytokinesis. but  in contrast with the  mitosis, the meiosis has two  nuclear divisions,not one. thus four nuclei are formed from a cell which under goes meiosis.                                                                                                                                                                     The first nuclear division of meiosis is called the first meiotic division and the second one the second meiotic division. For the sake of convenience the roman numeral1  is used as suffix for the phases or stages of the first meiotic division and 2 for the phases of the second meiotic division. like the meiotic division is also divided into prophase,metaphase,anaphaseand telophase.                                                                                                                 FIRST MEIOTIC DIVISION                                                                                  This division is the reduction division because the number of the chromosomes is halved; each daughter nucleus recieves half the number of chromosomes of the parent nucleus. it is longer than the second meiotic division because its prophase is much prolonged.                                                                                                  Prophase1. it is divided into the following 5 sequential substages;                                                                  1. leptotene                                                                                                                                                    2.zygotene                                                                                                                                                    3.pachytene                                                                                                                                                  4diplotene                                                                                                                                                      5.diakenesis                                                                                                                                                                                 SECOND MEIOTIC DIVISION                                                                                  1. Prophase2                                                                                                                                                2.metaphase2                                                                                                                                                3.Anaphase2                                                                                                                                                4.Telophase2                                                                                                                                                  Significance of meiosis;one  obvious effect of the reductional or heterotypic division is to reduce the number of the chromosomes from the somatic or diploid number, that is Sto half of the normal number, the haploid or S/2.by the union of the male and female gametes at fertilization the diploid number of the chromosomes of each species is kept constant . ther is a regrouping of the chromosomes in the daughter cells, so that all possible combination occurs in the germ cells. this is known as the segregation of the chromosomes.Moreover ,during the pairing of synapsis of the chromosomes ther is frequently an inter change of parts ,leading to the further mixing of herediatary characters .                                                                                                                                                                                                                                                                         

biology

Cells-Biological Units  

The living organisms,all bacteria,plants and animals,have cellular organization; their body is composed of  tiny microscopic units of the protoplasm,called cells. Their number varies one to many in the body of the protoplasm, called cells . Their number varies one to many in the body of the organisms ; the body composed of just one cell is called the unicellular body as it happens in bacteria,cyanobacteria and protozoans. The body composed of many cells is designated as the multicellular body which is characteristic of the metazoan animals and all eukaryotic plants which have a thlloid body or a body differentiated to shoot and root systems. The cells are as important as the body because these maintains the body and enable it to perform the biological activities. Loewy and siekevitz[1963] have defined cell as an unit of the biological activities , democrated by a semipermeable membrane and capable of self multiplication in an organism-free medium.                                                                                                                                                                                                                                                                                        

Discovery of Cells

    The discovery of the cells dates back to the year 1665 when Robert hooke ,an English scientist ,published a report titled ‘Micrographia’. In this, he described  his microscopic ex- animation of a thin piece of cork sheet . his observation showed that the cor piece was made of a mass of tiny cavities . Each cavities was enclosed  by walls , reminding  him of honey comb cells . these  tiny cavities were designated by him as cells meaning hollow spaces [l.cella =hollow space ] quite soon N.Grew and Malpighi made repeated observation on thin slice of the cork and reported the cellulose cell walls which separated one cell from another . in 1672,  grew presented  illustrations of a cellular structures of many plant matertials . Leeuwenhoek[1723] studied and described cells of microscopic organisms such as protozoans and bacteria and rbc. After a gap of about  100years, dutrochet[1824] deascribed that mimosa leaves are made of globular cells . in 1831, Robert brown noticed a spherical body in the cell  for which he coined the name nucleus[=kkaryon].quite soon dujardin[1835],a german botanist  noticed and described  the nucleolus in plant cells Three years later, schleiden[1838] again that all plants  are made of cells, ayears later, theoder scwann[1839], a german zoologist, declared that all plants are composed of cell.Considering their observations,Mathias J.Shleiden[1838] and theodor Schwann[1839] separately formulated the cell theory according to which,the cells are structural and functional units of the body of all living objects or organisms. This theory is a generalization about the architectural organization of the body of the living organisms membrane on the surface, usually one nucleos in the centre and cytoplasm in between them. The nucleus is,usually spherical and bounded by a nuclear membrane. It is filled with the nucleoplasm which has chromatin and nucleolus. The cyto plasm contains specialized living parts ,called cell organellesor cell organoids,and non living substances as particles called cell illusions . the organelles include mitochondria,golgi body and centrosome and some vacuoles as hollow spaces. The inclusions mostly occur as glycogen granules.