CANCER

            Uncontrolled, rapid amitotic proliferation of cells of the body is termed as cancer. The cancer cells exhibit less adhesion with other cells and show great motily.Thiscould be the result of mutationinthe genes regulating growth and mitosis.The persons exposed to some specific chemicals such as alcohol, tobacco, etc, ultra violet radiation. ionizing radiation, etc have great risk of development of cancer. In animal some viruses or frequent injury to a certain point may develop cancer.
               The cancerous cells complete with the normal cells for nutrition. Since they divide rapidly and indefinitely so the normal cells are deprived of the food and as a result start dying.

MEIOSIS (REDUCTION DIVISION)

                 The term meiosis (Gr, meion - to reduce)   was coined by J.B Farmer (1906). It takes place only in GERM CELLS. By the process of this division the number of chromosomes of the germ cell reduce to half, so the resulting cell (gametes in animals whereas spore in plants) have HAPLOID number of chromosomes. The fusion of haploid male and female gametes result in a cell called as ZYGOTE. It has same number of chromosomes as the parent germ cell.
Process of meiosis
       
           The meiotic division includes two complete divisions.

FIRST MEIOTIC (HETEROTYPIC) DIVISION:

              This is the first meiotic division and starts after an interphase. It consists of the following four phases.
PROPHASE-I
            This first prophase is the longest stage of meiosis. It includes the following substages.
(i) Leptotene:
          In this sub stage, the chromosomes are extremely thin, long, uncoiled, slender, thread-like structure. This stage resembles with the early mitotic prophase except that the chromosomes are longitudinally single instead of double. During this stage the chromosomes become more uncoiled and take up a specific oriebtation (to give a new direction) inside the nucleus. The chromosomes show beaded structure. CHROMOMERES at irregular intervals along their entire lenght. The centriole duplicates and each daughter centriole migrate towards opposite poles.

(ii) Zygotene:
              During this stage, homologous chromosomes are attracted toward each other to form a BIVALENT. This process is called as SYNAPSIS. The homologous chromosomes touch at each other at one or more points along their entire lenght.


(iii) Packytene:
              During the pachytene sub stage, the pair of chromosomes become twisted spirally around each other and more readily distinguishable. Later homologous chromosomes start splitting lengthwise so that each pair consists of four chromatids known as TETRADS.


(iv) Diplotene:
               During this sub stage an important genetic phenomenon called as CROSSING OVER takes place. It in volves reshuffling, redistriution and mutual exchange of hereditary material of two homologous chromosomes. The point of contacts and interchange between two homologous chromosomes is called CHIASMA or CHIASMATA. The nucleolus remains prominent up to this stage.


(v) Diakinesis:
               During this stage, the bivalent chromosomes become more condense and evenly distributed in the nucleus. The nucleolus and nuclear membrane disappear. The chiasmata move from the centromere towards the end of the chromosome so the intermediate chiasmata diminish. The chromatids still remain connected by the terminal chiasma and these exist up to the metaphase. Finally the bivalents appear to be thicker so they are become more prominent.

METAPHASE-I
            The bivalents get arranged themselves at the EQUATORIAL PLANE in a way that their centromeres are attached to the spindle fibers. Now the bivalent chromosomes are ready to separate.

ANAPHASE-I
             This is the actual reduction stage. Due to the disassembly of the chromosomal fibers, each homologous chromosome with its two chromatids and undivided centromere move towards the opposite poles. This stage lasts till they are reached at their respective poles.

TELOPHASE-I
              As soon as the chromosomes reach their respective poles, a nuclear membrane is formed around them. The two daughter nuclei are formed. The chromosomes now become uncoil and the nucleolus reappears. After a brief interphase, the two daughter haploid cells proceed to the second meiotic division.

SEOND MEIOTIC (HOMOTYPIC) DIVISION:

               The second meiotic division is also known as the HOMOTYPIC DIVISION. This is actually mitotic division which result in four haploid daughter cells. It consists of following four phases.

PROPHASE-II
                The centrioles duplicate to form two pairs of centrioles so that each piar move towards the opposite pole consequently forming "spindle". The nuclear membrane and nucleolus disintegrate.

METAPHASE-II
                 The chromosomes arrange themselves at the equatoial plane. The centromere of each diad becomes attached with a chromosomal fiber of spindle. They are now ready to separate from each other.

ANAPHASE-II
                  The chromatids of each diad begin separating and keep moving towards the opposite poles eventually touching it. As soon as they are completely separated from each other, they are termed as MONADS.

TELOPHASE-II
                     In the telophase-II a nuclei membrane is formed around them. The chromosomes become uncoil and diffused. CYTOKINESIS follows producing four daughter haploid cells.


Significance of Meiosis

1. ) Meiosis maintains constant number of chromosomes in sexually reproducing organisms.
2. ) By crossing over, meiosis provides an opportunity for the exchange of genes and thus causes the genetical variations among the species. The variation are the materials of the evolutionary process.