Meiosis Cell Division
Meiosis is the type of indirect cell division in which one cell divide into four cells in two sucessive division and each daughter cell have number of chromosome then parent cell. It was first discovered by Van Bendan in1883 but the term meiosis was given by Farmer and Moore in 1905.Meosis Occurs is special cell and at the special period. It is found in deploid old mother cell of another and megaspore mother cell in ovule in flower in plant haploid gamet.In meiosis two sucessive cell division are formed i.e: Meosis I and Meosis II.
The meosis I division is also known ad reductional division or heterotypic division because it involves the formation of two daughter cell with just half number of chromosome as compared to parent cell. It is completed in following phase:Prophase(I), Metaphase(I), Anaphase(I), Telophase(I). Prophase I: It is more complicated and longer as compared to mitosis. It can be divided into five sub-phase. Leptotene: The size and the volume of the nucleus is increased. The chromosome appear to be long, visible due to the condensation, shortening and thickning.Chromosomes are single stranded and thread like structure. Chromosome contains many dark areas called centromers. Each chromosome has its own pair of homologus chromosome and at the end of leptotene all chromosome are directed toward a small area on one side of nucleus. Nuclear membrane and nucleolus are present. Zygotene: The chromosome are more visible by further shortening and thickening. The homologus chromosome start to pair to form bivalents. The process of paring is called synapsis.A nucleo protin complex called synaptonemal complex is formed between the homologus chromosomes. Nuclear membrane and nucleolus are present. Pachytene: The bivalents become shorter and thicker. Each homologus chromosome divides longitudinally except in the centromere region into two sister chromatids and bivalent appears as tetrads. exchaange of chromatids segment between the homologus chromosome take place.This process is called crossing over and the point of crossing over is called chaismata. Due to the crossing over in the organism variations and recombination takes place. Nuclear membrane and nucleolus are present. Diplotene: Uncoling of the homologus chromosome take place and the repulsion force develops which saperates the two two chromosome of the bivalent apart. The homologus chromosome start to separate each other but joined at certain points called chaismata. chromosome more shorter and thicker. Nuclear membrane and Nucleolus are present. DIKINESIS: The chaismata move towards the end of the chromosome. This type of movement is called termenalization of chaismata. Nuclear membrane and nucleolus starts to disappear , appearence of the spindle fibre takes place. Metaphase I: The nuclear membrane and nucleolus disappear and formation of the spindle fibre completed. The homologus pair arrange themselves in two parallel metaphase plate. The chromosome or bivalents are attached to the spindle fibres by their centromere. Anaphase I: Each bivalent of two homologus centromeare seperate. The homologus chromosome move towards the opposite poles to the cell by the contraction of the spindle fibres. Centromere is not divided. In meosis the actual reduction division of chromosomes takes place in anaphase I. Telophase I: Each pole receive the one haploid set of chromosome having two chromatids. The chromosome become uncoiled and form chromatin fibre. Spindle fibre disappears and nuclear membrane and nucleolus appears around daughter chromosome at each pole. Due to the crossing over two chromatids of chromosome not similar.
In this division the chromosome numbers remains same. Therefore meosis II is also called equational division or homotipic division. It is also divided into four Stages.They are: Prophase II: Nuclear membrane and nucleolus start to degenerate. Chromosome become shorter and thicker. Centriols starts moving towards opposite poles. Appearence of spindle fiber start. Metaphase II: Nuclear membrane and Nucleolus are absent. Centroils move to the opposite pole. Spindle fibre attached to centromere of chromosome. Chromosome arranged at equator and form a metaphaic plate. Anaphase II: Spindle fibre starts stretching and pulling the centromeres. Centromere divide longitudinally and each chromatid start to move opposite p poles. Separated chromatid are now called chromosome. At the end of anaphase II four haploid chromosome are formed. Telophase II: At each pole chromosome uncoil get elongated and form a network chromatin fibre. Spindle fibre are disappeared. Centriole replicates. The nuclear membrane appears around the chromosome and the nucleolus reappears.
Significance of Meiosis
Meiosis is essintial for sexually reproducing organism at the time of gamet formation. It maintains the fix number of chromosome of species from generation to generation. Crossing over takes place in pachytene substage of prophase I which cause variation in genetic character. Meosis is helpful in evolution of organism. It helps in the alternation of generation of haploid and diploid generation of plants and animals.