•MEIOSIS : Meiosis consist of two successive division Meiosis – I and Meiosis –II . Thus a parent cell produces 4 daughter cell with half the number of chromosomes hence meiosis is also known as reduction division
MEIOSIS –I : In this the two homologous chromosome of each pair separate from each other and go to separate daughter cell hence diploid cell become haploid.This is called heterotypic division.
Phases of meiosis-I are: PROPHASE-I , METAPHASE-I , ANAPHASE-I , TELOPHASE-I
•PROPHASE-I : the longest one ,it take 90% of the time required for meiosis-I .It has 5 stages: Leptotene ,zygotene ,pachytene , diplotene ,diakinesis:
•Leptotene: Centriole move apart astral rays appear,Spindle begins to appear, chromatids are indistict at this stage.Each chromosome attached to the matrix by a structure called attachment plate.
•Zygotene:Pairing of homologous chromosomes takes place known as synapsis, pair of homologous chromosomes is called bivalent. Synaptonemal complex a fibrillar ladder like organelle develop between paired chromosome that stablizes pairing till crossing over takes place. The chromatids are not still visible. Pachytene: Chromatid of each synapse become visible and slightly separate, two chromatid of a chromosome are called dyad & group of 4 homologous chromosome as tetrad. Chromatids of same chromosome are called sister chromatid & chromatids of homologous chromosomes are called non sister chromatids. The exchange of segments of non sisterchromatids takes place in this stage called crossing over that produces . Breaking of segments done by enzyme endonuclease and annealing by ligase.
Diplotene: The synaptic forces holding the pair of chromosomes together comes to an end hence the homologous chromosomes separate from each other this is called dysjunction except at chiasmata where crossing over takes place at this point they do not separate from each other. Now the chromatids are more distict & tetrads very clear.
Diakinesis: Chromosome condensation takes place, chiasmata disappear by sliding towards the tip of chromosomes due to tight condensation called terminalization. Nucleoli , nuclear envelop disappear. Setting the tetrad free in the cytoplasm.
•METAPHASE-I: The tetrad move to the equator of the spindle & comes to lie in two parallel metaphasic plates. Each homologous chromosome has two kinetochore connect to the same spindle pole .The two kinetochore of its homologue join the opposite spindle pole.
•ANAPHASE-I: From each tetrad two chromatids of a chromosome move as a unit to one pole of spindle and other two to the other pole hence homologous chromosome of a pair separate from each other hence in this phase independent assortment takes place.
TELOPHASE-I :Chromosome at each pole elongate and uncoil. Nuclear membrane & nucleolus appear the spindle and astral rays gradually disappear.
•CYTOKINESIS—-
•Animal cell— Animal cell divide by furrowing also called cleavage . Constriction is brought about by a peripheral band of actin– myosin microfilaments just within the plasma membrane & involve enlargement of cell membrane.Constriction deepens and completely divide into two daughter cells.
•Plant cell—Plant cell form cell plate , the formation begins during late anaphase or early telophase . Small vesicle from golgi apparatus or ER line up right across the middle of the spindle here they fuse to form phragmoplast which form plasma membrane of two daughter cells.The content of phragmoplast form cell plate which form middle lamella then primary and secondary wall by the deposition of celluluse hemicellulose and pectin.
•NOW THE DAUGHTER CELLS WILL UNDERGO MEIOSIS-II
