Скачать презентацию Meiosis is a special cell division which takes Скачать презентацию Meiosis is a special cell division which takes

09-Meiosis.ppt

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Meiosis is a special cell division which takes place in the reproductive organs of Meiosis is a special cell division which takes place in the reproductive organs of living things.

• Characters of living things are carried by means of a pair of • Characters of living things are carried by means of a pair of chromosomes. One of them comes from father and other comes from mother. • These pair of chromosomes is called homologous chromosomes. Homologous chromosomes carry similar characters in the same order.

• Cell that has full number of chromosomes which characterises the species is • Cell that has full number of chromosomes which characterises the species is called diploid (2 n) cell. • At the end of the meiosis 4 different haploid (n) cells are formed. • These cells have half chromosome number of their parent cells.

STAGES OF MEIOSIS • The Meiosis consists of two cell divisions: –MEIOSIS-II STAGES OF MEIOSIS • The Meiosis consists of two cell divisions: –MEIOSIS-II

MEIOSIS-I • At the beginning of meiosis, cells have the diploid number of chromosomes. MEIOSIS-I • At the beginning of meiosis, cells have the diploid number of chromosomes. • There is interphase before the beginning of the first meiotic division. • DNA is replicated in interphase.

PROPHASE-I • Spindle fibers are formed by centrioles. • Nuclear membrane and nucleolus disappear. PROPHASE-I • Spindle fibers are formed by centrioles. • Nuclear membrane and nucleolus disappear. • DNA are shortened and thickened to form chromosomes. • Each chromosome lines up exactly with its homologous chromosome. • Homologous chromosomes attach to their pairs and tetrads are formed.

CROSSING-OVER • Pairs of homologous chromosomes forms the TETRADS. • The gen exchange between CROSSING-OVER • Pairs of homologous chromosomes forms the TETRADS. • The gen exchange between chromatids of homologous chromosomes pairs is called crossingover. • Crossing-over provide the variaty in species.

METAPHASE-I • Homologous chromosome pairs line up on the equator. • The chromosomes attach METAPHASE-I • Homologous chromosome pairs line up on the equator. • The chromosomes attach to the spindle fibers at their centromers.

ANAPHASE-I • The homologous chromosomes of each tetrad separate from each other. • They ANAPHASE-I • The homologous chromosomes of each tetrad separate from each other. • They move to opposite poles of the cell. • The set of chromosomes around each pole is haploid.

TELOPHASE-I • Nuclear membranes are formed. The cytoplasm divides forming two daughter cells. TELOPHASE-I • Nuclear membranes are formed. The cytoplasm divides forming two daughter cells.

MEIOSIS-II • The second division of the meiosis is similar to the mitotic cell MEIOSIS-II • The second division of the meiosis is similar to the mitotic cell division.

PROPHASE-II • Each of the daughter cells forms a spindle and the double stranded. PROPHASE-II • Each of the daughter cells forms a spindle and the double stranded. • Chromosomes move towards the middle of the cell.

METAPHASE-II • The chromosomes become attached to the spindle fibers at their centromers. • METAPHASE-II • The chromosomes become attached to the spindle fibers at their centromers. • And the chromosomes line up on the equator.

ANAPHASE-II • The Centromers divide and the sister chromatids seperate. • The chromatids move ANAPHASE-II • The Centromers divide and the sister chromatids seperate. • The chromatids move towards the opposite poles of the cells.

TELOPHASE-II • Both daughter cells divide forming 4 haploid cells. • The nuclear membrane TELOPHASE-II • Both daughter cells divide forming 4 haploid cells. • The nuclear membrane and nucleolus reappear.