Nondisjunction during the first mitotic division of a zygote leads to the formation of an abnormal embryo that has trisomy in half the cells, and monosomy in the remaining cells. Mitotic nondisjunction results in mosaicism, that is characterized by the presence of normal as well as genetically abnormal cells.
The cells with an extra chromosome are termed trisomic, while the ones lacking the corresponding chromosome are termed monosomic. The resultant cells are aneuploid, and have an abnormal set of chromosomes. The most common error that occurs during cell division processes is nondisjunction, a failure in the separation of homologous chromosomes (during meiosis I) or sister chromatids (during meiosis II or mitosis). Telophase II, leads to the formation of four genetically distinct haploid cells. Two haploid cells with duplicate copies of chromosomes are formed after telophase I. Spindle fibers also begin to form at this stage. Prophase involves the coiling and shortening of chromosomes, disintegration of the nuclear envelope, and movement of centrioles towards opposite ends of the cell. The major purpose is to facilitate sexual reproduction through gametogenesis.Ĭertain specialized cells called meiocytes, that are involved in sexual reproduction, undergo meiosis. The major purpose is vegetative growth and asexual reproduction. Genetic variation is introduced during meiotic divisions.Ĭentromeres spilt during anaphase, resulting in the separation of sister chromatids.Ĭentromeres and the sister chromatid pairs remain intact during meiosis I, but separate during meiosis II. The genetic identity is retained after a mitotic division. Synapsis and crossing over between homologous chromosomes occurs during meiosis I. Synapsis and crossing over events do not occur during mitosis. Ploidy reduction occurs giving rise to haploid daughter cells. The ploidy of the daughter cell remains the same as that of the mother cell. Only a diploid mother cell can undergo meiosis. One mother cell undergoes two successive divisions, and gives rise to four daughter cells.Ī haploid or diploid mother cell can undergo mitosis. One mother cell undergoes a single division, and gives rise to two daughter cells.
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