16. Meaning of meiosis The process of nuclear division that reduces the number of chromosomes in new cells to half the number of chromosomes in the parents cells. Humans Meiosis produces haploid reproductive cell or gametes. Meiosis occurs in the reproductive organs.. Testes in male sand ovaries in females. Meiosis occurs in the anthers and ovaries of the flowers. Plants
17. Sexual reproduction involves the fusion of two haploid gametes during fertilisation. This results in the formation of a diploid zygote with 46 chromosomes. The offspring will inherit traits from both the parents to ensure the continuation of life. The species that produces sexually,meiosis ensures that the diploid number of chromosomes is mantained from one generation to the next.
18. Meiosis Ovum (n) Sperm (n) Fertilisation Diploid zygote 2n=46 Testis Ovary Female Male Haploid gametes (n=23) Multicellular diploid adults (2n=46) Mitosis and development The human life cycle
21. Prophase I The chromosomes begin to condense. They become shorter, thicker and clearly visible. Unlike mitosis, the homologous chromosomes come together to form bivalents through a process called synapsis. Each bivalent is visible under the microscope as a four-part structure called a tetrad. A tetrad consists two homologous chromosomes, each made up of two sister chromatids.
22. Non-sister chromatids exchange segments of DNA in a process known as crossing over. Crossing over results in a new combination of genes on a chromosome. The points at which segments of chromatids cross over are called chiasmata. In the end of prophase I, the nucleolus and nuclear membrane disappear. The two pairs of centrioles migrate to the opposite poles of the cells. Each pair of centrioles acts as a central point from which the spindle fibres radiate. These features are similar to those of prophase during mitosis.
23. Metaphase I The chromosomes are lined up side by side as tetrads on the metaphase plate. The chromosomes are still in homologous pairs. One chromosome of each pair is attached to the spindle fibre from one pole while its homologue is attached to the fibre from the opposite pole. The centromere does not divide.
24. Anaphase I The spindle fibres pull the homologous chromosomes away from one another and move them to the opposite poles of the cell. Each chromosomes still consists of two sister chromatids which move as a single unit. Although the cell started with four chromosome, only two chromosomes (each with two sister chromatids) move towards each pole.
25. The chromosomes arrive at the poles. Each pole now has a haploid daughter nucleus because it contains only one set of chromosomes. The spindle fibres disappear. The nuclear membrane reappears to surround each set of chromosomes. The nucleolus then reappears in each nucleus. Telophase I
27. Prophase II The chromosomes, each still made up of sister chromatids, are positioned randomly on the metaphase plate with the sister chromatids of each chromosome pointing towards the opposite poles. Each sister chromatid is attached to the spindle fibres at the centromere. Metaphase II The nuclear membranes of the daughter cells disintegrate again. The spindle fibres re-form in each daughter cell.
28. Anaphase II The centromeres of the sister chromatids finally separate,and the sister chromatids of each chromosome are now individual chromosomes.The chromosomes move towards the opposite poles of the cell.
29. Telophase II Finally, the nucleoli and nuclear membranes re-form. The spindle fibres break down. Cytokinesis follows and four haploid daughter cells are formed,each containing half the number of chromosomes and is genetically different from the parent diploid cell. These haploid cells will develop into gametes.
30. At the end of meiosis II, four haploid daughter cells are produced, each genetically distinct from the other and from the parent cell. In meiosis, although cells undergo two nuclear divisions, the DNA of each chromosome only replicates once.
31. Mitosis Meiosis Comparison between mitosis and meiosis In plants, meiosis occurs in the anthers and ovaries of the flowers. In plants,mitosis occurs actively in the meristems at the tips of the roots and shoots. Involves two nuclear divisions to produce four haploid daughter cell which are not identical. Involves one nuclear divisions to produce two identical diploid daughter cells.
