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Meiosis in Pollen Mother Cells

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Introduction

TITLE: MEIOSIS IN POLLEN MOTHER CELLS OBJECTIVE: To observe meiosis in Lillium using pollen squash method. INTRODUCTION: Meiosis is the process in cell division in sexually reproducing organisms that reduces the number of chromosomes from diploid to haploid (2n(n). Meiosis involves two consecutive divisions of the nucleus and leads to the production of reproductive cells (gametes) in animals and to the formation of spores in plants, fungi, and most algae (the spores grow into organisms that produce gametes by mitosis). This enables organisms to reproduce sexually as gametes (sperm and eggs are haploid and thus the fusion of two gametes doubles the number of chromosomes. During the first division, meiosis begins when the chromosomes, which have already duplicated, condense along the center of the nucleus, and pairs of homologous chromosomes undergo crossing over, whereby some of their genetic material is exchanged. The pairs of chromosomes then separate and move to opposite ends of the cell, and the cell itself divides into two cells. In the second stage, each of these two cells also divides into two cells. Meiosis thus produces four cells, each of which contains half the number of chromosomes as the original cell. Some or all of the four cells may become functional gametes or spores. ...read more.

Middle

Lillium anthers were chosen in this experiment because they have nice meiotic figures as they produce the male gametophyte, that is, the chromosomes are more easily observed than in many other species. 2. Meiosis I (description of process in detail): a) Prophase I * The chromosomes become visible as single threads. * Homologous chromosomes pair up along their length (synapsis). * More condensation makes them appear shorter and fatter and it is possible to distinguish the chromatids of each chromosome of a pair. * In some regions the members of a pair repel each other but they remain attracted to each other. * At points of attraction, non sister chromatids break and rejoin. This process is known as crossing over, resulting in the formation of chiasmata. * The nucleoli disappear, the centrioles migrate to opposite poles of the cell and the nuclear envelope breaks down. b) Metaphase I * Pairs of homologous chromosomes (bivalent) become attached to the spindle. * The centromere attach to individual spindle fibres and the bivalents are arranged across the equator of the cell * The centomere of each homologous pair repel each other, the sister chromatids remain associated. * The order of the chromosomes on the equator is entirely random. ...read more.

Conclusion

7. The squashed anthers must first be examined under the microscope at low power to ensure the cells are spread into a single layer. When the cells are successfully squashed, examine under high power. 8. Some students fail in observing the stages of meiosis of a pollen grain. This happens because some of the pollen grains has already matured and no longer undergo meiosis. There is also another reason for this, some stages of pollen grain cannot be observed because it is not squashed evenly, to form a single layered cell. 9. The importance of meiosis in plants: i. Meiosis halves the number of chromosomes in the gametes, so that when two haploid gametes fuse, the diploid number is restored. ii. Genetic variation is introduced in the offspring. There are three causes of variation: * Crossing over - crossing over creates a new combination of genes on one chromosome. * Reduction and fusion of gametes - when haploid gametes fuse at fertilization, they combine different genetic material of the two parents * Independent (random) assortment - during metaphase II, homologous chromosomes arrange themselves randomly and so they are re-sorted in the daughter cells. CONCLUSION: Pollen grains are produced in pollen sacs. The diploid pollen mother cells divide by meiosis to form four haploid cells, each of which will develop into a pollen grain. ...read more.

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