The Life Cycles of Sexually-Reproducing Organisms - Lab Report Example

Introduction Cell division or cell multiplication is the process of formation of new or daughter cells from the pre-existing or parent cells. It can be of three types, namely amitosis, mitosis and meiosis. Amitosis is a simple method of cell division which is of common occurrence in many bacteria. Also called the direct cell division it occurs just by elongation and constriction of the nucleus and cytoplasm resulting in two daughter cells. Mitosis is a division of a parent cell into two identical daughter cells having the same number and kind of chromosomes as in the parent cell. Mitosis was first observed by Strasburger (1870) in plant cells. Boveri and Flemming studied it in 1879 in animal cells. The term "mitosis" was coined by Flemming in 1882. Mitosis involves a series of events both in nucleus and cytoplasm. Therefore, it consists of two stages- Karyokinesis or nuclear division and Cytokinesis or cytoplasmic division. This is followed by separation of daughter cells. Meiosis is the type of cell division by which germ cells (eggs and sperm) are produced. Meiosis was first observed by the Belgian cytologist Pierre-Joseph van Beneden in 1887. Meiosis involves a reduction in the amount of genetic material. It comprises two successive nuclear divisions with only one round of DNA replication. One parent cell, at the end of one meiotic division, produces four daughter cells. Daughter cells have half the number of chromosomes found in the original parent cell and with crossing over, are genetically different. Meiosis differs from mitosis primarily because there are two cell divisions in meiosis, resulting in cells with a haploid number of chromosomes. Cell division is especially rapid in the growing root tips of sprouting seeds. The chromosomes in dividing root tip cells can be demonstrated if, after sprouting seeds or bulbs,and harvesting the young root tips, and then fix, acid digest, stain, squash, and view them under a microscope. Methods Background The root tip is the place of intense mitotic activity. This region is three dimensional and individual cells cannot be seen. It is therefore necessary to separate the cells out into a thin layer. Plant cells are glued together by the middle lamella of calcium pectate. Hydrochloric acid will dissolve this calcium pectate but leave the cellulose cell wall unaffected. In addition the acid kills and fixes the cell contents in position. Acetic orcein stains nuclei and not the cytoplasm. 1. The apical 5mm from the tip of a growing lateral root of a broad bean was carefully cut. 2. The root tip was placed in a watch glass containing acetic orcein stain and 1mol dm-3 hydrochloric acid in the approximate proportions 10 parts stain to 1 part acid. 3. Allowed to warm for five minutes by passing repeatedly through a low Bunsen flame. 4. The fixed root tip was placed on a microscope slide and two drops of acetic orcein were added. 5. The root tip was broken up without destroying the arrangement of the cells, and the root tip was spread out as thinly as possible. 6. A cover slip was placed over the root tip, and covered it with blotting paper and squashed gently by pushing down on the cover slip. 7. The slide was warmed gently to intensify the stain, by quickly passing through a low Bunsen burner flame a few times. 8. The slide was examined for identifying any stage of mitosis. Results Mitosis The four phases of mitosis- Prophase, Metaphase, Anaphase and Telophase- were observed under the microscope. The different phases were identified based on the unique chromatin organization features: 1. Interphase: Each chromosome undergoes replication, making an identical copy of itself. At this point, the chromosomes are still long and thin, and are not visible inside the nucleus. Cells spend most of their life in this non-reproductive phase. 2. Prophase: The chromosomes coil and shorten, and become visible. It becomes apparent that the chromosomes have duplicated. Pairs of identical chromosomes remain attached to each other at the centromere and each chromosome is called a chromatid. 3. Metaphase: Chromosomes line up along the centre of the cell. A pair of structures called centrioles form at the poles of the cell, and produce spindle fibres which attach to the centromeres of each chromosome pair. 4. Anaphase: The paired chromosomes split at the centromere and the two halves migrate along the spindle fibres to opposite sides of the cell. At the same time, the centre of the cell begins to pinch. 5. Telophase: Cell division occurs, and each is indentical to the original. Cells return to Interphase and prepare for another round of division. Meiosis Meiosis comprises two successive nuclear divisions with only one round of DNA replication. Four stages can be described for each nuclear division: Interphase I: Identical to Interphase in mitosis. Prophase I: Identical to Prophase in mitosis. Metaphase I: Instead of all chromosomes pairing up along the midline of the cell as in mitosis, homologous chromosome pairs line up next to each other. This is called synapsis. Homologous chromosomes contain the matching alleles donated from mother and father. This is also when meiotic recombination, also know as "crossing over" occurs. This process allows for a genetic shuffling of the characteristics of the two parents, creating an almost infinite variety of possible combinations. See the close-up diagram below. Anaphase I: Instead of chromatids splitting at the centromere, homologous chromosome pairs (now shuffled by crossing over) move along the spindle fibres to opposite poles. Telophase I: The cell pinches and divides. Prophase II: It is visibly obvious that replication has not occurred. Metaphase II: The paired chromosomes line up. Anaphase II: The chromatids split at the centromere and migrate along the spindle fibres to opposite poles. Telophase II: The cells pinch in the centre and divide again. The final outcome is four cells, each with half of the genetic material found in the original. In the case of males, each cell becomes a sperm. In the case of females, one cell becomes an egg and the other three become polar bodies which are not used. Discussion Mitosis is the process of cell division, and occurs only in "somatic" or body cells. When haploid sperm meets haploid egg, a chain of events that begins with a single diploid cell and ends with an adult organism made of billions of cells is set in motion. The single cell divides into 2, and each of those 2 divide again, and this process continues geometrically along the following progression: 1, 2, 4, 8, 16, 32, 64, and 128, and so on into the billions. Therefore, the first purpose of mitosis is growth. The second function of mitosis is repair. Cells are constantly wearing out and getting damaged and unless an organism replaces them at least as fast as they are lost, a gradual deterioration will occur. Meiosis is a special version of cell division that occurs only in the testes and ovaries; the organs that produce the sperm and eggs. Normal body cells have a complete set of chromosomes. If normal body cells from mother and father fused to form a baby, the fertilized egg would have twice as many chromosomes as it should. Meiosis is sometimes called "reduction division" because it reduces the number of chromosomes to half the normal number so that when fusion of sperm and egg occurs, baby will have the correct number. Therefore the purpose of meiosis is to produce gametes; the sperm and eggs. Meiosis produces four haploid daughter cells. Meiosis produces daughter cells that are not genetically identical. Mitosis produces two diploid daughter cells. Mitosis produces daughter cells that are genetically identical. . Meiosis differs from mitosis primarily because there are two cell divisions in meiosis, resulting in cells with a haploid number of chromosomes. The biological significance of mitosis can be simply put as to ensure identicality of daughter cells while that of meiosis is to ensure variation in the daughter cells. Thus cells forming part of the same tissue or performing the same function are identical due to mitosis while enormous genetic variation in population is brought about by meiosis. The entire process of cell division either by mitosis or meiosis can be observed in terms of the changes in the nucleus. This is made possible by the nucleus-specific acetic orcein stain. Because not all the cells undergo divisions at the same time, the different stages of mitosis/meiosis could be observed in a single slide preparation (onion root squash/frog testis). Conclusion The life cycles of all sexually-reproducing organisms follows the same basic pattern- Haploid cells or organisms alternate with diploid cells or organisms: Reference 1. Raven, P. H. and Johnson, G. Biology. 6th Edition. Columbus, USA: McGraw-Hill, 2001. 2. Rastogi, V. B. Modern Biology. Volume I. New Delhi, India: Pitambar Publishing Co. Pvt. Ltd., 1997. Read More