Mitosis experiment free essay sample

Introduction The practical will involve the preparation, observation, and classification, of the various stages of mitosis by observing garlic root tips using a microscope. The study comes under the heading of cytogenetics which is the visual characterisation of genetic material and the study of how that material reacts during the life of the cell. Strictly speaking, mitosis is the division of the nucleus into two daughter nuclei that are identical to one another and to the parent nucleus. Mitosis is part of the cell cycle which also includes chromosome replication and division of the cytoplasm i.e. cytokinesis. The chromosomes are always present in the nucleus but they are most easily seen when the nucleus divides. During this division process chromosomes contract significantly after which they can be seen under the light microscope as distinct thread-like structures. Chromosomes are usually studied by obtaining actively dividing tissue, fixing the material to kill the cells, and to preserve their structure, staining the chromosomes with a dye, and then observing them with a microscope. We will write a custom essay sample on Mitosis experiment or any similar topic specifically for you Do Not WasteYour Time HIRE WRITER Only 13.90 / page By studying mitosis in this way, changes that occur in the chromosomes can be observed and the visible events that bring about the process of heredity in somatic (body) cells can be seen. Furthermore, using certain chemicals it is possible to arrest the division process at the end of prophase and catch the chromosomes in their most condensed form (see karyotyping). This technique permits the study of number, size, and form, of the chromosomes, and enables us to see how these aspects of chromosome organisation vary from one species to another. General Procedures for making Slides to study Mitosis Cells, dividing by mitosis, can be found in the actively growing (meristems) part of an organism and in tissues where the cells require continual replacement. It is usual to work with meristematic regions of the roots although various other parts of plants can be used. In animals the source of material may be young embryos, cells from the testes or in humans, white blood cells grown in culture medium. For most purposes, study of the meristematic or actively dividing region of plant root tips is the most convenient. A simple method, and the one that is used in this case, is to suspend a clove of garlic (Allium sativum) on a beaker full of water so that  the bottom of the clove is covered by the water. Within a day or two, roots will have begun to grow into the water and these can be used when they are about one centimeter in length. The resultant preparations should allow study of all the stages of mitosis. Procedure for root tip preparations 1.Garlic root tips have been heated for 1h at 60 ºC in aceto-orcein stain contained in boiling tubes 2.Transfer an undamaged root tip (i.e. one with an intact point to the tip) to a clean microscope slide using a pair of forceps 3.Gently angle a cover slip over the specimen and lower it on 4.Place three squares of blotting paper over the cover slip and gently, then with increased pressure, press your thumb vertically down on the blotting paper 5.This removes excess stain and helps separate the chromosomes to provide increased clarity Studying mitosis under the microscope 6.Place the slide on the stage of the microscope and focus on typically dividing cells using the X10 objective. The morphology of cells in the dividing region of the root is quite specific, by being relatively square and in uniform long lines. 7.Once the region of division has been located change to the X40 objective to view the chromosomes and identify the various stages of division. 8.It is important not to look just for the various stages in isolation but to fit them into the pattern of the cell cycle starting with prophase and ending with telophase. 9.Draw and label each stage noting on your diagram the magnification (use half a page for each drawing. 10.If possible note the number of chromosomes present in the somatic cells of garlic by counting the chromosomes in ten representative cells. You should ask yourself the following questions: †¢Are the chromosomes present as discrete structures? †¢Are chromatids visible and are they discrete? †¢Are the chromatids orientated in any particular way? †¢Have the chromatids separated into two distinct groups? †¢What mitotic stage(s) is/are most common? †¢Can I identity the various stages of thickening in prophase? †¢Can I assess the relative positions of the centromeres? †¢Can I follow the progress of the chromosomes at anaphase by studying various cells in anaphase by noting the approximate positions of the centromeres? †¢Can I identify cytokinesis by improving the clarity of the telophase stage (see appendix)? †¢Do I observe any polyploidy cells? Stages of mitosis Interphase Strictly speaking, interphase is not a phase of mitosis, but is the normal resting state of a cell. The chromosomes are present as an amorphous stained material. Prophase The chromosomes gradually shorten and thicken by contraction or condensation and so become more obvious. This reduces the chances of chromosomal entanglement or breakage in the later stages of mitosis. Careful observation of prophase and metaphase under high power will verify the fact that each chromosome is a double structure composed lengthways of a pair of sister chromatids that will eventually move to opposite ends of the cell during anaphase. Metaphase When the chromosomes have migrated or congressed to the equator of the cell they are said to be in the metaphase condition. The chromosomes become attached at their centromeres to the spindle such that the sister chromatids are attached to opposite poles. The spindle will not be visible using the aceto-orcein staining method but the centromeres can be located lying along the equator. Anaphase Sister chromatids are held together by a cohesive force of unknown nature up until the beginning of anaphase. At the beginning of anaphase the cohesive force lapses to allow the chromatids to move to opposite poles of the spindle. In these anaphase cells the approximate position of the centromeres  can be determined because they now point directly to the poles, whilst the chromosome arms trail behind.