Mitosis - A Quick Overview of Cell Division

MITOSIS A Quick Overview of Cell Division The genetic information of plants, animals and other eukaryotic organisms resides in several individual DNA molecules, or chromosomes. For example, each human cell possesses 46 chromosomes, while each cell of an onion possesses 8 chromosomes. All cells must replicate their DNA when dividing. During DNA replication, the two strands of the DNA double helix separate, and for each original strand a new complementary strand is produced, yielding two identical DNA molecules. A process by which a parent cell divides into two or more daughter cells is called cell division. Cell division is a small part of the cell cycle. In normal eukaryotic cells, the type of cell division is known as mitosis. Another type of cell division is present in reproductive cells of eukaryotes and is known as meiosis. Cell cycle is mainly classified into two segments: M-phase and Interphase. Interphase is the longer period of cell division. During this phase the cell prepares for its next stage. Mitosis Mitosis refers to a type of cell division (cell cycle) through which the cell (parent cell) produces two identical daughter cells. Unlike Meiosis, which is also a type of cell division, mitosis results in the production of two diploid daughter cells. The two daughter cells contain the same number of chromosomes as the parent cell. Given that the process results in the proliferation of cells, it's important for general growth and replacement of damaged cells (e.g., the wound healing process). In general, mitosis occurs through several stages that include: Prophase (divided into prophase and prometaphase), Metaphase, Anaphase, & Telophase Onion Root Tip Mitosis Because of the rapid rate at which onion root tips grow as a result of rapid cell division, it's possible to observe and identify the different stages of mitosis. An onion has a total of 8 pairs of chromosomes. This is especially beneficial for this experiment given that fewer chromosomes are slightly easier to see when they condense. Laboratory Objectives • Explain the process and stages of mitosis. • Identify specimens of onion root to visualize all of the stages of mitosis. • Apply an analytical technique by which the relative length of each stage of mitosis can be estimated. Experiment For this section, the experiment will be divided into two main parts. These include growing onion root tips. Growing fresh root tips is recommended given that older or dried roots may produce poor results. As mentioned, onion root tip cells divide rapidly as the roots elongate to absorb water and various minerals from the soil. For this reason, it's possible to identify different stages of cell division by mitosis based on chromosomal distribution. Requirements • • • • • • Uncut red onions Clear glass or plastic jars Clean water Toothpicks Masking tape Permanent markers Procedure 1. Pour clean water into the clean (and clear) glass/plastic jars - about three-quarters full. 2. Carefully place the onion bulbs in the glass/plastic jars so that only the lower surface of the onion comes in contact with the water. 3. In a case where the onions are too small, they can be supported using toothpick/splints. 4. Let the onions rest on water for 3-4 days - only the lower surface (base from which roots emerge) should be in contact with water. I. Illustrate the diagrammatic representations of this setup. II. Why use onion roots for viewing mitosis? _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ III. Observations and Inferences Timeline After 24 hours Observations Inferences After 48 hours After 36 hours Conclusions: IV. Illustrate the diagrammatic representations of the results and observations. In an actual laboratory set-up, under 10X magnification, students will be able to observe several single layers of cells. Depending on how well the slide was prepared, the cells are spread out without any overlapping or with very little overlapping. Some root cells have ceased dividing and are only increasing in size, whereas others have reached their final, mature size and function, and are said to be in the GO stage. Generally, a row of cells (single layer) may consist of between 2 and 5 cells. When viewed under the microscope, a properly prepared slide will yield the following results: The following is a diagrammatic representation of onion root tip cells under 10X magnification MITOSIS AND THE CELL CYCLE INTERPHASE ∞ Cell that are actively dividing but not yet in mitosis are said to be in interphase, during which time the DNA is copied and the cell is otherwise preparing for replication. ∞ While this is not necessarily one of the main stages of mitosis, cells in this state can easily be identified by their prominent nucleoli. Diagrammatic representation of an onion root tip cell during interphase: