Botany - B.E. Yakubenko 2017

Part One. Plant Anatomy and Morphology
Chapter I. Cytology
Laboratory Class Topic 3.5. Nuclear Division (Mitosis) and Cell Division (Cytokinesis)

General Remarks. Plant GROWTH AND REPRODUCTION are associated with The formation of new Cells. The most common mechanism for this is the division of The Nucleus and The Cell itself. The following types of nuclear division are distinguished: amitosis, mitosis (karyokinesis), and Meiosis (reductional division). During the physiological maturity of a cell, division occurs periodically through distinct stages, phases, and steps that reflect major structural changes within the cells, primarily in the nucleus. One of the key features is the formation of Chromosomes, which transmit hereditary traits from ancestors to descendants.

Mitosis comprises the following phases: interphase, prophase, metaphase, anaphase, telophase, and cytokinesis. As a result of the division of a somatic mother cell, two daughter cells are produced, each containing an identical set of chromosomes. This type of division takes place in the apical Meristems of the ROOT and SHOOT, at the Base of the leaf blade, and during the division of cambial cells.

Meiosis is a more complex process that combines two successive divisions: 1) reductional (where the chromosome number is halved); 2) equational, resembling standard mitosis.

Object: root tip of the common onion (Allium cepa L.)

Tasks:

1. Examine the structural changes characteristic of individual phases of mitosis and cytokinesis using a prepared permanent slide of a Cytology/practical/54.html">Longitudinal section of an onion root tip.

2. Make detailed, large-scale drawings of the structural changes occurring in the individual phases of mitosis and cytokinesis.

Equipment and Materials: MBR-1 or Biolam microscopes, prepared slides, instructional charts, and other laboratory accessories.

Microscopic Examination of Mitosis in the Onion Root Tip. Using low magnification and reference charts of mitosis, locate the most characteristic phases of nuclear division and study their structural changes. In your laboratory notebook, draw six separate rectangles that will represent large Cells of the apical meristem. Label them underneath in the following order: interphase; prophase; metaphase; anaphase; telophase; cytokinesis. As you locate and examine each phase, draw it within the corresponding cell diagram, illustrating its structural features and changes. The fine structural changes of individual phases can only be studied under high magnification. In an interphase cell, all structural components are clearly visible: The Cell wall, Cytoplasm, nucleus, and vacuole. Draw such a cell in the first diagram.

Cells in prophase are characterized by the presence of the following components: the cell wall, cytoplasm, and nearly fully formed chromosomes. At this stage, neither the nucleolus nor the nuclear envelope is visible (Fig. 21).

During metaphase, the cell retains the cell wall and cytoplasm, while the chromosomes acquire a specific characteristic shape with clearly defined centromeres and arms. By the end of metaphase, each chromosome splits into two chromatids. The chromosomes align in the center of the cell, forming the equatorial plate. Their centromeres attach to the fibers of the achromatic spindle (mitotic spindle). The spindle fibers extending from one pole to the other are called interpolar (or supporting) fibers, whereas those connecting the chromosomal centromeres to the poles are known as chromosomal (or traction) fibers. Anaphase is characterized by the migration of chromatids toward opposite poles as a result of the shortening of the spindle fibers.

The chromatids appear to glide along the supporting fibers of the mitotic spindle. This phase is easily identified because the opposite poles contain the same number of chromosomes as seen in prophase. The cell still contains the spindle fibers, chromosomes, cytoplasm, and cell wall.

Telophase is readily recognized: during this phase, chromosomes lose their individual outline and merge into a general mass of Chromatin, from which two daughter nuclei are formed, complete with nucleoli, nuclear envelopes, and karyolymph. The slide reveals various stages of daughter nucleus formation. Locate and examine small thickenings—achromatic nodes—along the spindle fibers in the center of the cell. Through centrifugal growth from the center toward the periphery, a primary cell plate forms, dividing the mother cell into two daughter cells.

Cytokinesis is characterized by the division of a single mother cell into two daughter cells. Each daughter cell inherits the same chromosome number as the mother cell.

Conclusion. Mitosis is characteristic of the division of vegetative, or somatic, cells. During this process, the nucleus undergoes profound changes accompanied by the formation and division of chromosomes. Chromosome division involves the formation of chromatids, chromonemata, chromomeres, spindle fibers, achromatic nodes, the equatorial plate, the phragmoplast, and the primary cell plate. Ultimately, two genetically identical daughter cells are produced from a single mother cell.

Review Questions

1. Name the Methods of Plant Cell Formation.

2. What are the phases of Mitosis and Meiosis?

3. Explain The Essence of the changes that occur during Direct and Indirect Cell Division.

4. What is The Structure of a chromosome? Name its constituent parts.

5. In which phase do two daughter nuclei form?

6. In which phase does Chromosome Replication take place?