Cell Division In Eukaryotic Cells

Cell Division in Eukaryotic CellsI. Introduction to the Cell CycleThe cell cycle includes all of the events in the life of an individual cell, from celldivision to the period when a cell is not dividing while it carries out it’s regular functions.We can thus divide the cell cycle into:A. Interphase: Period of cell cycle when cell is not dividing1. G1 Phase: Cellular organelles begin to duplicate.2. S-Phase: DNA replication (chomosomes become doubled).3. G2 Phase: Cell growth and protein synthesis.B. Mitotic-Phase: Period of Cell Division1. Mitosis (or Nuclear Division):This includes Prophase, Metaphase, Anaphase & Telophase.2. Cytokinesis (Cytoplasmic Division):This includes the formation of a cleavage furrow in animal cells and aPartition called a cell plate in plant cells during telophase of celldivision.Today we will observe actual eukaryotic cells during mitosis, in which a parent (ormother) cell divides into two identical daughter cells that have the same number ofchromosomes as the parent cell. To accomplish this, we will observe prepared slides oforganisms (or parts of organisms) that were actively dividing.II. ObjectivesUpon completion of today’s lab, you should be able to:1) Distinguish between chromatin, replicated chromosomes, sister chromatids, singlechromosomes, and a homologous pair of chromosomes.2) Given a diagram or prepared slide of an Allium root tip or whitefish embryo, identifythe various stages of the cell cycle.3) Describe the events that occur during each stage of the cell cycle.4) Demonstrate how a diploid mother cell produces two diploid daughter cells.5) Determine the relationship between the number of cells in each stage of the cell cycleand the time required for each stage.6) Compare and contrast cytokinesis in plant and animal cells.1

III. Observations of the Cell Cycle in PlantsIn order to observe plant cells during mitosis, we have to observe a part of a plantthat is actively dividing. Meristematic regions (or meristems) are the parts of plantswhere cell division actively occurs. Plant meristems are at the tips of most roots, stems,branches, and buds. Thus, the only parts where plants can increase in length are at thetips of roots, stems, and branches. We will observe the tips of young onion (Allium) rootsto observe plant cells during the different stages of the cell cycle.Obtain a prepared slide of Allium root tip. Place the slide on the microscope stageand focus the prepared slide under scanning (4x) power. Locate the meristematic regionof the root tip. Center the meristematic region in the center of the field of view. Usingthe proper technique, focus the meristematic region under high (40x) power. Proceed tothe exercise below, using Figure 1 to help you identify cells during the different stages ofthe cell cycle.Figure 1. Diagram of plant cells during the different stages of the cell cycle.2

A. InterphaseDuring interphase, the cell is not actively dividing. The nucleus contains darklystained chromatin, a term that applies to all of the chromosomes (DNA) collectively. Atthis stage the chromosomes are threadlike and are not visible as distinct bodies. Anucleolus is clearly visible inside the nucleus. This body is composed of ribosomalRNA. Prior to cell division, two pairs of protein bodies called centrioles are present inthe cytoplasm at one end of the cell. Centrioles are not typically present in plant cells.Locate a cell that is undergoing interphase and sketch it below, labeling the nucleus,chromatin, and nucleolus:What happens to the DNA by the end of the “S” phase of interphase?Count and record the approximate number of all cells observed in the field of view underhigh power:Of these cells, how many are in interphase?Approximately what percentage of the cells in your field of view are undergoinginterphase?B. ProphaseWith the onset of prophase, the nuclear membrane and nucleolus begin todisintegrate and the chromatin shortens and thickens so that they are visible as distinctrod-shaped bodies called chromosomes. At this time each chromosome is doubled and3

consists of two identical sister chromatids. Each chromatid is essentially composed of agreatly coiled DNA molecule and protein. The chromatids are attached in a region knownas the centromere.Locate a cell that is undergoing prophase and sketch it below, labeling the nuclearmembrane (if present) and chromosomes:Of the cells in your field of view under high power, how many are in prophase?Approximately what percentage of the cells in your field of view are undergoingprophase?C. MetaphaseThe chromosome (recall, each chromosome is now in the form of two, identicalsister chromatids) become arranged in the central region of the cell known as theequator. We refer to this as the formation of the metaphase plate. Protein threadscalled the spindle fibers connect the centromere region of each chromosome with thecentrioles at the poles of the cells.Locate a cell that is undergoing metaphase and sketch it below, labeling chromosomes,metaphase plate, and spindle fibers:4

Of the cells in your field of view under high power, how many are in metaphase?Approximately what percentage of the cells in your field of view are undergoingmetaphase?D. AnaphaseDuring anaphase, the two identical sister chromatids that make up eachchromosome separate from each other at the centromere region. The sister chromatidswill now form the single chromosomes that move to opposite ends (poles) of the cell.When the chromatids separate from each other they are no longer called chromatids.They are now referred to as single chromosomes. The single chromosomes are actuallybeing pulled to opposite ends of the cell as the spindle fibers shorten.Locate a cell that is undergoing anaphase and sketch it below, labeling the chromosomes,and spindle fibers:5

Of the cells in your field of view, what percentage are in anaphase?E. TelophaseDuring telophase, the chromosomes at each end of the cell begin to organize intoseparate nuclei, each surrounded by a nuclear membrane. Cytoplasmic division(cytokinesis) in a plant cell is accomplished by the formation of a partition or cell platebetween the two newly formed daughter nuclei (see Figure 1 on page 2).Locate a cell that is undergoing telophase and sketch it below, labeled chromosomes, cellplate, and spindle fibers:6

Of the cells in your field of view, what percentage are in telophase?F. InterphaseNow we are back to interphase again, but now there are two daughter cells. Eachdaughter cell is chromosomally identical with the original (mother) cell. They each havea nucleus that contains a nucleolus and chromatin. During this phase the chromosomeswill replicate and become distinct pairs of sister chromatids as each daughter cell entersprophase.IV. Observations of the Cell Cycle in AnimalsWe will observe the embryo of an animal to get a “snapshot” of animal cells asthey actively divide. The prepared slides we will observe are of embryos during a stageof embryological development when the animal is nothing more than a ball ofundifferentiated (unspecialized) cells. At one time, all animals (including humans) gothrough such a stage. At this stage, each cell is unspecialized and can potentially developinto any type of cell (or even a separate organism). Figure 4 shows a highly magnifiedview of a whitefish early embryo showing several stages of mitosis.7

Figure 4. A. Magnified view of an entire whitefish embryo during the 32-cell stage ofembryological development; B. Highly magnified view of a whitefish embryo showing severalstages of mitosis: 1 prophase, 2 metaphase, 3 anaphase, 4 telophase.Place the slide of the whitefish embryo on your microscope and observe under highpower (using the proper focusing technique). Remember, always start with the scanningobjective and increase power from there Find animal cells undergoing prophase, metaphase, anaphase, and telophase, and sketcheach in the spaces provided below. For each, list the characteristics of each cell thatallowed you to identify the stage of mitosis. Use your observations to answer the reviewquestions in the next section.8

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V. Review Questions1.Which stage of mitosis was most common in the meristematic region of your Alliumroot tip slide?2. Using your answer from #1, which stage of the cell cycle takes the longest to occur?3. Approximately what % of the time in the cell cycle do you think interphase occupies?4. Approximately what % of the time in the cell cycle do you think prophase occupies?Metaphase? Anaphase? Telophase?5. Compare and contrast mitosis and cytokinesis in plant and animal cells. You maywant to use your text or class notes to help with this answer.11

The cell cycle includes all of the events in the life of an individual cell, from cell division to the period when a cell is not dividing while it carries out it’s regular functions. We can thus divide the cell cycle into: A. Interphase: Period of cell cycle when cell is not dividing 1. G1 Phase: Cellular organelles begin to duplicate. 2.