Cell Division TWO major types of cell division take place in flowering plants. They are mitosis and meiosis. Mitosis, along with cytokinesis, involves the division of cells for growth and development. Cells produced through mitosis and cytokinesis make up the plant body, which consists of the roots, stem, leaves, and flower parts. Meiosis is associated with the production of sex cells and sexual reproduction. Objective: Describe the processes of mitosis and meiosis. Key Terms: anaphase chromatid clone crossing over cytokinesis diploid gamete haploid homologous chromosomes interphase meiosis metaphase mitosis mitotic spindle polyploid prophase E-unit: Cell Division Page 1 AgEdLibrary.com
synapsis telophase zygote Mitosis Plant cells either stop growing or divide once they reach a certain size. Growth and division of cells is how a plant increases in size. Cell division takes place through two main processes, mitosis and cytokinesis. Mitosis is a complex process in which the nucleus of a cell divides. The resulting two nuclei have the same number and types of chromosomes as the original nucleus. Cytokinesis is the process by which the cytoplasm of a cell divides, providing each new cell with the essential organelles. The life of a cell follows a cycle of growth and division. The cell cycle can be broken down into stages, or phases. Most of the cell s life cycle is spent in interphase, a time between periods of cell division. During interphase the cell grows and synthesizes needed materials, such as proteins. Interphase is further divided into three phases. The first is the G 1 phase, during which the cell increases enzyme activity for DNA synthesis. This stage is followed by the S phase, a period when chromosomes duplicate. Each half of duplicated chromosomes is called a chromatid. The third phase, the G 2 phase, sees an increase in protein synthesis. Toward the end of interphase, mitosis begins. Mitosis is divided into four distinct stages. The first stage is prophase. During prophase, the chromatin condenses, or masses, in the form of chromosomes. Sister chromatids, identical units created during S phase, are drawn together at a region called the centromere. Note that a dividing cell is typically described as being similar to a globe, in that it has an equator and polar regions. While the chromatins are massing, fibers are forming at the poles of the cell. These fibers are referred to as mitotic spindles. Toward the end of prophase, the nuclear membrane breaks down, and spindles begin to attach to chromatids. Prophase is followed by metaphase. During metaphase, the mitotic spindles attach to the centromeres of the chromosomes. The chromosomes also line up along the equator. MITOSIS PROPHASE No synapsis of homologous chromosomes METAPHASE Individual chromosomes line up on metaphase plate ANAPHASE Sister centromeres move to opposite poles TELOPHASE Two 2n cells identical to the parent cell FIGURE 1. Mitosis is a complex process in which the nucleus of a cell divides. E-unit: Cell Division Page 2 AgEdLibrary.com
During the third stage, called anaphase, the chromatids part at the centromere. One set of chromosomes moves toward a pole, and an identical set of chromosomes moves toward the other pole. The final stage of mitosis sees the reformation of two nuclei. This phase is known as telophase. Cell division is completed during cytokinesis. The result is two cells identical to the parent cell in every way except size. These cells start the cycle anew. All the cells of plants propagated by asexual reproduction are produced through mitosis. The cells of the new plants are identical. These genetically identical offspring are said to be clones of the parent. Meiosis Meiosis is the mechanism behind sexual reproduction. In meiosis, four gametes are produced from a single cell. Gametes are specialized sex cells, the microspore (sperm) and megaspore (egg). An egg fertilized by a sperm is a zygote. The significance of meiosis is that genetic material is recombined during the process. All offspring differ genetically from all others. Chromosomes normally exist in pairs. Each set of two chromosomes is called homologous chromosomes. Homologous chromosomes share a similar size, shape, and position of the centromeres. When a cell contains the normal two sets of two chromosomes, it is said to have a diploid (2n) number of chromosomes. A haploid (n) cell has a single set of chromosomes. Gametes contain a haploid number of chromosomes. Many grasses and flowering plants have three or more sets of chromosomes. They are called polyploid. Plants with extra sets of chromosomes are typically larger and more vigorous than diploid members of the same species. Bread wheat is a good example of a polyploid plant used in agriculture. It is hexaploid. Meiosis differs from mitosis in some important ways. Meiosis involves two cell divisions that result in four cells. Each of the four cells produced has a haploid number of chromosomes. During meiosis, the genetic information from each parent is rearranged so that the resulting gametes have a unique combination of genes. The two nuclear and cytoplasmic divisions of meiosis are named meiosis I and meiosis II. Each division includes a prophase, metaphase, anaphase, and telophase. FIGURE 2. The polyploid Stella de Oro daylily on the left is larger than the diploid Stella de Oro daylily on the right. (Courtesy, Klehm Nursery) E-unit: Cell Division Page 3 AgEdLibrary.com
During the S phase of interphase, the chromosomes duplicate just before meiosis I. The chromosome pairs join at their centromeres. Since a diploid number is duplicated, there are four chromatids, or two homologous pairs of chromosomes. The four chromatids are referred to as a tetrad. In prophase I, these chromosomes lie side by side, a process called synapsis. At this time, genetic material may be exchanged between the homologous chromatids in a process called crossing over. Enzymes cut each chromatid, sections are switched, and then the sections are fused together. In this way, genetic material is altered, leading to greater genetic variation among the offspring. Also, the nuclear envelope vanishes. Metaphase I begins with the tetrads aligned along the equatorial plane. Spindle fibers attach to homologous chromosomes. In anaphase I, the homologous chromosomes of each pair separate and are pulled toward the opposite poles. In telophase I, the nuclear envelope reappears, and cytokinesis takes place. Meiosis II begins with a diploid number of chromosomes in each of the two cells. No further duplication PROPHASE I Synapsis of homologous chromosomes to form tetrads METAPHASE I Tetrads line up on metaphase plate ANAPHASE I Homologous centromeres move to opposite poles TELOPHASE I Each chromosome is still duplicated MEIOSIS PROPHASE II Chromosomes are still duplicated METAPHASE II Chromosomes line up on the metaphase plate ANAPHASE II Sister centromeres move to opposite poles TELOPHASE II Four n cells FIGURE 3. In meiosis, four gametes are produced from a single cell. of genetic material takes place prior to meiosis II. Prophase II is brief, since the chromatids are already condensed. Spindle fibers form, and the nuclear envelope breaks down. In metaphase II, the chromosomes line up along the equator. Anaphase II sees the chromatids pulled to opposite poles by spindle fibers. The nuclear envelope reforms, and the cytoplasm divides. The two divisions of meiosis result at telophase II in four haploid cells, each with a different genetic makeup. When fertilization takes place during sexual reproduction, the haploid gametes fuse to form a diploid cell called the zygote. The zygote develops into the embryo. It inherits a single set of chromosomes from the female parent and a single set from the male parent. E-unit: Cell Division Page 4 AgEdLibrary.com
Summary: Cells undergo division. The two major types of cell division that take place in flowering plants are mitosis and meiosis. Mitosis, along with cytokinesis, involves the division of cells for growth and development. Roots, stems, leaves, and flower parts are made of cells produced through mitosis and cytokinesis. Meiosis is cell division that results in sex cells, or gametes. The two divisions of meiosis result in four haploid cells, each with a different genetic makeup. When fertilization takes place during sexual reproduction, the haploid gametes fuse to form a diploid cell called the zygote. Checking Your Knowledge: 1. Differentiate between mitosis and cytokinesis. 2. What are the steps involved in mitosis? 3. During what stage of mitosis is most of a cell s life cycle spent? 4. What are the steps involved in meiosis? Web Links: The Biology Project Mitosis Tutorial http://www.biology.arizona.edu/cell_bio/tutorials/cell_cycle/main.html Computer Animations by John Kyrk Meiosis http://www.johnkyrk.com/meiosis.html E-unit: Cell Division Page 5 AgEdLibrary.com