Lab 8. Mitosis and Meiosis (revised Fall 2009) Lab 8 - Biol 211 - Page 1 of 24 Lab 8. The Modeling and Microscopic Observation of Mitosis and Meiosis in Plant and Animal Cells Prelab Assignment Before coming to lab, read carefully the introduction and the procedures for each part of the experiment, and then answer the prelab questions at the end of this lab handout. Hand in the prelab assignment just before the start of your scheduled lab period. Bring your textbook to class when you do this lab. Goals of this Lab After completing this lab exercise you should be able to... - Identify and describe the stages of the cell cycle, mitosis, and meiosis, recognizing the events that occur during each stage. - Distinguish between mitosis and cytokinesis as they take place in animal and plant cells. - Identify the structures involved in mitosis and meiosis and describe the role each plays. - Describe the significance of crossing over, independent assortment, and segregation in meiosis. - Indicate the differences and similarities between mitosis and meiosis. - Describe the importance of mitosis and meiosis in the life cycle of an organism. Introduction “All cells arise from preexisting cells” is one tenet of the cell theory. It is easy to understand this concept when thinking about unicellular organisms such as Amoeba and bacteria. Each cell divides to give rise to two entirely new individuals. But it is quite fascinating that each of us began life as one single cell and developed into an astonishingly complex animal. This one cell has all the hereditary information we’ll ever need. In somatic cells (body cells) of multicellular critters and in single-celled eukaryotic organisms, the nucleus divides by mitosis into two daughter nuclei, which have the same number of chromosomes and the same genes as the parent cell. Multicellular organisms prepare for sexual reproduction by producing gametes (egg or sperm cells) by another type of nuclear division, meiosis. In meiosis, nuclei of certain cells in ovaries or testes (sporangia in plants) divide twice, but the chromosomes are duplicated only once. Meiosis results in the formation of four daughter nuclei each with half the number of chromosomes with differing alleles (alternate forms of a gene) as the parent cell. Eggs and sperm (spores in plants) eventually form from the cells produced by meiosis. In sexually reproducing higher plants and animals, fertilization, the fusion of egg and sperm nuclei, produces a single-celled zygote. The zygote divides by mitosis into two cells, these two into four, and so on to produce a multicellular organism. During cell division each new cell receives a complete set of hereditary information and organelles. The hereditary material of both eukaryotes and prokaryotes is DNA (deoxyribonucleic acid). In prokaryotes, the DNA is organized into a single chromosome. Prior to cell division, the chromosome duplicates. Then the cell undergoes prokaryotic fission, the spitting of the cell into two, with each new cell receiving a full complement of the genetic material. This exercise, however, will consider cell division in eukaryotic organisms only.Lab 8. Mitosis and Meiosis (revised Fall 2009) Lab 8 - Biol 211 - Page 2 of 24 In eukaryotes, the process of cell division is more complex, primarily because of the more complex nature of the chromosomes. Chromosomes in eukaryotes consist of a complex of DNA and structural proteins. These proteins are involved with the folding and condensation of the DNA within the chromosomes. The nuclei in eukaryotic cells contain chromosomes with clusters of genes, discrete units of hereditary information consisting of DNA that codes for a particular trait. Cell division is preceded by duplication of the chromosomes and usually involves two processes: mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm). Whereas mitosis results in the production of two nuclei, both containing identical chromosomes, cytokinesis ensures that each new cell contains all the metabolic machinery (enzymes, organelles, nutrients, etc.) necessary to sustain life. Dividing cells pass through a regular sequence of events called the cell cycle (fig.1). Notice that the majority of time is spent in interphase and that actual nuclear division, mitosis, is but a brief portion of the cycle. Interphase is divided into three parts: the G1 period, during which cytoplasmic growth occurs; the S period, when DNA is duplicated; and the G2 period, when the structures involved with mitosis are synthesized. Figure 1. The cell cycle of eukaryotic cells. Interphase is a metabolically active part of the cell cycle: new DNA is synthesized (S-phase), proteins are assembled from amino acids, carbohydrates are actively synthesized, while others are broken down to provide energy for the various cellular processes (G1 and G2). Meanwhile all of the normal day-to-day activities of the cell are taking place. In short interphase is a very busy time in the life of a cell. In this lab activity you will.... - Use pop bead models of chromosomes to model the cell cycle, mitosis, and meiosis. - Observe prepared slides of onion cells and whitefish blastula with a compound microscope to study mitosis and cytokinesis in plant and animal cells.Lab 8. Mitosis and Meiosis (revised Fall 2009) Lab 8 - Biol 211 - Page 3 of 24 Part 1. Modeling the Cell Cycle and Mitosis Important Note!! To get an overview of this laboratory activity and to use your lab time efficiently read the following procedure before attending lab. If you and your group members are not familiar with the procedure before coming to lab you will have great difficulty completing this exercise during the lab period. Introduction Within the nucleus of an organism each chromosome contains genes, which are units of inheritance. Genes may exist in two or more alternate forms called alleles. Chromosomes come in look-alike pairs called homologous chromosomes. Homologous pairs have the same length, staining pattern, and possess the genes for the same characteristics at the same loci (location on the chromosome). One homologue is inherited from the organism’s mother, the other from the father. Thus each homologue contains genes for the same traits.