Grade 9 Science Unit #3 – Reproduction Topic #2 – Cell Division
A. Cell Theory
The discovery of the cell was made possible by the invention of the microscope, which was made possible by improved lens-grinding techniques. Antoni van Leeuwenhoek (1632 – 1723), a Dutch tradesman, learned to grind lenses and assemble them into simple microscopes. His contemporary Robert Hooke (1635 – 1703) used such an instrument to observe cork cells, sketches of which appeared in his 1665 publication “Micrographia”. Inspired by Hooke’s work, Leeuwenhoek began making microscopic examinations of his own. In 1678, he reported to the Royal Society that he had discovered “little animals” – bacteria and protozoa – in various samples. The society asked Hook to confirm Leeuwenhoek’s findings, and he did. This paved the way for wide acceptance that a hidden world existed just beyond the limits of human vision and encouraged many scientists to take up the microscope in their investigations. One such scientist was German botanist Matthias Jakob Schleiden (1804 – 1881), who looked at numerous plant samples. Schleiden was the first to recognize that all plants, and all the different parts of plants, are composed of cells. While having dinner with zoologist Theodor Schwann (1820 – 1882), Schleiden mentioned his idea. Schwann, who came to similar conclusions while studying animal tissues, quickly saw the implications of their work. In 1839, he published “Microscopic Investigations on the Accordance in the Structure and Growth of Plants and Animals,” which included the first statement of the cell theory: All living things are made up of cells. Then, in 1858, Rudolf Virchow (1821 – 1902) extended the work of Schleiden and Schwann by proposing that all living cells must rise from pre-existing cells. This was a radical idea at the time because most people, scientists included, believed that nonliving matter could spontaneously generate living tissue. The inexplicable appearance of maggots on a piece of meat was often given as evidence to support the concept of spontaneous generation. But a famous scientist by the name of Louis Pasteur (1822 – 1895) set out to disprove spontaneous generation with a now-classic experiment that both firmly established the cell theory beyond doubt and solidified the basic steps of the modern scientific method.
The steps of Pasteur’s experiment are outlined below: 1. First, Pasteur prepared a nutrient broth similar to the broth one would use in soup. 2. Next, he placed equal amounts of the broth into two long-necked flasks. He left one flask with a straight neck. The other he bent to form an “S” shape. 3. Then he boiled the broth in each flask to kill any living matter in the liquid. The sterile broths were the left to sit, at room temperature and exposed to the air, in their openmouthed flasks. 4. After several weeks, Pasteur observed that the broth in the straight-neck flask was discoloured and cloudy, while the broth in the curved-neck flask had not changed. 5. He concluded that germs in the air were able to fall unobstructed down the straightnecked flask and contaminate the broth. The other flask, however, trapped germs in its curved neck, preventing them from reaching the broth, which never changed colour or became cloudy. 6. If spontaneous generation had been a real phenomenon, Pasteur argued, the broth in the curved-neck flask would have eventually become reinfected because the germs would have spontaneously generated. But the curved –neck flask never because infected, indicating that the germs could only come from other germs. Pasteur’s experiment has all of the hallmarks of modern scientific inquiry. It begins with a hypothesis and it tests that hypothesis using a carefully controlled experiment. This same process – based on the same logical sequence of steps – has been employed by scientists for nearly 150 years. Over time, these steps have evolved into an idealized methodology that we now know as the scientific method
B. Cell Cycle The cell cycle is an ordered set of events, ending in cell growth and division into two daughter cells. Non-dividing cells not considered to be in the cell cycle. The stages, pictured below are G1-S-G2-M. The first three stages are considered the growing stages, called Interphase, no cell division occurs in this section of the cell cycle. The G1 state stands for “Gap 1”, in this stage the cell grows in size. The S state stands for “Synthesis”, in this stage the DNA replicates. The G2 State stands for “Gap 2”, in this stage the cell prepares to divide. The M state stands for “Mitosis”, in this stage the cell divides
In addition to the four stages of the cell cycle, there are two checkpoints that are built in to help insure the survival of the cell. The first checkpoint is at the end of G1. At this point the cell decides if it should continue to prepare to replicate by determining if its size is large enough and if conditions are suitable to divide (enough nutrients). If the cell decides it should not replicate it enters the G0 state, which just gives the cell time continue to grow or if conditions are not suitable to replicate, the cell dies. The second checkpoint is at the end of G2, this checkpoint is similar to the first, but here the cell also checks that the DNA has been properly replicated. If a cell did not have these checkpoints, the cells would replicate uncontrollably, which is what happens during the formation of cysts or tumours. The major effects of cancer are caused by cells whose DNA has been improperly replicated, but the cell has not properly check the replication. The cell skips all checkpoints and continues to replicate. This causes abnormal growths on or inside the organism.
C. Mitosis Mitosis is nuclear division plus cytokinesis, and produces two identical daughter cells during prophase, metaphase, anaphase, and telophase. Cells begin the cell cycle as a diploid cell (contains both copies of each chromosome) and end as two identical diploid cells. Interphase is often included in discussions of mitosis, but interphase is not part of mitosis, but rather encompasses stages G1, S, and G2 of the cell cycle. Interphase The cell is engaged in metabolic activity and performing it’s preparations for mitosis (the next four phases that lead up to and include nuclear division). Chromosomes are not clearly discerned in the nucleus, although a dark spot called the nucleolus, the area in the nucleus that the DNA is organized, may be visible. The cell may contain a pair of centrioles (or microtubule organizing centers in plants) both of which are organizational sites for spindle fibers. Prophase DNA in the nucleus begins to condense and becomes visible in the light micoroscope as chromosomes. The nucleolus disappears. Centrioles begin moving to opposite ends of the cell and fibers begin to extend toward the center. Prometaphase is a subphase at the end of prophase. During this phase the nuclear membrane dissolves. Proteins attach to the center of the chromosome creating kinetochores. The fibers attached to the kinetochores and begin to move the chromosomes toward a line across the cell. Metaphase Spindle fibers align the chromosomes along the middle of the cell nucleus. This line is referred to as the metaphase plate. This organization helps to ensure that in the next phase, when the chromosomes are separated, each nucleus will receive one copy of each chromosome.
Anaphase The paired chromosomes separate at the kinetochores and move to the opposite sides of the cell. Motion results from kinetochore movement along the spindle fibers.
Telophase Chromatids arrive at opposite poles of the cell, and new membranes form around the daughter nuclei. The chromosomes disperse and are no longer visible under the light microscope. The spindle fibers disperse, and cytokinesis, the partitioning of the cell begins. In animal cells, cytokinesis results when a protein called actin around the center of the cell contracts pinching the cell into two daughter cells, each with one nucleus. In plant cells, the ridid wall requires that a cell plate be synthesized between the two daughter cells.
D. Meiosis In meiosis, cells divide to create male and female gametes needed for sexual reproduction. The cell begins as one diploid cell and divides into four haploid cells. Haploids only have one of each chromosome pair, therefore when the two gametes combine they create a diploid cell with a complete set of chromosomes. Meiosis is divided into two cell divisions. In meiosis I, one cell divides into two haploid cells. In meiosis II, the two haploid cells divide again into four haploid cells. Both of these stages have similar stages to Mitosis and the nucleus and fibers follow the same pattern in each stage. The only difference between the two types of cell division is the amount of DNA being separated. Meiosis I Prophase I DNA replication occurs during Interphase as it did during the cell cycle described before. Although with meiosis, the starting cell has two copies of each pair of chromosomes. During prophase, DNA condenses and identical chromosomes pair and fuse forming bivalents. Prometaphase I is indicated by the disappearance of the nuclear membrane. Fibers form and attached to kinetochores on each chromosome. The bivalents begin to move to the center of the cell. Metaphase I Bivalents align at the metaphase plate. Anaphase I Bivalents separate and chromosomes are pulled to either side of the cell. Each daughter cells is now a haploid (23 chromosomes), but each chromosome has two chromatids. Telophase I Nuclear envelopes may reform or the cell may quickly start meiosis II. Cytokinesis occurs and the cell becomes a complete two complete daughter cells. Meiosis II These four stages are similar to mitosis. However, there is no “S” phase in the interphase between Meiosis I and Meiosis II. Also cells entering Meiosis II are no longer diploids as they would be with mitosis. The end result of Meiosis II is each of the haploid cells dividing into two haploid cells with only one chromatid from the 23 chromosomes.