Grade 7 Standard: Life Science 1e Students know cells divide to increase their numbers through a process of mitosis, which results in two daughter cells with identical sets of chromosomes. Major concepts: 1. One way that cells can propagate is by dividing into two identical copies. Eukaryotic cells undergo a process called mitosis. Most importantly, during mitosis each resulting cell must have a complete copy of the genetic information present in the parental cells DNA. 2. Mitosis generates exact copies of each of chromosomes present in the parental cell and places one copy of each into each daughter cell. By so doing mitosis creates exact genetic replicas of the original cell. 3. Cell division is an elaborate process. During mitosis the cell first replicates the entire DNA present in the chromosomes, generating a second copy of each one. Later, these chromosomes segregate to the two daughter cells. In eukaryotic cells the processes of DNA replication and division are separated in time. Cells have a schedule for accomplishing these two processes; this schedule is called the cell cycle. Notes: Cell Reproduction: From One Cell to Two Why do Cells Reproduce? 1. to create the next generation 2. for growth - multicellular organisms 3. for repair - replace damaged cells Types of cell division 1. Binary fission - bacteria and other monerans 2. Mitosis - eukaryotes; protista, fungi, plants, and animals Chromosomes Chromosomes contain linear molecules of DNA-one per chromosome-packaged into a complex structure with special protein molecules. Before a cell can divide, its DNA content must be duplicated (so that identical copies can be passed on to the next generation). This creates the familiar X-shaped structures that you probably associate with chromosomes. In these are two identical structures, called sister
chromatids. The linear structure and the X-shaped structure (before and after replication) are called chromosomes. Sister chromatids refer to the two halves of the duplicated chromosome. When mitosis occurs sister chromatids separate, move to separate cells, and become chromosomes. The place where the sister chromatids touch (the crossing point of the X) is called the centromere. This is an important structure since it is the place at which the machinery responsible for moving DNA into the new daughter cells attaches. Ploidy and chromosome number Most organisms have two parents--a mother and a father-both of whom contribute genetic material. These organisms must have a maternal and a paternal copy of each chromosomes, the highest level of organization of genetic material. A cell carrying two copies of each chromosome is called diploid. (One with four copies, which occurs in some plants, is called tetraploid, and one with only a single copy, as occurs in germ cells, is called haploid). Human cells have 23 pairs of chromosomes, or 46. Some species have very few chromosomes (four pairs in the fruit fly, Drosophila melanogaster), while some have enormous numbers (such as the 108 pairs in the Horsetail). But each is still matched with one and only one identical partner. Binary Fission Bacteria have just one circular chromosome and no organelles except ribosome and the plasma membrane. The dividing cell replicates its chromosome and attaches the original and its copy to the plasma membrane. As the cell and its membrane elongate the two sister chromosomes are teased apart. Finally the cell pinches in two. Each new cell has one chromosome and about half of the cytoplasm and ribosomes. This entire process can be completed in about 20 minutes. The cell cycle in eukaryotes Eukaryotic cells require a much more involved process for cell division. The periods between 2 cell divisions is called the cell cycle. This cycle is subdivided into 2 main steps 1. Interphase 2. Mitosis
Interphase has three periods 1. G1 phase during which the cell replaces organelles and grows in size. 2. S phase in which the cell replicates its DNA. 3. G2 phase during which the cell prepares for cell division. Mitosis Chemical signals at the end of interphase trigger the beginning of mitosis Mitosis has four stages: Prophase, Metaphase, Anaphase, and Telophase. Prophase (Examples shown are from plant cells) 1. Prophase is noticeable when the chromosomes coil, becoming so thick that they are visible under the light microscope. This process also shortens the chromosomes to a manageable length. Each "chromosome" at this stage is actually 2 sister chromatids held together at a special junction called the centromere. 2. The nuclear membrane is disassembled 3. The nucleolus disappears
4. In animal cells 2 centrioles begin to separate and move to opposite poles of the cell. 5. Spindle fibers take shape Metaphase The spindle fibers attached to the centromeres jostle and tug the chromosomes around for awhile, each side competing with the other, until all the chromosomes are aligned at the center of the cell -- an area called the metaphase plate (the cell's equator). Metaphase may be identified by the tight line of sister chromatids located on or near the metaphase plate. Anaphase Anaphase begins when the centromeres of each chromosome divide followed by the rapid untangling of sister chromatids as the move toward opposite poles of the cell (see above). Each chromatid is an exact copy of the original chromosome. Anaphase is distinguished by two distinct rows of V-shaped chromosomes. The space between them increases until they are moved to the poles of the cell. Telophase
The final (telo) phase in which chromosomes arrive at the polar regions of the cell is called telophase. During this phase the chromosomes uncoil, the nucleus and nucleolus reform, and the spindle disassembles. Most steps are just the opposite of prophase. Cytokinesis Concurrently or at some time after telophase the cell completes division by forming two new daughter cells, replacing the original cell. This step is called cytokinesis. Animal cells accomplish this by pinching apart at the metaphase plate. Plant cells manage cell division by making a new cell wall at the metaphase plate.