Cells must continually grow and divide in order for an organism to grow, maintain its structure, and reproduce. Cell division involves the replication, or copying, of the complete set of hereditary information. It also involves the equal distribution of the genetic material in the resulting cells. The hereditary information of organisms is contained in large molecules called deoxyribonucleic acid, DNA.
Upon completion of this laboratory you will be able to: 1. list the similarities and differences between mitosis and meiosis. 2. describe and recognize the stages of mitosis under a microscope or on models. 3. know the events that occur during each stage of mitosis and meiosis. 4. define the boldface terms.
Chromosomes (Figure 1) are tiny rod-shaped structures in the cell’s nucleus that carry the genetic message. A single complete set of chromosomes for an organism is referred to as the haploid number of chromosomes or 1n. Most organisms are diploid, having two complete sets of chromosomes, 2n. All species contain a specific number of chromosomes; for example humans have 23 pairs of chromosomes (for a total of 46). The matched chromosomes are called homologous chromosomes. They are alike in their size, structure, and the genes that they carry. Thousands of genes may be located on a single chromosome. A gene is the set of instructions for one protein product. It is estimated that humans have approximately 30,000 genes. |[pic] |[pic] |[pic] | |Electron micrograph |Artist’s rendition |Chromosome under increasing magnification |
Figure 1. Several renditions of the structure of a chromosome. Genetic material doesn’t always appear as it does in Figure 1. Usually it exists as loose strands of DNA and protein. A completely uncoiled human DNA strand in a single cell can be up to three feet long! Just before cell division the DNA strand coils, condensing 100X to form a chromosome. This structure helps prevent tangling and breakage of the genetic material during cell division.
Two Cells From One – Mitosis
Even after you are completely grown your somatic cells continue to grow and divide. Somatic cells are those cells that make up the structure of the body and include all the cells in the body except reproductive cells. Thus somatic cells are not involved in passing the genetic information from one generation to the next. A somatic cell’s nucleus divides through a process called mitosis. Following a mitotic cell division the two resulting daughter cells are genetically identical to each other and to the parent cell. The original parental cell and the two resulting daughter cells contain the same number of chromosomes and, barring mutations, possess identical copies of the same genetic blueprint. Procedure:
1. Watch the animations on mitosis:
Plant cell mitosis: http://web.grcc.cc.mi.us/biosci/pictdata/mitosis/planmito.htm Animal cell mitosis: http://www.cellsalive.com/mitosis.htm 2. Obtain a prepared slide of Allium (onion) root tip. Root tips in plants contain cells that are constantly undergoing mitotic divisions. 3. Using the 430x objective select a region of the root tip that is one cell thick from the area designated in Figure 2; the mitotic stages will be clear here. Locate a region that contains the mitotic stages shown in Figure 3. 4. Work with your partner to quiz each other on your ability to identify the different mitotic stages. [pic]
Figure 2. Allium (onion) root tip.
Interphase Prophase Metaphase Anaphase Telophase Figure 3. Stages of Plant Cell Mitosis
5. Draw in the labeled space provided the appropriate mitotic stage (prophase,...