It was discovered in 1858, by Rudolf Virchow, that new cells can only arise from previously existing cells. This is done in two ways: mitosis and meiosis. Somatic (body) cells divide exclusively by mitosis followed by cytokinesis, while germ cells produce gametes by the process of meiosis. Plant cells grow by enlargement, essentially by taking up water. When they reach a certain size, they divide, forming two identical daughter cells. The various parts of the cell are divided in such a way that the new daughter cell is identical to the parent cell.
Strictly speaking, mitosis implies only the division of the nucleus, and is therefore distinct from cell division, in which the cytoplasm is divided. In most organisms, cells divide by ingrowth of the cell wall, if present, and the contraction of the cell membrane, a process that cuts through the spindle fibers. In land plants (bryophytes and vascular plants) and a few algae, cell division takes place by the formation of a cell plate. Small droplets appear across the equatorial plate of the cell and gradually fuse, forming a disc that grows outward until it reaches the wall of the dividing cell, which completes the separation of the two daughter cells.
The DNA of prokaryotes is simply replicated before division. In eukaryotes, however, the hereditary material is part of their complex chromosomes. Equal division of this material requires a more complex method by which the chromosomes are replicated, separated, and apportioned precisely between the daughter cells.
Mitosis, or nuclear division, ensures the equal division of the nuclear material between the daughter cells in eukaryotic organisms. During mitosis the chromosomes condense, and move to the center of the cell where they fully contract. They then split longitudinally into two identical halves that appear to be pulled to opposite poles of the cell by a series of microtubules. In these two genetically identical groups, the coiling of the chromosomes relaxes again, and they are reconstituted into the nuclei of the two daughter cells. It is a continuous process that can be divided into five major phases: interphase, prophase, metaphase, anaphase, and telophase.
In this experiment, you will
- Examine and compare the phases of mitosis in animal and plants cells.
- Determine the relative time cells spend in each phase of mitosis.
- Prepare microscope slides of mitotic cells using onion Allium root tips.
- Follow the processes of mitosis and meiosis in the life cycle of Sordaria.
- Examine the arrangement of Sordaria ascospore microscopically to determine the frequency of crossing over.
- Calculate the distance, in map units, between a specific gene and the chromosome centromere.