13a. Bacteria and other microorganisms - Cell division

Bacteria and other microorganisms are microscopic organisms and thus are only seen when examined by a microscope. For many years my passion was the study of the structure of bacteria and to relate that structure with their function in the environment. The following descriptions are of various representative bacteria, yeast & fungi, and bacterial viruses. Most of the pictures were taken by me with cameras associated with electron microscopes. Those not taken by me will include proper credit. Most of what you will see are magnifications of 50-100,000 times or more, as in the case of the bacterial viruses. In order to "see" these bacteria and othe microorganisms, they have to be prepared in very special ways. Two ways are used in this discussion. First is chemically-fixed cells that are embedded in epoxy and then sliced with a diamond knife into extremely thin sections. A second method freezes the cells very quickly using freon and liquid nitrogen,. This is followed by a fracturing of the frozen cells, etching them, and then making a carbon replica of the fractured and etched surface. This is called Freeze-fracturing or freeze-etching.


In its normal growth phase, prior to any trigger for cell division, the DNA or genetic material of the cell is compact and condensed together as shown in this freeze-etching of Bacillus subtilis. This also illustrates the benefit of freeze-etching as the DNA is clearly visualized within the cytoplasm of the cell. Furthermore there has been no chemical "damage" to the cell.








This cell, also Bacillus subtilis, shows the same cell as above, but is chemically fixed and illustrates the appearance of many different bacteria. In the center of the cell is a lighter material that is the DNA of the cell - its genetic material. Surrounding it is all the cellular material required to grow and reproduce. And surrounding everything is a cell membrane and a cell wall that maintains the integrity of the cell.



This picture shows a similar cell of B. subtilis, but this time it is a freeze-etching. Again, comparing this picture to the one just above it shows the difference between a chemically fixed cell and one that has been feerze-etched. Similar but different!





As the bacterial cell begins to grow by utilizing food material in its environment, it goes through several stages. When it reaches a certain point in growth or size, it triggers a process known as cell division. At this point, the cell begins to make a copy of its DNA . Once made, the two separate strands of DNA are separated by specific parts of the cell membrane. As this happens, another part of the cell membrane begins to fold inward eventually dividing the cell into two equal parts, as illustrated in this picture.








This final pictured of a chemically fixed cell shows a close-up of the final stages in the process of cell divivsion. If you look closely, you can see membranous swirls in close proximity to the closing cell membrane and dell wall (the dark outer layer). These membranous swirls that originated from the cell membrane are believed to contain the machinery to build and maintain membranes and cell walls.