Concept 7 All cells arise from pre-existing cells.
Walther Flemming explains the phases of mitosis.
Hello, I am Walther Flemming. In 1882, I published a detailed description of the process of mitosis — cell division. Mitosis was seen as early as 1844 by Herr Carl Nageli. However, he and many of my colleagues weren't sure if what they were seeing in stained, fixed (dead) cells really happened in living organisms. I showed that mitosis is a natural event. I looked at the cells of developing salamander embryos. These cells had big, thick chromosomes made visible by aniline dyes I used for staining. A good thing about these cells is they divide simultaneously in a fixed amount of time. I took cell samples at increasing time intervals and figured out the sequence of events for mitosis. Mitosis is a continous process, but I described it as a series of phases, or stages. My terminology is still in use today. INTERPHASE METAPHASE PROPHAS ANAPHASE TELOPHASE When a cell is not undergoing mitosis, I call it the resting stage or interphase. During interphase, the nucleus stains as a dense granular body. Of course, the cell is not really resting. Nuclear material is duplicating. Prophase is the first stage of mitosis. The duplicated nuclear material condenses out of the nucleus as fibers. In my day, I called these fibers chromatin threads. After 1888, they were known as chromosomes. Each duplicated chromosome can be seen as a pair of sister chromatids. Although I didn't note this myself, sister chromatids are joined at a structure called the centromere. Let's zoom in and take a look. SISTER CHROMATIDS CENTROMERE The next stage is metaphase. The nuclear membrane disappears and the sister chromatids move in line with the cell equator. Then the centromere of each chromatid pair divides and the daughter chromosomes begin to move apart. In my stained slides, I saw that spindle fibers were connected with these chromosome movements. During anaphase, the daughter chromosomes are pulled to opposite poles of the cell. The last stage is telophase. The chromosomes become less distinct and membranes form around the new nuclei. The cell pinches at the equator and splits into two daughter cells. So, mitosis starts with one mother cell and produces two daughter cells. The cells all have the same amount of genetic material. MOTHER CELL DAUGHTER CELLS and so on . . . Once I published my paper, there was no doubt that mitosis is a natural process. As Rudolph Virchow said previously in 1858, omnis cellula e cellula, which is Latin for "all cells arise from other cells."
When chromosomes were first identified, there was some debate about whether they were artifacts or real cellular components. Some scientists thought that chromosomes might be contaminants or some type of staining imperfections.
Chromosome number is unrelated to size and the biological complexity of the species. It can range from 2 in roundworm (Ascaris sp.) to 1,260 in Indian fern (Ophioglossum reticulatum). Why is there such a variation?