In 1882, biologist Walther Flemming was studying salamander larvae under a light microscope. He observed that each cell's nucleus contained thread-like structures that were dividing lengthwise. This was the first time that scientists had seen chromosomes. Later, scientists discovered that the threads Flemming had seen were sister chromatids separating as the cell went through mitosis. (Mitosis is the Greek word for thread.)
It is important to understand that while chromosome duplication occurs during the interphase section of the cell cycle, the actual separation of chromatids happens during mitosis. How do sister chromatids actually separate? Watch this video to see the process in action.
What actually happens during mitosis is one of the most amazing aspects of cell life. Cell division that results in two genetically identical daughter cells occurs very quickly, but involves five separate phases or steps. Phase 1 is called prophase. During prophase, duplicated chromosomes are visible, and mitotic spindles begin to form. Phase 2 is the prometaphase. During prometaphase, proteins called kinetochore form around the cell’s centromere. Phase 3 is metaphase. During metaphase, the nuclear envelope breaks down, and the spindle fibers attach to the sister chromatids. Phase 4 is Anaphase—when the sister chromatids form two sets of chromosomes as they are pulled apart by spindle fibers. The fifth and final phase of mitosis is telophase. During telophase, the nuclear envelope re-forms, but two nuclei form within the cell. Then the cell separates into two cells, during a process known as cytokinesis. After mitosis is complete, the cell enters the G0 [G-0] phase, which is where the cell remains until it wants to divide again. After the G0 phase, the cell enters the G1, S and G2 phases. During G1, the cell prepares for the S phase. During the S phase replication of DNA happens and during the G2 phase the cell grows in size, adding more cytoplasm and organelles. After the G2 phase the cell re-enters prophase. Let’s take a look at the whole mitosis process. First the duplicated chromosomes are visible, and mitotic spindle begin to form. Then proteins called kinetochore form around the centromere. Next, the nuclear envelope breaks down, and the spindle fibers attach to the sister chromatids. Then the sister chromatids form two sets of chromosomes as they are pulled apart by spindle fibers. Finally, the nuclear envelope re-forms, and two nuclei form within the cell. After mitosis there are two daughter cells with identical chromosomes.
Question
If you are a scientist looking to stop cells from growing, which phase in the cell cycle would be of interest to you?
