Cells spend most of their time in a stage called interphase. During this phase, the nuclear envelope surrounds the nucleus. There may be one or more nucleoli (dark, condensed regions) visible within the nucleus. The material around the nucleoli, contained within the nuclear envelope is DNA in the form of chromatin. This will not pick up a stain well and so will not appear as distinct shapes within the nucleus. Find these indicators of interphase in cell A in the image below.
Figure \(\PageIndex{1}\): Cells in an onion root in interphase and prophase. Cell A has a large, dark nucleolus surrounded by greyish material (chromatin) that is enclosed within the nuclear membrane. A cell wall makes a box around each cell and the plasma membrane would be located just inside this box, though we cannot easily see it. In cell B, the chromatin is condensing and begins to look like dark, thick strands. It is still contained in the center of the cell, as the nuclear envelope has not finished dissolving.Photo by Maria Morrow, CC BY-NC.
In contrast, when a cell begins the process of division, the chromatin condenses into visible chromosomes that will pick up a stain and look like dark strings within the nuclear envelope, as seen in cell B in the image above.
In order to begin dividing, the cell needs to go through several processes that take place during interphase, including replicating the DNA (occurs in S-phase) and all of the cell contents.
Mitosis
Mitosis is the process of dividing the nucleus. To see cells in meiosis, we look in areas of a plant that would be actively growing. These areas where cells are actively dividing are called meristems, such as the root apical meristem.
Figure \(\PageIndex{2}\): A long section of an onion root tip. The edge of the growing tip has been outlined, attempting to show the location of the protoderm cells (located approximately on the black line) vs. the root cap, which would be located to the exterior of the black line. The root apical meristem is indicated by an arrow that points to the center of the U-shaped line. All cells in the root are derived initially from the root apical meristem. Photo by Maria Morrow, CC BY-NC.Figure \(\PageIndex{3}\): Cells in this onion root tip were caught in various stages of the cell cycle. The white arrow indicates the location of the root apical meristem. The box highlights an area where cells can be seen in most stages of mitosis. A stain has been used to show regions of dense nucleic acids, such as chromosomes and the nucleolus. Photo by Maria Morrow, CC BY-NC.
Prophase
Figure \(\PageIndex{4}\): Cell A is in late prophase. The chromatin has condensed into visible chromosomes that form dark strands and the nucleolus is no longer visible. In cell B, the chromatin is condensing and some chromosomes are visible, but the nucleolus has not yet dissolved. In cell C, the nuclear envelope is gone, no nucleoli are visible, and the chromosomes are separating from each other. In cell D, the nuclear envelope and nucleoli are distinct and the chromatin has not yet condensed. Can you put these four cells in order by where they are in the process of mitosis? Photo by Maria Morrow, CC BY-NC.
Metaphase (including Prometaphase)
Figure \(\PageIndex{5}\): Cell A is in prometaphase. The chromosomes are no longer contained within a nuclear envelope. The spindle fibers are attaching to the kinetochores, but the chromosomes have not yet been pulled into a line. Cell B is in metaphase and the chromosomes are loosely aligned on the metaphase plate. Photo by Maria Morrow, CC BY-NC.
Anaphase
Figure \(\PageIndex{6}\): Cell A is in anaphase. The sister chromatids have been pulled apart and now there are two distinct groups of chromosomes on either side of the cell. Photo by Maria Morrow, CC BY-NC.
Telophase
Figure \(\PageIndex{7}\): Cell C is in telophase. There are two dark regions where the chromosomes are clustered and decondensing, becoming indistinguishable from each other. A fuzzy line is forming between them, indicated by a black arrow, showing cytokinesis is happening as a new cell wall forms. Cell A is the same cell that is shown in Figure \(\PageIndex{6}\). In cell B and D, the chromosomes are grouped together in the center of the cell, though they appear much more orderly in cell D. In cells E and F, the chromosomes are in two distinct groups on either side of the cell, but are still distinguishable as individual strands. In the two cells indicated by G, there are distinguishable nucleoli and a clear nuclear envelope. Can you place these cells in order for where they are in the cell cycle? Photo by Maria Morrow, CC BY-NC.
Cytokinesis
Figure \(\PageIndex{8}\): Cell D is in telophase. Cytokinesis is not part of mitosis, but happens concurrently with telophase. There are two dark regions where the chromosomes are clustered and decondensing, becoming indistinguishable from each other. A fuzzy line is forming between them, indicated by a white arrow, showing cytokinesis is happening as a new cell wall forms. Cell A has distinguishable chromosomes and a nucleolus. Cell B has partially condensed chromosomes, two nucleoli, and a clear nuclear envelope. Cell C has two groups of chromosomes being pulled to opposite sides of the cell. Can you place these cells in order for where they are in the cell cycle? Photo by Maria Morrow, CC BY-NC.
