Let’s take a second look at mitosis and focus on the phases of chromosome segregation. As we concentrate on chromosomes, you’ll learn the tricky dance that takes place through all five phases of mitosis.
Okay, so we’ve been talking about a lot of different strategies of how to make it easier for a cell to segregate or separate its chromosomes into two daughter cells during mitosis.
We know before mitosis, the chromosomes condense into compact structures that are going to be easier to maneuver. The chromosome movement is orchestrated by microtubules, which organize into a structure known as the mitotic spindle apparatus. This complex structure is held together by the centrosomes at the spindle poles and the kinetochores at the chromosomes.
Now that we have a pretty good idea of how these tricks work individually, let’s see how they all fit together in mitosis.
The Phases of Mitosis
Although mitosis, like cell division, is a continuous process, it’s easier for us to understand if we divide the process into several steps. Mitosis is typically divided into five phases, known as prophase, prometaphase, metaphase, anaphase and telophase.Let’s see what happens to the chromosomes of a simple two chromosome organism in each phase of mitosis.
If this is the diploid organism, recall that means that the organism has two copies of each chromosome, known as homologs, giving the organism a total of four chromosomes in each cell.Actually, before we start talking about mitosis, let’s briefly review the relevant steps that occur during interphase.
During interphase, DNA replication occurs, and after DNA replication is complete, each chromosome is going to be composed of two sister chromatids. However, during interphase, the chromosomes are still loosely packed and not condensed. The centrosomes, which will serve as microtubule organizing centers during mitosis, also duplicate during interphase.
Prophase is the first step in the mitotic process. The prefix ‘pro’, in Greek, means ‘before’. During prophase, the chromosomes condense. The centrosomes begin to form spindle and move into position on opposite sides of the cell.
Sister chromatids are held together by a protein called cohesin at the centromere.
Prometaphase is the second step in mitosis. Prometaphase literally means ‘before the middle stage’ in Greek. During prometaphase, the nuclear membrane breaks down. The centrosomes move to opposite sides of the cell.
Chromosomes are beginning to attach to the mitotic spindle.Remember the guy with the winch at each of the kinetochores? Well, they allow the chromosomes to adjust the length of the kinetochore fibers. By adjusting the length of kinetochore fibers, the chromosomes begin to orient in the middle of the cell at the metaphase plate. The metaphase plate is an imaginary line in the middle of the cell that is halfway between the spindle poles. It is also sometimes referred to as the equatorial plate.You can think of the balanced tension that’s holding the chromosomes at the metaphase plate kind of like two equally strong guys that are in a tug of war. Since the pole on each side is relatively the same, the flag in the middle isn’t really going to move.
This is basically what the case is with the chromosomes. Since the tension on each side of the chromosome is essentially the same, chromosomes are held in place at the metaphase plate.
Metaphase is the third step in mitosis. Metaphase means ‘middle stage’ in Greek. By metaphase, all of the chromosomes are aligned on the metaphase plate.
Since chromosomes are maximally condensed during this phase, scientists often study the karyotype of metaphase chromosomes.
Recall that we were thinking about metaphase chromosomes as being held in place by equal tension exerted by the kinetochore microtubules. The only thing preventing each chromatid from shooting toward its respective spindle pole is the protein cohesin, which is holding the chromatid’s centromeres together.
Anaphase is the fourth step in mitosis. Now, once all the chromosomes are at the metaphase plate, we’re ready to begin segregating the chromosomes. A protein called separase cleaves the cohesin protein that’s been holding the centromeres together.
You can think of separase as kind of like a pair of molecular scissors that cleaves the cohesin protein.And with the physical connection between the chromatids gone, the chromatids are free to move toward each pole. Separation of the sister chromatids marks the beginning of anaphase. Anaphase ends when the chromosomes reach the spindle poles.
This marks the beginning of telophase, the fifth and final step in mitosis.
The prefix ‘telo’ means ‘end’ in Greek. During telophase, the spindle breaks down. The nuclear membrane reforms around the chromosomes, establishing two nuclei. Chromosomes begin to uncoil back to the level of packaging found in chromatin during interphase.
The end of telophase also marks the end of mitosis.
To summarize, during interphase, DNA replication occurs and the centrosomes duplicate.Prophase is the first step in mitosis. During prophase, the chromosomes condense, the centrosomes move to opposite sides of the cell and the mitotic spindle begins to assemble.Prometaphase is the second step in mitosis. During prometaphase, the nuclear envelope breaks down and the chromosomes attach to the mitotic spindle and begin to orient toward the metaphase plate.
The metaphase plate is an imaginary line in the middle of the cell halfway between the spindle poles where the chromosomes align during mitosis. The metaphase plate is also sometimes referred to as the equatorial plate.Metaphase is the third step in mitosis. At metaphase, all of the chromosomes are aligned at the metaphase plate.
Anaphase is the fourth step in mitosis. During anaphase, the physical connection between sister chromatids is severed, allowing the chromatids to move toward their respective spindle pole.Telophase is the fifth and last step in mitosis. During telophase, the mitotic spindle breaks down, the chromosomes uncoil to the diffuse interphase state, and the nuclear membrane reforms.Oh, boy! Is this still all Greek to you? Well, here’s another little mitosis tune for your studying pleasure:Prophase is step number oneIn step two, alignment’s just begunAttach, align, where is the middle?Mitotic spindle solves this riddleAnaphase, the sisters severTelophase ends this endeavor