Artem has a doctor of veterinary medicine degree.
What Is Prometaphase?
Don't you hate it when one science textbook tells you one thing and another science textbook doesn't even mention that one thing at all? Science is hard enough to understand, but it becomes even more confusing when scientists can't come to a consensus on what to call or label something. Such is the case with prometaphase, the phase of mitosis that begins when the nuclear envelope starts to break down.
Some textbooks identify prometaphase as a distinct phase of mitosis, the part of the cell cycle that involves the separation of chromosomes and other cellular constituents into two daughter cells. Other textbooks completely avoid mentioning prometaphase as a term. Instead, they include the concepts involved in prometaphase as a part of late prophase or late prophase and early metaphase. So just keep that tidbit in mind in case you are cross-referencing things.
In any case, this lesson focuses on prometaphase as a distinct phase of the cell cycle. Just know that as described herein, prometaphase of mitosis follows open mitosis, which applies to plants and animals. Closed mitosis, which applies to the likes of yeast and slime molds, is not discussed.
The difference between the two lies, in part, with the nuclear envelope. In closed mitosis, chromosome separation occurs in an intact nuclear envelope. In open mitosis, the chromosomes separate after the nuclear envelope disintegrates. That latter sentence should remind you of our definition for prometaphase, as it is a phase of mitosis that begins when the nuclear envelope disassembles into vesicles and begins to sort of look like the smooth endoplasmic reticulum as it does so. If it hasn't already, prior to prometaphase, the nucleolis completely disappears at this point as well. Thus, the first part of early prometaphase involves the disassembly of the nuclear envelope.
The nuclear envelope now looks like a really old t-shirt full of holes and tears. While not pretty, it's pretty important. That's because the holes allow for the penetration of the nuclear envelope by microtubules, cylindrical structures made of a protein called tubule, which supports cellular structures and functions as tracks for the movement of intracellular components. These microtubules grow from the spindle poles towards the nuclear envelope and penetrate the envelope, so the second part of early prometaphase involves the growth of microtubules.
Once the microtubules make their way into the disintegrating nuclear envelope, they superglue themselves onto structures called kinetochores, button-like protein-based structures located on the surface of the centromere. The centromere is a structure that holds together the two sister chromatids of a replicated chromosome. Each sister chromatid has its own kinetochore located at opposite sides of the chromosome. Thus, the third part of early prometaphase involves the attachment of microtubules to kinetochores.
At this point, only one kinetochore is attached to a microtubule. This means the entire chromosome begins to quickly move towards one pole. The first part of late prometaphase involves the movement of a chromosome towards one pole, following the track laid out by the attached microtubule.
But the dividing cell loves to play a game of tug-of-war. It sends out microtubules from the opposite pole to attach to the sister kinetochore of the chromosome. The second part of late prometaphase involves the attachment of a microtubule from the opposite pole of a cell onto the other kinetochore on a chromosome.
This is where that game of tug-of-war can now begin. Since the chromosome is now attached to microtubules emanating from both poles, the third part of prometaphase, the movement of the chromosome to the midpoint between the two spindle poles, begins.
At this point in mitosis, metaphase begins. But our lesson on prometaphase must obviously end. Let's review what we've learned. Prometaphase is the phase of mitosis that begins when the nuclear envelope begins to break down and mitosis is the part of the cell cycle that involves the separation of chromosomes and other cellular constituents into two daughter cells. Prometaphase can be split up into early and late phases. During early prometaphase, the following occurs:
- The nuclear envelope disassembles.
- Microtubules begin to grow. Microtubules are cylindrical structures made of a protein called tubule, which supports cellular structures and functions as tracks for the movement of intracellular components.
- The microtubules attach to kinetochores. Kinetochores are button-like protein-based structures located on the surface of the centromere.
The late phase of prometaphase includes the following key events:
- The movement of a chromosome towards one pole using the track laid out by the microtubule it's attached to.
- The attachment of a microtubule from the opposite pole to the other kinetochore on a chromosome.
- The alignment of chromosomes roughly midway between the two spindle poles.
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