Then, in the second part of anaphase - sometimes called anaphase B - the astral microtubules that are anchored to the cell membrane pull the poles further apart and the interpolar microtubules slide past each other, exerting additional pull on the chromosomes (Figure 2). For example, human DNA sums to almost 2 m, whereas most of our cells are only less than. More specifically, in the first part of anaphase - sometimes called anaphase A - the kinetochore microtubules shorten and draw the chromosomes toward the spindle poles. Meanwhile, changes in microtubule length provide the mechanism for chromosome movement. Upon separation, every chromatid becomes an independent chromosome. Enzymatic breakdown of cohesin - which linked the sister chromatids together during prophase - causes this separation to occur. Metaphase leads to anaphase, during which each chromosome's sister chromatids separate and move to opposite poles of the cell. Kinetochore microtubules attach the chromosomes to the spindle pole interpolar microtubules extend from the spindle pole across the equator, almost to the opposite spindle pole and astral microtubules extend from the spindle pole to the cell membrane. In addition, the spindle is now complete, and three groups of spindle microtubules are apparent.
At this point, the tension within the cell becomes balanced, and the chromosomes no longer move back and forth. Every chromosome has at least two microtubules extending from its kinetochore - with at least one microtubule connected to each pole. Some textbooks list five, breaking prophase into an early phase (called. This process ensures that each daughter cell will contain one exact copy of the parent cell DNA.Īs prometaphase ends and metaphase begins, the chromosomes align along the cell equator. Mitosis consists of four basic phases: prophase, metaphase, anaphase, and telophase. As they move, they pull the one copy of each chromosome with them to opposite poles of the cell. The spindle tubules then shorten and move toward the poles of the cell.
As mitosis progresses, the microtubules attach to the chromosomes, which have already duplicated their DNA and aligned across the center of the cell. These tubules, collectively known as the spindle, extend from structures called centrosomes - with one centrosome located at each of the opposite ends, or poles, of a cell. Early microscopists were the first to observe these structures, and they also noted the appearance of a specialized network of microtubules during mitosis. The word "mitosis" means "threads," and it refers to the threadlike appearance of chromosomes as the cell prepares to divide. Mitosis is the process in which a eukaryotic cell nucleus splits in two, followed by division of the parent cell into two daughter cells.