Prophase: Function, Tasks, Role & Diseases

Mitosis proceeds in several phases. Among them, prophase represents the beginning of mitosis. Disruptions in the prophase process prevent the initiation of cell division.

What is prophase?

Both mitosis and meiosis begin with prophase. In both cases, cell division occurs. However, while in mitosis the identical genetic material is passed on to the daughter cells, in meiosis the formation of germ cells takes place with the genetic information being halved. However, germ cells formed in meiosis can also continue to divide via mitosis in the same way as normal somatic cells. The actual mitosis does not include cell division but is characterized by the process of multiplication of identical genetic information with the formation of new cell nuclei. In most cases, however, cell division of the entire cell is associated with it. In a few cases, however, mitosis proceeds without further cell division (cytokinesis). Then multinucleated cells are formed, which, among other things, perform various functions in the formation of new cells of the hematopoietic system. The process of mitosis is divided into prophase, prometaphase, metaphase, anaphase and telophase. Prophase always serves to initiate mitosis. Often, prometaphase is counted as prophase because the processes of both subphases occur in parallel.

Function and task

Prophase is followed by the so-called interphase, in which an identical copy of a chromatid is replicated and this is linked to the identical sister chromatid via the centromere. With the completion of interphase, mitosis is prepared. In this phase, chromatin is loosely packed and appears filamentous. Thus, interphase represents the phase between two cell divisions and is not part of mitosis. Mitosis proper then begins with prophase, in which chromatin becomes increasingly condensed by folding. Visible structures can now be detected under the light microscope. These more compact structures make the chromatin transportable, thus creating the conditions for the division of the identical chromatids into the gradually emerging cell poles. At this stage, chromosomes consist of two identical chromatids held together at least at a constriction site, also known as the centromere. A longitudinal cleft appears between the two identical chromatids of the chromosomes. In this compact form, the chromatin is transportable but no longer readable. Therefore, no new proteins are formed during this phase. The nucleoli (nuclear corpuscles) required for this dissolve. In parallel, two centrosomes are formed by division, each positioned on opposite sides of the nucleus, where they begin to form their spindle apparatus. The spindles are composed of microtubules built by polymerization from tubulin subunits. These spindle fibers must make contact with the centromere of the chromosomes during the further stages of mitosis in order to dissolve the centromere and pull the two identical chromatids to their respective poles. In order for the spindle fibers to get there, the nuclear envelope must first be temporarily degraded. The nuclear envelope consists of lamins. These are dissolved by the process of phosphorylation. This occurs during the prometaphase, which is partly associated with the prophase and partly regarded as a separate phase. Protein structures known as kinetochores are located at the centromeres, to which the spindle fibers can dock. Thus, kinetochore-microtubule structures are formed, which arrange themselves parallel to the pole fibers and are responsible for the subsequent transport of the separated chromatids to the poles. During this phase, the spindle apparatus completes itself as the stellate fibers originating from the centrosomes make contact with the remaining components of the cytoskeleton. The assembly of these structures causes the centrosomes to move further and further toward the cell poles. In metaphase, which follows prometaphase, the chromosomes become centrally aligned. In the following anaphase, the separation of identical chromatids occurs at the centromeres. The final phase (telophase) begins with the arrival of chromatids at the poles and ends with the decondensation of chromosomes.

Diseases and disorders

Cell divisions occur in both unicellular and multicellular organisms.In humans, animals and plants, mitosis is the prerequisite for growth and general functioning of the organism. Old cells die and must be constantly renewed. In the course of mitosis, however, it can happen that no completely identical copies of the genetic material are passed on. These are so-called mutations, which can influence the functionality of the newly formed cells. This can result in severe diseases. Cancer also develops as a result of deregulation of cell division due to genetic changes or hormonal misregulation. However, genetic alterations occur mainly between the individual mitoses in interphase or also in the case of incorrect separation of the chromatids in anaphase. In prophase itself, the occurrence of mutations is not possible, since here only structural changes occur due to condensation of the chromosomes. However, disruptions during prophase are always lethal because they prevent the initiation of mitosis. No more cell divisions could take place. Old cells would merely die and would not be replaced by new cells. There are also no known congenital diseases that result from disruption of prophase during mitosis.