Mitosis is a biological process through which a single cell divides to produce two genetically identical daughter cells. It ensures each new cell receives a complete set of genetic information. Mitosis is essential for the growth and development of multicellular organisms. It also repairs damaged tissues and replaces old cells.
Setting the Stage: Early Prophase
Mitosis begins with prophase, the first stage of nuclear division. Prior to prophase, during interphase, the cell has already duplicated its DNA, so each chromosome consists of two identical copies called sister chromatids, joined together at a region called the centromere. In early prophase, the diffuse genetic material, chromatin, undergoes a remarkable transformation, coiling and condensing into compact, visible chromosomes.
Concurrently, structures called centrosomes, which serve as microtubule-organizing centers, begin to move apart. As the centrosomes migrate towards opposite poles of the cell, they initiate the formation of the mitotic spindle, a network of protein fibers called microtubules. This early spindle apparatus starts to assemble between the separating centrosomes.
Transformations of Late Prophase
As the cell progresses from early to late prophase, sometimes referred to as prometaphase, several defining events unfold. A significant change is the breakdown of the nuclear envelope, the membrane that encloses the nucleus. This disassembly allows the spindle microtubules, which have been forming in the cytoplasm, to access the condensed chromosomes. The nuclear envelope breaks into small vesicles, which can resemble parts of the endoplasmic reticulum.
With the nuclear barrier removed, the mitotic spindle continues to grow and its microtubules begin to interact with the chromosomes. Specialized protein structures called kinetochores develop on each sister chromatid, specifically at the centromere region. These kinetochores act as attachment points for the spindle microtubules. Each chromosome, with its two sister chromatids, develops two kinetochores, one on each chromatid, positioned to face opposite poles.
Microtubules emanating from the centrosomes actively seek out and “capture” these kinetochores. Once attached, these kinetochore microtubules begin to pull and push the chromosomes. This dynamic interaction, driven by motor proteins, facilitates the movement of chromosomes towards the cell’s equatorial plane. The chromosomes are maneuvered into position for the next stage of mitosis.
Why Late Prophase Matters
The events of late prophase are important for ensuring accurate distribution of genetic material to daughter cells. The breakdown of the nuclear envelope is a prerequisite, as it grants the mitotic spindle direct access to the chromosomes. Without this step, the spindle microtubules would be unable to engage with the chromosomes.
The formation of kinetochores and their attachment to spindle microtubules are important. These attachments are important for aligning chromosomes correctly at the cell’s center during metaphase and for their separation in anaphase. Each sister chromatid must attach to microtubules originating from opposite poles to ensure that one copy of each chromosome goes to each new daughter cell.
Errors during late prophase, such as incorrect kinetochore-microtubule attachments or failure of the nuclear envelope to break down properly, can lead to missegregation of chromosomes. Such errors can result in daughter cells with an abnormal number of chromosomes (aneuploidy), which can contribute to developmental problems or diseases like cancer. The transformations in late prophase prepare the cell for chromosome segregation in subsequent stages of mitosis.