Mitosis: Cell Division and Chromosome Segregation

Mitosis is a type of cell division that results in two daughter cells, each with an identical set of chromosomes to the original cell, or mother cell. This process is essential for growth, development, and tissue repair in multicellular organisms and is also used by single-celled eukaryotes like yeast for reproduction. Mitosis occurs in four main phases: prophase, metaphase, anaphase, and telophase.

Prophase

In prophase, the chromosomes begin to condense, making them easier to separate during mitosis. The mitotic spindle, a structure made of microtubules that help organize and move chromosomes, starts to form between the two centrosomes, which are located at the opposite ends of the cell. The protein "glue" that holds the sister chromatids (the two identical halves of a chromosome) together is broken down, allowing them to separate.

Metaphase

During metaphase, the chromosomes align along the cell's equator. The spindle microtubules attach to the chromosomes' kinetochores, which are protein structures at the centromere of each chromatid. The chromosomes are pulled by these microtubules until they are balanced, with equal tension on both sides of the cell. The spindle is now complete, and three groups of spindle microtubules are apparent: kinetochore microtubules attach the chromosomes to the spindle pole, interpolar microtubules extend from the spindle pole across the equator, and astral microtubules extend from the spindle pole to the cell membrane.

Anaphase

In anaphase, each chromosome's sister chromatids separate and move to the opposite poles of the cell. This separation occurs through the enzymatic breakdown of cohesin, which holds the sister chromatids together during prophase. The kinetochore microtubules shorten, and the astral microtubules exert additional pull on the chromosomes. Once the sister chromatids are separated, each becomes an independent chromosome.

Telophase

In telophase, the chromosomes begin to decondense, returning to their long, stringy, decondensed form. The nuclear envelope reforms around the two groups of chromosomes, and the mitotic spindle disassembles. The two groups of chromosomes are now ready for cytokinesis, the process of dividing the cell's contents, to create two daughter cells.

Mitosis is a highly regulated process that ensures the accurate segregation of chromosomes, and any errors in this process can lead to issues such as aneuploidy, which is associated with diseases like Down syndrome. The accuracy of mitosis is crucial for the proper functioning of cells and the overall health of an organism.