Mitosis is a fundamental biological process where a single parent cell divides to produce two daughter cells that are generally identical to itself. This process is essential for the growth and maintenance of living organisms. While mitosis is highly accurate, ensuring genetic continuity, rare instances occur where perfect cellular identity is not achieved. This article explores the reasons for this identity, its maintenance mechanisms, and potential exceptions.
The Purpose of Mitosis
Identical cell division is central to multicellular life and the propagation of many organisms. Mitosis drives the increase in cell number necessary for growth and development, transforming a single fertilized egg into a complex organism with trillions of cells.
Mitosis also plays a role in the repair and replacement of cells throughout an organism’s life. Tissues like skin, blood, and the digestive tract lining constantly shed old or damaged cells, which are then replenished by new, identical cells through mitotic division. The liver, for example, can regenerate lost tissue primarily through mitotic division. For many single-celled organisms, such as bacteria and yeast, and some plants, mitosis serves as the primary method of asexual reproduction, generating new individuals that are genetically exact copies of the parent.
The Mechanism of Identical Duplication
The precise duplication in mitosis stems from a series of carefully orchestrated steps. Before mitosis, a cell copies its entire DNA. This DNA replication process is highly accurate, with specialized enzymes called DNA polymerases using proofreading mechanisms that reduce errors to as low as one mistake per billion nucleotides. This copying ensures each new cell receives a complete set of genetic instructions.
Following DNA replication, the long, thread-like DNA molecules condense into compact structures known as chromosomes, each consisting of two identical copies called sister chromatids, joined together. These duplicated chromosomes then align along the cell’s central plane. Microtubules, which are protein fibers, attach to specific regions on the chromosomes and pull the sister chromatids apart to opposite ends of the cell. This segregation ensures each daughter cell receives one complete set of chromosomes. The final step, cytokinesis, involves the division of the cell’s cytoplasm, separating the two newly formed nuclei into two distinct, genetically identical daughter cells.
When Cells Aren’t Quite Identical
While mitosis is a highly regulated and accurate process, it is not entirely infallible. Errors can occasionally arise, leading to daughter cells that are not perfectly identical to the parent. One source of deviation is mutations that occur during DNA replication. Although DNA replication machinery has proofreading capabilities, rare errors can still slip through, resulting in slight changes to the genetic code in the daughter cell.
More significant deviations can occur through chromosomal abnormalities, where entire chromosomes or large parts are incorrectly distributed during cell division. This happens if chromosomes fail to separate properly, a phenomenon known as non-disjunction, leading to daughter cells with an incorrect number of chromosomes, a condition called aneuploidy. Down syndrome, for example, results from an extra copy of chromosome 21. These mitotic errors, though infrequent in healthy cells, can have significant implications, including cell death or contributing to the development of diseases like cancer if affected cells continue to divide.