Mitosis is a biological process where a single cell divides to produce two genetically identical daughter cells. This division is essential for an organism’s growth, tissue repair, and the replacement of old cells. While mitosis is a continuous process, it is understood by dividing it into stages, each marked by specific cellular events.
Understanding Mitosis
Mitosis plays a role in the maintenance and development of multicellular organisms. It increases cell numbers, underpinning growth from a single fertilized egg into a complex organism. This process also facilitates the renewal of cells in tissues where cells are regularly shed and replaced. It creates two daughter cells that are exact genetic copies of the parent cell, ensuring genetic stability.
Getting Ready: Interphase
Before a cell undergoes mitosis, it prepares during interphase. Interphase is a crucial preparatory period involving cell growth and genetic material duplication. It is divided into three sub-phases: G1, S, and G2.
In G1, the cell grows, synthesizes proteins, and increases organelles. In S phase, its entire DNA content is replicated, resulting in two identical sister chromatids for each chromosome. In animal cells, the centrioles, which organize the mitotic spindle, also duplicate during this phase. G2 involves further cell growth and the synthesis of additional proteins, ensuring the cell has all necessary components for successful division.
The Stages of Mitotic Division
Mitosis consists of four distinct stages: Prophase, Metaphase, Anaphase, and Telophase. These stages involve the organization and separation of the duplicated chromosomes into two new nuclei. The coordinated movements during these phases ensure that each daughter cell receives a complete and identical set of chromosomes.
Prophase
In prophase, chromatin begins to condense and coil into visible, compact chromosomes. Each chromosome is seen as two identical sister chromatids joined at a region called the centromere. The nuclear envelope, which encloses the genetic material, starts to break down, and the nucleolus often disappears. The mitotic spindle, a structure made of microtubules that guides chromosome movement, also begins to form between the centrosomes as they move towards opposite ends of the cell.
Metaphase
In metaphase, the condensed chromosomes align along the cell’s equatorial plane, known as the metaphase plate. This alignment is a critical checkpoint to ensure accurate chromosome segregation. Spindle fibers attach to a protein structure called the kinetochore, located at the centromere of each sister chromatid. The opposing pulling forces from the spindle fibers attached to each sister chromatid help maintain their position at the cell’s center.
Anaphase
Anaphase is characterized by the separation of the sister chromatids. The centromeres holding the sister chromatids together divide, transforming each chromatid into an individual chromosome. These newly separated chromosomes are then pulled by the shortening spindle fibers towards opposite poles of the cell. Motor proteins associated with the spindle fibers facilitate this movement. As chromosomes move, the cell elongates.
Telophase
Telophase represents the final stage of nuclear division. As the chromosomes arrive at the cell poles, they begin to decondense and uncoil. A new nuclear envelope reforms around each set of chromosomes at both poles, creating two distinct daughter nuclei. The nucleoli also reappear within these newly formed nuclei, and the mitotic spindle disassembles.
Completing the Split: Cytokinesis
Following the nuclear division of mitosis, the cell undergoes cytokinesis, which is the physical division of the cytoplasm and its contents. This process typically begins during late anaphase or telophase and results in the formation of two separate daughter cells. The mechanism of cytokinesis differs between animal and plant cells due to their structural variations.
In animal cells, cytokinesis occurs through the formation of a cleavage furrow. This furrow is an indentation that deepens around the cell’s equator, pinching the cell into two. A contractile ring forms beneath the cell membrane, tightening to create this furrow. Plant cells, with their rigid cell walls, divide differently, forming a cell plate in the middle of the cell. Vesicles move to the cell’s center and fuse to create this new cell wall, which expands outwards until it connects with the existing side walls, dividing the parent cell into two.