Mitosis: Videos & Practice Problems

Mitosis is a crucial process of cell division that results in the formation of two identical daughter cells, playing a vital role in the growth and repair of tissues in the body. The majority of cells, including skin, kidney, and liver cells, undergo mitosis to produce more cells of the same type. The process can be divided into several key phases, starting with interphase, which is technically not part of mitosis but is essential for preparing the cell for division.

Interphase consists of three main stages: G1, S, and G2. The G1 phase, or the first growth phase, is where the cell grows significantly before DNA replication. During this phase, the cell assesses whether it has reached the appropriate size to divide. If it has not, it may enter the G0 phase, a resting state where the cell will not proliferate until it is ready. If the cell is sufficiently large, it proceeds to the S phase, where DNA replication occurs, resulting in the formation of sister chromatids—two identical copies of a chromosome. Following this, the G2 phase serves as a brief growth period to ensure that DNA replication was successful before entering mitosis.

The first stage of mitosis is prophase, where structures called centrioles, made of microtubules, move to opposite ends of the cell. This movement is crucial as it leads to the formation of spindle fibers, which will later help separate the chromosomes. During prophase, the nuclear envelope begins to break down, and chromatin condenses into visible chromosomes, held together by a protein called cohesion.

Next is prometaphase, where the chromosomes begin to move towards the cell's equatorial plane. This movement is completed in metaphase, where chromosomes align at the metaphase plate. Here, a protein complex known as the kinetochore attaches to the centromere of each chromosome, linking them to the spindle fibers. This attachment is critical for the subsequent separation of sister chromatids.

During anaphase, the sister chromatids are pulled apart as cohesion degrades, allowing them to move toward opposite poles of the cell. If this separation occurs incorrectly, it can lead to nondisjunction, resulting in chromosomal abnormalities such as Down syndrome. Following anaphase is telophase, where the separated chromosomes reach the poles and begin to de-condense back into chromatin. The nuclear envelope re-forms around each set of chromosomes, preparing for the final separation of the two daughter cells.

Cytokinesis, which occurs after mitosis, is the process that divides the cytoplasm and completes cell division. In animal cells, this is achieved through the formation of a cleavage furrow that pinches the cell membrane, while in plant cells, a cell plate forms to create a new cell wall. This final step ensures that two distinct daughter cells are formed, each with a complete set of chromosomes, ready to enter their own interphase and potentially undergo mitosis again.

The cell cycle is a crucial process that governs growth, reproduction, and death in organisms. Its regulation is vital because unregulated cell growth can lead to diseases such as cancer, characterized by the formation of tumors due to uncontrolled cell division. To prevent such issues, the cell cycle incorporates several checkpoints that ensure proper progression through its phases.

Three primary checkpoints are essential for understanding cell cycle regulation: the G1/S checkpoint, the G2/M checkpoint, and the M checkpoint. The G1/S checkpoint occurs before the synthesis (S) phase and verifies that the cell size is adequate for DNA replication. The G2/M checkpoint takes place after the G2 phase and before mitosis, ensuring that DNA has been accurately replicated. Finally, the M checkpoint, which occurs during metaphase, confirms that spindle fibers are correctly attached to chromosomes, facilitating proper chromosome separation during cell division.

These checkpoints function similarly to a security system, where proteins act as regulatory agents. Two key proteins involved in this regulation are cyclin-dependent kinases (CDKs) and cyclins. CDKs are enzymes that add phosphate groups to other proteins, a process that can either activate or deactivate the cell cycle. This phosphorylation process is akin to a police officer checking identification—either allowing progression or halting it based on the cell's status.

Cyclins serve as regulatory proteins that control the activity of CDKs. They are released in response to specific cellular events, activating CDKs when conditions are favorable for cell cycle progression. Conversely, if errors occur, different cyclins can signal CDKs to inhibit the cell cycle, preventing further division until issues are resolved.

The cell cycle is often depicted in a circular diagram, illustrating its phases: G1, S (DNA replication), G2, and mitosis. Interphase, which includes G1, S, and G2, is significantly longer than mitosis. Each phase is monitored by checkpoints, including the G1 checkpoint, S checkpoint, G2 checkpoint, and the spindle assembly checkpoint (M checkpoint), ensuring that the cell only progresses when all conditions are met for successful division.

In summary, the intricate regulation of the cell cycle through checkpoints and the action of cyclins and CDKs is essential for maintaining cellular integrity and preventing diseases such as cancer. Understanding these mechanisms provides insight into how cells control their growth and division, ensuring proper function and health.