Mitosis: Videos & Practice Problems

Mitosis is a crucial process of cellular division that involves the separation of chromosomes into two daughter cells. Before a cell can enter mitosis, it must progress through the cell cycle, which consists of several phases: G1, S, and G2. During the G1 phase, the cell grows and prepares by accumulating nutrients and growth hormones. The S phase is where DNA replication occurs, resulting in two identical copies of each chromosome, known as sister chromatids. Finally, the G2 phase serves as a preparatory stage before the cell transitions into the M phase, or mitosis.

To initiate mitosis, the cell relies on M Cyclins and cyclin-dependent kinases (CDKs). M Cyclins are proteins that increase in concentration leading up to and during mitosis. They bind to CDKs, activating them to trigger the necessary processes for mitosis. Specifically, the protein cdc25, a phosphatase, removes inhibitory phosphate groups from CDKs, allowing them to bind to M Cyclins and become active. The term "cdc" stands for Cell Division Cycle protein, indicating its role in regulating the cell cycle.

One of the primary functions of activated MCDKs is to instigate chromosomal condensation, which is essential for the orderly separation of chromosomes during mitosis. Chromosomes must be tightly packed to prevent entanglement and ensure efficient movement within the cell. This condensation is facilitated by two key protein complexes: condensins and cohesins. Condensins are responsible for compacting the chromatin into a condensed chromosome, while cohesins hold sister chromatids together at the centromere, ensuring they remain attached until the appropriate stage of mitosis.

Sister chromatids are formed during the S phase when DNA is replicated, resulting in two identical copies of each chromosome. These chromatids are connected by the centromere and held together by cohesins, which play a critical role in maintaining their structure. If cohesins fail to function properly, it can lead to improper separation of sister chromatids, resulting in genetic abnormalities in the daughter cells.

As mitosis progresses, the chromosomes undergo further condensation, becoming more compact and easier to move. This fully condensed structure is referred to as a metaphase chromosome, which is essential for the subsequent anaphase, where sister chromatids are pulled apart and transported into the daughter cells. After the separation is complete and the nuclear envelope reforms around the new nuclei, the condensins detach, allowing the chromosomes to begin uncondensing within their respective nuclei.

In summary, the processes of chromosomal condensation and the roles of M Cyclins, CDKs, condensins, and cohesins are vital for the successful completion of mitosis, ensuring that genetic material is accurately distributed to daughter cells.

Mitosis is a crucial process of cell division that ensures the equal distribution of genetic material to daughter cells. It consists of several distinct phases, each with specific events that facilitate this process. The first phase, prophase, marks the formation of the mitotic spindle, a structure composed of microtubules and centrosomes that plays a vital role in chromosome movement. During prophase, the chromosomes condense and become visible, transitioning from a less organized chromatin state to distinct structures. The centrosomes, which are made up of microtubules, duplicate during the preceding interphase and migrate to opposite poles of the cell, establishing the spindle apparatus.

The next phase, prometaphase, involves the disassembly of the nuclear envelope, which allows the microtubules of the mitotic spindle to interact with the chromosomes. Each chromosome has a specialized region called the kinetochore, where microtubules attach. This attachment is crucial as it ensures that each sister chromatid is connected to opposite spindle poles, facilitating their eventual separation.

Following prometaphase is metaphase, characterized by the alignment of the chromosomes along the cell's equatorial plane, forming what is known as the metaphase plate. This alignment is critical for the subsequent separation of sister chromatids. A key regulatory mechanism during this phase is the spindle assembly checkpoint, which verifies that all chromosomes are properly aligned before allowing the cell to proceed to anaphase.

In anaphase, the cohesin proteins that hold sister chromatids together are cleaved by the enzyme separase, allowing the chromatids to move toward opposite poles of the cell. This movement is facilitated by the microtubules of the spindle apparatus, which also elongate, pushing the spindle poles further apart. The anaphase promoting complex plays a role here by degrading proteins that would otherwise promote mitosis, ensuring that the cell cycle progresses correctly.

The final phase, telophase, involves the reformation of the nuclear envelope around each set of separated chromatids, now considered individual chromosomes. The mitotic spindle disassembles, and the cell prepares for cytokinesis, the process that will ultimately divide the cytoplasm and organelles into two distinct daughter cells.

Understanding these phases of mitosis is essential for grasping how cells replicate and maintain genetic integrity. Each step is intricately linked, ensuring that the process is tightly regulated and efficient, ultimately leading to successful cell division.