The Cell Cycle
The cell cycle is a series of regulated steps that occur in cells on the way to cell division. These stages of growth, DNA replication, and chromosome separation are timed in such a way that they produce two genetically identical daughter cells.
Regulation of the Cell Cycle
The cells of eukaryotes, like humans and most other organisms, have genes that control the cell cycle. These genes, called cyclins and cyclin-dependent kinases (CDKs), have important roles in controlling how quickly the cell divides. They also ensure that the cell's DNA doesn't become damaged or incomplete during these stages.
These regulatory molecules are essential for the cell cycle to run smoothly and without errors. They work in conjunction with other cell-cycle control systems and help the cells navigate through each phase of the cell cycle.
G1 Phase (Gap 1)
During this phase, the cell grows physically larger and makes the molecular building blocks necessary for DNA synthesis. It synthesizes proteins and RNA, makes organelles such as the ribosome and spindle, and prepares for the next phase of the cell cycle.
S Phase (Synthesis)
DNA is replicated during this phase, making one copy of each chromosome. The cells then form a centriole from the duplicated chromosomes, and attach the chromatids to each other at a point on the chromosome called the centromere. The centriole then forms a pair of centrosomes that will help the chromosomes separate during mitosis.
M Phase
During this phase, the cells examine whether all the chromosomes that have been duplicated during S phase are correctly attached to the microtubules that make up the spindle. If not, the cell stops dividing and tries to repair any damaged chromosomes. If the cell is unable to repair the chromosomes, it enters a resting state known as G0.
If the cell is able to repair its chromosomes, it enters the S phase again, which continues the process of DNA synthesis and re-attaches the duplicated chromosomes. The DNA is then replicated again, creating two new copies of the chromosome and forming two genetically identical daughter cells.
During this second growth phase, the cell checks that all the replicated DNA is complete and correct. If it fails, the cell pauses the cell cycle and retries DNA replication again until all the replicated DNA is perfect.
M Checkpoint (Spindle Checkpoint)
The M checkpoint, also known as the spindle checkpoint, occurs near the end of the metaphase stage of mitosis. This checkpoint determines if the kinetochores of all the sister chromatids are correctly attached to the spindle microtubules that separate them.
After this, the cell is ready to re-enter the M phase and begin the process of separating and dividing chromosomes into two genetically identical daughter cells.
Importance of the Cell Cycle
This phase is a critical part of the cell cycle because it controls how fast the cell reaches its destination and the number of daughter cells produced from each round of division. It is also the stage where the daughter cells are ready to receive nutrients from their parent cell.
