The cell cycle is the sequence of events a cell undergoes from its formation to its division into two daughter cells. This process is necessary for the growth, repair, and maintenance of tissues in multicellular organisms. The cycle occurs in an orderly series to ensure that cells only divide when necessary and that each new cell receives a complete copy of the parent cell’s genetic material. This process allows a single-celled zygote to develop into a complex organism.
Interphase: The Growth and Preparation Period
Most of a cell’s life is spent in interphase, a period of growth and preparation for division. During this phase, the cell carries out its metabolic functions, increases in size, and duplicates its internal components. Interphase is divided into three sub-phases that prepare the cell for division.
The first stage is the G1 phase, or first gap. The cell is biochemically active, growing physically larger and synthesizing proteins and other molecules. It also accumulates the building blocks for DNA and the energy reserves needed for replication. This phase is a point of decision where the cell commits to divide or delay division.
Following G1, the cell enters the S phase, which stands for synthesis. The primary event of this stage is the replication of the cell’s entire set of DNA. During this phase, a structure called the centrosome is also duplicated to aid in separating the DNA later on.
The final stage of interphase is the G2 phase, or second gap. The cell continues to grow and produces proteins and organelles needed for division. It replenishes its energy stores and dismantles parts of its cytoskeleton to provide resources for the upcoming division. Some cells can also enter a quiescent state called G0, exiting the cycle and ceasing preparations for division.
M Phase: The Division Process
Following interphase, the cell enters the M phase to physically divide. This phase involves a reorganization of cellular components to partition the duplicated contents into two new cells. The M phase consists of two overlapping events: mitosis, the division of the nucleus, and cytokinesis, the division of the cytoplasm.
Mitosis is a continuous process divided into four stages.
- Prophase: The replicated DNA condenses into visible chromosomes, each consisting of two identical sister chromatids. The mitotic spindle begins to form, and the nuclear envelope breaks down.
- Metaphase: The fully condensed chromosomes align along the cell’s equator, called the metaphase plate. Spindle microtubules from opposite poles attach to each chromosome.
- Anaphase: The connection between the sister chromatids breaks, and they are pulled apart by the spindle microtubules. The separated chromatids, now individual chromosomes, move to opposite poles of the cell.
- Telophase: The chromosomes arrive at the poles, decondense, and a new nuclear envelope forms around each set, creating two distinct nuclei.
Overlapping with the later stages of mitosis, cytokinesis begins. In animal cells, a contractile ring of protein filaments pinches the cytoplasm, cleaving the cell into two separate daughter cells.
Regulation and Checkpoints
Progression through the cell cycle is a controlled process directed by internal and external signals. The cell uses internal control mechanisms called cell cycle checkpoints to ensure each phase is completed correctly. These checkpoints act as decision points, monitoring the order and integrity of major events.
Three major checkpoints govern the cycle. The G1 checkpoint assesses if conditions are favorable for division, checking for cell size, energy reserves, and DNA integrity. The G2 checkpoint bars entry into the M phase if DNA replication is incomplete or if the DNA is damaged. The M checkpoint occurs during metaphase and ensures all chromosomes are correctly attached to the mitotic spindle before separation.
The proteins behind this regulation are two groups: cyclins and cyclin-dependent kinases (Cdks). Cdks are enzymes that, when active, drive the cell forward through the checkpoints. However, Cdks are only active when bound to their partner proteins, the cyclins. The concentrations of cyclins fluctuate in a predictable pattern throughout the cycle, rising and falling to activate the appropriate Cdks at the correct time, pushing the cell from one phase to the next.
When the Cell Cycle Goes Wrong
A breakdown in cell cycle regulation can have severe consequences. Cancer is a disease of uncontrolled cell proliferation caused by mutations in genes that regulate the cycle. When checkpoint mechanisms fail, they can allow cells with damaged DNA to divide, leading to the propagation of genetic errors and the accumulation of more mutations in subsequent generations.
This loss of control is linked to mutations in two types of genes: proto-oncogenes and tumor suppressor genes. Proto-oncogenes produce proteins that encourage cell division, but when mutated, they can become overactive oncogenes. Tumor suppressor genes create proteins that halt the cell cycle, such as those that enforce checkpoints. Mutations that inactivate these suppressor genes remove the brakes on cell division.