Eukaryotes and Cell Cycle

The cellular life cycle, also called the cell cycle, includes many processes necessary for successful self-replication. Beyond carrying out the tasks of routine metabolism, the cell must duplicate its components — most importantly, its genome — so that it can physically split into two complete daughter cells. The cell must also pass through a series of checkpoints that ensure conditions are favorable for division.

In eukaryotes, the cell cycle consists of four discrete phases: G₁, S, G₂, and M. The S or synthesis phase is when DNA replication occurs, and the M or mitosis phase is when the cell actually divides. The other two phases — G₁ and G₂, the so-called gap phases — are less dramatic but equally important. During G₁, the cell conducts a series of checks before entering the S phase. Later, during G₂, the cell similarly checks its readiness to proceed to mitosis.

Together, the G₁, S, and G₂ phases make up the period known as interphase. Cells typically spend far more time in interphase than they do in mitosis. Of the four phases, G₁ is most variable in terms of duration, although it is often the longest portion of the cell cycle (Figure 1).

In order to move from one phase of its life cycle to the next, a cell must pass through numerous checkpoints. At each checkpoint, specialized proteins determine whether the necessary conditions exist. If so, the cell is free to enter the next phase. If not, progression through the cell cycle is halted. Errors in these checkpoints can have catastrophic consequences, including cell death or the unrestrained growth that is cancer.

Each part of the cell cycle features its own unique checkpoints. For example, during G₁, the cell passes through a critical checkpoint that ensures environmental conditions (including signals from other cells) are favorable for replication. If conditions are not favorable, the cell may enter a resting state known as G₀. Some cells remain in G₀ for the entire lifetime of the organism in which they reside. For instance, the neurons and skeletal muscle cells of mammals are typically in G₀.

Another important checkpoint takes place later in the cell cycle, just before a cell moves from G₂ to mitosis. Here, a number of proteins scrutinize the cell's DNA, making sure it is structurally intact and properly replicated. The cell may pause at this point to allow time for DNA repair, if necessary.

Yet another critical cell cycle checkpoint takes place mid-mitosis. This check determines whether the chromosomes in the cell have properly attached to the spindle, or the network of microtubules that will separate them during cell division. This step decreases the possibility that the resulting daughter cells will have unbalanced numbers of chromosomes — a condition called aneuploidy.

The cell cycle and its system of checkpoint controls show strong evolutionary conservation. As a result, all eukaryotes — from single-celled yeast to complex multicellular vertebrates — pass through the same four phases and same key checkpoints. This universality of the cell cycle and its checkpoint controls allows scientists to use relatively simple model organisms to learn more about cell division in eukaryotes of all types — including humans. In fact, two of the three scientists who received Nobel Prizes for cell cycle research used yeast as the subject of their investigations.

The eukaryotic cell cycle includes four phases necessary for proper growth and division. As a cell moves through each phase, it also passes through several checkpoints. These checkpoints ensure that mitosis occurs only when environmental conditions are favorable and the cellular genome has been precisely replicated. Collectively, this set of checks on division helps prevent chromosomal imbalance in newly produced daughter cells.