Thecellular life cycle, also called the cellcycle, includes many processes necessary for successful self-replication.Beyond carrying out the tasks of routine metabolism, the cell must duplicateits components — most importantly, its genome — so that it can physically splitinto two complete daughter cells. The cell must also pass through a series ofcheckpoints that ensure conditions are favorable for division.
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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).
Figure 1:The eukaryotic cell cycle
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How Do Cells Monitor Their Progress through the Cell Cycle?
Inorder to move from one phase of its life cycle to the next, a cell must passthrough numerous checkpoints. At each checkpoint, specialized proteinsdetermine whether the necessary conditions exist. If so, the cell is free toenter the next phase. If not, progression through the cell cycle is halted.Errors in these checkpoints can have catastrophic consequences, including celldeath or the unrestrained growth that is cancer.
Eachpart of the cell cycle features its own unique checkpoints. For example, duringG1, the cell passes through a critical checkpoint that ensuresenvironmental conditions (including signals from other cells) are favorable forreplication. If conditions are not favorable, the cell may enter a restingstate known as G0. Somecells remain in G0 for the entire lifetime of the organism in whichthey reside. For instance, the neurons and skeletal muscle cells of mammals aretypically in G0.
Anotherimportant checkpoint takes place later in the cell cycle, just before a cellmoves from G2 to mitosis. Here, a number of proteins scrutinize thecell"s DNA, making sure it is structurally intact and properly replicated. Thecell may pause at this point to allow time for DNA repair, if necessary.
Yetanother critical cell cycle checkpoint takes place mid-mitosis. This checkdetermines whether the chromosomes in the cell have properly attached to the spindle, or the network of microtubulesthat will separate them during cell division. This step decreases thepossibility that the resulting daughter cells will have unbalanced numbers ofchromosomes — a condition called aneuploidy.
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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.