When do mitotic and meiotic cell division occur in the life cycles of eukaryotes?

When do mitotic and meiotic cell division occur in the life cycles of eukaryotes?

1. In eukaryotes, the cell cycle consists of four discrete phases: G1, S, G2, and M.

2. 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.

3. 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.

4. 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.

5. 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.

   • 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.

6. 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.

7. 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.

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