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4.2: The Life of a Cell

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    The Life of a Cell

    Cells go through different phases during their lives. These phases may involve preparing for cell division, or they may be in a phase where they are not moving toward a cell division. The sequence of phases or stages a cell goes through during its life is known as the cell cycle.

    This diagram shows the stages of the cell cycle where the two main stages are interphase and mitotic cell division. There is an inner circle showing that interphase is composed of G1 phase, S phase, and G2 phase. A set of arrows points away from the cell cycle during G1 to indicate cells can enter G0 and not continue toward division.

    Above: This diagram summarizes the life phases a cell goes through (the cell cycle). The abbreviations are as follows: I= interphase, G1 = G1 phase, S = S phase, G2 = G2 phase, G0 = G0 phase, and M = mitotic cell division. The outer circle shows that G1, G0, S, and G2 phases are all considered part of interphase. The inner circle shows the specific phases occurring during interphase. At the end of mitotic cell division, the cell divides into two. Both cells continue in the cell cycle after division - meaning that both cells are in the G1 phase after completing mitotic cell division.

    The bulk of the cell's life in the cell cycle is spent in the “living phase”, known as interphase, a phase when a cell is not dividing. The cell might not be progressing toward a cell division at all - a phase known as G0 phase where cells do not move through the other stages of interphase preceding a cell division, but these cells typically are conducting normal cellular processes (undergoing metabolic reactions, conducting gene expression, responding to their environmental conditions etc.). If a cell in interphase is progressing toward cell division, then interphase involves normal cellular processes as well as three stages that prepare the cell for division. Preparation for cell division occurs in three phases occurring in the following order: G1 phase, S phase, and G2 phase:

    • G1 phase: a phase of growth when the cell is accumulating resources to live and grow. After attaining a certain size and having amassed enough raw materials, a checkpoint is reached where the cell uses biochemical markers to decide if the next phase should be entered.
    • S phase: a phase when metabolism is shifted towards the replication (or synthesis) of the genetic material (DNA replication). During S phase, the amount of DNA in the nucleus is doubled and copied exactly in preparation for cell division. Before S phase, the chromosomes exist as single chromatids. At the end of S phase, each chromosome consists of two identical sister chromatids (identical twin sister chromatids) attached to each other at a centromere. These identical sister chromatids are separated from each during the cell division process to produce two genetically identical cells at the end of cell division.
    • G2 phase: a phase of growth and final preparation before the cell division process begins. Another checkpoint takes place at the end of G2 to ensure the fidelity of the replicated DNA and to re- establish the success of the cell’s capacity to divide in the environment. If conditions are favorable, the cell continues on to mitotic cell division.

    One single chromatid appears as one single line whereas two sister chromatids attached to each other appears as an X-like structure.

    Above: The diagram and chromosome images above show the distinctions between single chromatids and two identical sister chromatids attached at a centromere. Please note that this figure shows the DNA in these chromatids as condensed (the DNA is tightly coiled and organized into the structures shown above). Throughout interphase, the DNA is decondensed (loose) in the nucleus and cannot be observed in this condensed structure until mitotic cell division begins.

    Attributions

    This page titled 4.2: The Life of a Cell is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Rosanna Hartline.