The Phases of the Cell Cycle

The cell cycle is commonly described as having four phases: M (mitosis), Gap 1 (postmitotic interphase), S-phase (period of DNA synthesis), and Gap 2 (postsynthetic interphase). Gaps 1 and 2 were initially thought to be resting stages between mitosis and S-phase. This description is a misnomer because numerous genes regulate cell growth in these phases. Appropriately, these terms became abbreviated to G1 and G2. Moreover, networks of cell cycle gene products constitute molecular checkpoints that in G1 determine whether a cell is competent to replicate its chromosomes during S-phase, and that in G2 sense whether the cell is ready to partition its chromatids during mitosis. Uniquely in plant cells, in late G2 an array of microtubules known as the preprophase band appears and chromosomes separate in a plane perpendicular to it.

Only in mitosis do chromosomes become visible by light microscopy; each one appears as two sister chromatids constricted at a specific point along their length, the centromere. At mitosis, a diploid parent cell passes through four phases: prophase, metaphase, anaphase, and telophase. During late prophase, the nuclear envelope disintegrates and spindles of mi-crotubules span the cell. Unlike animal mitosis where the spindles attach to centrioles (and associated polar asters), there is no obvious anchoring structure for higher plant spindles. This led to the botanical term "anastral cell division." At metaphase, the chromosomes align at the cell's equator and attach to mitotic spindles via kinetochores, discs of structural protein that also meristematic related to cell division at the tip diploid having two sets of chromosomes, versus having one (haploid)

Arrows point to a developing cell plate, which divides the new daughter cells.

vesicle a membrane-bound cell structure with specialized contents

ATP adenosine triphosphate, a small, water-soluble molecule that acts as an energy currency in cells

Arrows point to a developing cell plate, which divides the new daughter cells.

vesicle a membrane-bound cell structure with specialized contents

ATP adenosine triphosphate, a small, water-soluble molecule that acts as an energy currency in cells bind to the centromere of the chromosome. During anaphase, sister chromatids are pulled apart and move to opposite ends of the cell. In telophase, nuclear envelopes reform around each new diploid set of chromosomes followed by cytokinesis when a new wall forms between sibling cells. Cytokinesis requires the formation of a cell plate or phragmoplast that spans the cell center, and becomes dense with vesicles from the Golgi complex (also called the Golgi apparatus). The plasma membrane and the membrane surrounding the phragmoplast fuse, resulting in separation of the sibling cells. On the phragmoplast, cellulose forms the fibrillar component of the cell wall while hemicelluloses and pectins are added as a matrix. Trapped in the primary cell wall are cytoplasmic strands and microtubules that become plas-modesmata, the cytoplasmic connections between the new cells.

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