The cell cycle consists of four phases: G1, S, G2, and M (mitosis), with M further divided into prophase, metaphase, anaphase, and telophase. G1, S, and G2 are collectively known as interphase. During S, DNA is replicated, and during G2, cells prepare for mitosis. Mitosis begins with prophase, followed by metaphase, anaphase, and telophase. Cytokinesis completes cell division. Some researchers include a fifth phase, G0, between mitosis and G1.
The stages of the cell cycle, also called stages, are the components that make up the life cycle of a cell. There are four phases in a cell cycle, gap phase 1 (G1), synthesis (S), gap phase 2 (G2), and mitosis (M). Mitosis is further divided into four phases, prophase, metaphase, anaphase, and telophase. Some descriptions of cell cycle phases include a third gap phase, known as G0.
The G1, S and G2 stages are collectively known as interphase. A cell in the G1 phase is actively growing and undergoing the metabolic changes necessary for cell division. G1 phase ends at the point of restriction, when the cell “decides” to undergo division and moves on to S phase.
All the DNA of the cell is replicated during the synthesis phase of the cell cycle phases. During DNA synthesis or replication, special cellular components separate the double-stranded DNA helix into two single strands. An enzyme called DNA polymerase then runs through each template strand of DNA and relies on a second strand to copy the cell’s original DNA exactly. Other enzymes in the cell check that the DNA has been copied correctly. The end result of the synthesis is two exact double-stranded copies of the cell’s original DNA, called chromosomes.
Cells prepare for mitosis during the second gap phase. For mitosis, special materials are required in the cytoplasm of the cell. The cell undergoes metabolic changes during G2 to form these cytoplasmic materials.
After G2, mitosis begins with the prophase phase. During this stage, a structure known as the mitotic spindle is formed. Another structure called the centrosome duplicates and the duplicates move to opposite ends of the cell. Chromosomes move to a region of the mitotic spindle called the metaphase plate and centromeres attach to the spindle using structures known as kinetochores. This last phase of prophase is sometimes further divided into a phase called prometaphase.
During metaphase, chromosomes line up with the metaphase plate to help chromosomes separate properly during anaphase. Once the chromosomes are aligned, anaphase occurs when chromosomes separate and move to opposite ends of the cell. The separate chromosomes are called daughter chromosomes.
Telophase is the final phase of the mitosis and cell cycle phases. The daughter chromosomes each acquire their own nuclear membranes and the spindle fibers detach and disappear. Cell division is not complete, however, until cytokinesis occurs and the cell completely divides into two new cells. At this point the cell cycle restarts with G1.
Some researchers include a fifth phase of the cell cycle phases. The G0 phase is sandwiched between mitosis and G1. If cells enter the G0 phase, they no longer grow. They can, however, reactivate and enter the G1 phase again.
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