The cell cycle encompasses the changes a cell goes through to produce new, offspring cells. There are two major parts of the cell cycle: mitosis and interphase. Within these two parts are several other identifiable stages.
During interphase, three stages occur -- a first growth stage, a DNA synthesis stage and then another growth stage. The differences in these stages are subtle and not easily seen by the untrained eye. Traditionally, descriptions of interphase do not give specific attention to each stage.
When a cell undergoes mitosis, more pronounced alterations occur through a series of changes identified as prophase, metaphase, anaphase and telophase. The cells are split into two cells in a process known as cytokinesis.
A cell must prepare for mitosis. The preparations include growth, synthesizing DNA, then more cell growth and protein synthesis. These three stages are known as G1 or gap 1; S or synthesis; and G2 or gap 2. The gaps are so called because there is a break or gap between DNA synthesis and mitosis.
In general, interphase occupies almost all the cell cycle. A cell will spend 90% to 95% of its time in interphase. There are exceptions in some fast-reproducing cells, including embryonic cells, which skip the G1 and G2 phases. These cells undergo only DNA replication and mitosis. A microscopic viewing of cells in interphase would reveal normal-looking cells. There may be some noticeable structures, including the chromosomes bundled into the nucleolus and a pair of centrioles.
The changes that occur in prophase are more noticeable than in interphase. During interphase the chromosomes replicated in the aspect of individual chromatin. Now, in prophase, the nucleolus disappears and the chromatin come together into bundles as chromosomes. The centrioles, which are paired rods, begin to move away from the nucleus region. As the centrioles move apart, strands of fiber are stretched across the cell. The spindle fibers also connect to the chromatin by way of the kinetochore.
Remembering that "meta" means middle makes the changes in this stage easy to remember. The spindle fibers have been fully stretched across the cell by the centrioles. The chromosomes have been aligned in the center of the cell by these fibers.
Chromosomes are separated during anaphase. The spindle fibers are drawn in toward the centrioles. This pulling activity causes the chromosomes to separate at a central point called the centromere. The movements of the spindle fiber are important to ensure each cell gets the correct number of chromosomes.
The chromatids arrive at the poles of the cell. These two complete sets of chromatids will act as the basis for the new nucleus in each cell. The chromosomes will begin to disappear, and the nucleus begins to form. There is also a pinching motion in the center of the cell.
The final splitting of the cell is due to cytokinesis. In animal cells, there is a splitting motion similar to a bubble being pinched into two smaller bubbles. In the more rigid plant cells, the new cell plates are synthesized between the two nuclei.
