Stages of a Typical Cell Cycle

Cells reproduce using a process called mitosis. Mitosis is defined by Campbell and Reece's "Biology" as "a process of nuclear division in eukaryotic cells conventionally divided into five stages: prophase, prometaphase, metaphase, anaphase, and telophase." Prokaryotes undergo a much simpler process of division, called binary fission, in which a single DNA molecule replicates, attaches each copy to different parts of the cell, and then separates. All cells undergo cytokinesis, or separation, from the sister cell by dividing the cytoplasm at the end of the cycle.

Interphase is not grouped within the process of mitosis, as it is a longer phase of the cell cycle that alternates with mitosis. During interphase, cells prepare for mitosis by growing and copying chromosomes. This phase is broken up into three sub-phases: G1, S, and G2. Chromosomes are only produced during the S sub-phase, but the cell produces proteins, helping it grow throughout all three sub-phases.

During prophase, chromatin fibers condense into tight coils in this phase and become visible using a light microscope. Two sister chromatids appear attached to one another and identical. The nucleolus, a structure within the nucleus made by chromosomes, is lost. Within the cytoplasm of the cell, which is outside the nucleus, a mitotic spindle starts to form. This will eventually help move the chromosomes apart.

The nucleus begins to break apart, leaving only pieces of the nuclear envelope in prometaphase. These pieces will form new nuclei during telophase. The chromatids are even more condensed in this phase, and the center of each is called the centromere. The chromatids form a new structure at their centromeres called a kinetochore. This is where some microtubules of the mitotic spindle attach to the chromatids. The microtubules of the mitotic spindle that are not attached to chromatids interact with each other on opposite ends of the cell.

The end pieces of the mitotic spindle are called centrosomes, and they reach opposite poles of the cell in this phase. The chromatids line up across the center of the cell called the metaphase plate. The kinetochore of each sister chromatid is attached to a microtubule from the opposite of the cell as the other.

The sister chromatids are pulled apart from each other by the microtubules, and start moving towards their respective poles as the attached microtubules shorten. Each is considered a single chromosome at this point. They are pulled apart at their centromeres, and so travel centromere-first. The microtubules that are not attached to chromosomes begin lengthening, moving the poles of the cell away from each other.

The unattached microtubules continue to lengthen the cell, and everything begins to reverse the process of mitosis in each daughter cell. The pieces of the nuclear envelope left over from prometaphase form daughter nuclei at the poles of the cell. The rest of the nuclear system start to form in the new nuclei as well. The chromatin of the chromosomes begin loosening again. Shortly after mitosis is complete, cytokinesis divides the cytoplasm, creating two complete daughter cells. The cycle can now start all over in each of the daughter cells.