Mitosis and Cytokinesis

The cell cycle can be described in several ways.

Breaking it into G1, S, G2, and M phases emphasizes patterns in DNA^* replication and separation.

The amount of DNA remains stable during the two gap phases.

DNA synthesis occurs during S phase.

The separation of paired chromosome^*s happens in M phase.

Another way to look at it is as a process of growth and division.

From this perspective period^*s of growth - G1, S, and G2 - are all part of Interphase while DNA separation (mitosis) and cell division (cytokinesis) are part of cell division.

Interphase ends at the end of G2 with the transition into M Phase. At this point in the cycle the cell has two complete copies of its DNA and enough cellular material to support two cells.

The first part or M Phase is mitosis. During mitosis the DNA in the cell is divided so that each daughter cells receives a complete copy of the organism's genome.

Mitosis is subdivided into phases starting with:

Prophase

The begining of Prophase marks the transition from Interphase to cell division.

During this stage, the DNA in the cell condenses into x-shaped chromosomes.

Each x-shaped structure contains two copies of the chromosome connected at the centromere^*. The copies, called chromatid^*s, will separate later in mitosis with one chromatid going to each daughter cell.

As the cell proceeds through prophase, centrioles migrate to opposite sides of the cell and begin to synthesize spindle fibers that will attach to the chromatids and pull them apart later in the process.

Pro-metaphase

During pro-metaphase the nuclear envelope breaks down, and the spindle fibers continue to form connections between the centrioles and each chromosome.

Metaphase

Metaphase starts when the microtubules in the spindle fibers complete the connection between the centrioles and the chromosomes and begin to pull the sister chromatids towards opposite sides of the cell.

The pull of the spindle fibers cause the chromosomes to line up across the center of the cell along the metaphase plate - a perpendicular line approximately halfway between the two centrioles.

Metaphase is followed by

Anaphase

During Anaphase the sister chromatids separate and one of each gets pulled towards either side of the cell.

As the chromatids move towards the centrioles, the cell begins to elongate.

By late Anaphase, one complete copy of the organism's genome is present on either side of the elongated cell.

The final stage of mitosis is

Telophase

Once the chromatids have been pulled to the sides of the cell, nuclear envelopes form around each set.

The new nuclear envelope surrounds the DNA but not the centrioles.

after the nuclear envelope has reformed, the DNA begins uncoils, returning to its relaxed state.

The end of telophase represents the end of mitosis, but not the end of M Phase.

To complete the cell cycle the cell needs to divide. In animal cells this occurs through:

Cytokinesis

Cytokinesis ensures that one nucleus ends up in each daughter cell.

During cytokinesis, a contractile ring of protein filaments forms along the membrane at the center of the cell. As the contractile ring begins to shrink, it pulls the cell membrane along with it.

This forms a cleavage furrow.

By the end of the process, the contractile ring has pulled the membrane all the way into the center of the cell resulting in two separate cells, each surrounded by its own plasma membrane.

At this point, each newly formed cell re-enters Interphase to begin the cell cycle again.