Cells, which are the building blocks of all living things, reproduce by duplicating their contents and dividing into two new cells called daughter cells. This five-stage process is called mitosis, and is integral to a cell's life cycle. While single-celled organisms like bacteria duplicate to make two brand new organisms, many rounds of mitosis are required for the growth and development of multicellular organisms like humans and other mammals.
During interphase, the cell is preparing itself for division. Cells contain many proteins and structures called organelles that it must replicate in preparation for doubling. The DNA of the cell is also duplicated during this phase, creating two copies of each strand of DNA called a chromosome. At this time the cell also increases in size. The majority of the cell's lifespan is spent in interphase.
In the phase to follow, called prophase, the duplicated chromosomes from the previous phase condense, meaning they become compacted and more tightly wound. An apparatus known as a mitotic spindle forms on the edges of the dividing cell. The mitotic spindle is made up of proteins called microtubules that gradually lengthen during prophase, which drives the division of the cell by elongating it.
Preceding metaphase is a period called prometaphase, during which the membrane, or nuclear envelope, surrounding the chromosomes breaks down, allowing the condensed chromosomes to come into direct contact with the microtubules of the mitotic spindle. Upon entering metaphase, the pairs of condensed chromosomes line up along the equator of the elongated cell. Because they are condensed, they move more easily without becoming tangled.
During anaphase, the pairs of chromosomes, also called sister chromatids, are drawn to opposite poles of the elongated cell. Therefore, duplicate copies of the cell's DNA are now on either side of the cell and are ready to divide completely. At this stage the microtubules get shorter, which begins to allow the cell to separate.
The most important characteristic of telophase is that the nuclear envelope, which had previously broken down to allow the microtubules to access and recruit the chromosomes to the equator of the dividing cell, reforms as two new nuclear envelopes around the separated sister chromatids. The complete division of the cell, however, is not complete until cytokinesis takes place. Cytokinesis is the process by which the elongated cell is finally pinched into two brand new cells by a ring of proteins called actin and myosin, the same proteins found in muscle. At this point, the cytoplasm, or fluid in which all cell components are bathed, is equally divided between the two new daughter cells. Each daughter cell is identical, containing it's own nucleus and a complete copy of the organism's DNA.
Cytokinesis, or the separating of the two daughter cells, begins during or after the final phases of mitosis. It occurs in animal cells happens when a fiber ring containing a protein called actin, located in the cell’s center, contracts, pulling the cell membrane to it, and subsequently squeezing the two nuclei apart. Because cell walls are so rigid in plant cells, cytokinesis in plant cells occurs when a cell plate takes the place of the metaplate and grows between the two new cells. This plate becomes a membrane that inculcates itself into the cell wall in each of the daughter cells.