All living organisms are made from cells, the basic unit of biology. Cells are separated from their outside environment by a lipid and protein-containing membrane. The membrane acts to exchange nutrients from the outside and waste products from the inside of the cell. As the cell grows, its surface area increases by the square of the radius; however, the volume increases by the cube (third power) of the radius. There comes a point where the volume of the cell is too large for the surface area to efficiently exchange nutrients and waste products. This is the point where the cell must divide.
Cell growth and division are separated into different continuous phases of a cell cycle. Interphase is the non-dividing portion of the cell cycle where the cell becomes larger. The other major phase, consisting of cell division, is called the Mitotic phase. In terms of time, interphase is the longest phase. Interphase is divided into three parts: Gap1 (G1), Synthesis (S) and Gap2 (G2).
Cells consist of many membrane-enclosed organelles and structures, including the nucleus, containing genetic material, and two centrioles, which are important for cell replication.
G1 occurs when a newly-formed cell grows and conducts its normal metabolism. Synthesis (S) is where the chromosomes (genetic material in the nucleus) duplicate. The genetic material consists of DNA and protein. When the DNA duplicates, two sister chromatids are formed which are held together by centromeres. Thus, the amount of DNA doubles during S-phase. Gap 2 (G2) is the next part of interphase, where there is more growth and metabolism, but specifically geared toward cell replication. Once the cell reaches the S phase, the cell is committed to replicating.
The next phase is the Mitotic phase, which consists of mitosis (nuclear duplication) and cytokinesis (closing the cell membranes to form two new cells). Mitosis is divided into four major sections: prophase, metaphase, anaphase, and telophase.
Key steps during prophase include separation of the two centrioles toward opposite poles of the cell, condensation of the chromosomes, so that they are visible in a microscope, and the start of dissolution of the nuclear membrane. As the centrioles separate, the put out spindle fibers that can stick to the centromeres of the sister chromatids.
In metaphase, the nuclear membrane is completely dissolved, and the sister chromatids are lined up in the middle of the cell on a plane. These chromatids are attached by the centrioles to the spindle fibers on the opposite sides of this plane. The sister chromatids are now ready to be separated in the next phase.
During anaphase, the sister chromatids are separated and they move toward the centrioles on opposite sides of the cell, being pulled by the spindle fibers. They are now separated chromosomes for the new cells.
In telophase, the nuclear membrane reforms around the two sets of chromosomes, the chromosomes become more disperse, and the spindle fibers dissolve. The cell has two separate nuclei at this stage until cytokinesis starts (usually at the same time as telophase).
During cytokinesis, the cell membrane squeezes in between the new nuclei to form a furrow that eventually closes to form two new identical daughter cells. Plant cells have an outer cell wall in addition to the cell membrane, so they need to form a new cell wall (called a cell plate before it fully develops) in addition to new cell membrane during cytokinesis. (Note: see my article entitled: An overview of plant cell walls.)
Thus, mitotic cell division results in two genetically identical daughter cells from a single parent cell. Division of sex cells (gametes) consists of one single parent cell forming four daughter cells with half the DNA, in a process called meiosis. Meiosis is similar to mitosis, except it occurs as two cycles of mitosis-like steps. When two gametes from different individuals fuse together, the resulting zygote has the correct amount of DNA.
Reference: Campbell, Biology, 8th Edition, c. 2008