Cells are the fundamental units of all living things. Each of these microscopic entities contains structures with specialized functions, just as your body as a whole features specialized organs that perform everyday vital tasks. By the same token, just as you undergo different stages of life from beginning to end – infancy, childhood, adolescence, adulthood and old age – cells have their own life cycle including stages that are well-defined but blend smoothly into one another.
Prokaryotic organisms, which include the domains Bacteria and Archaea, consist of only a single cell with few specialized components and do not undergo a cell cycle; instead, the merely grow, split in two and repeat this process over and over. In contrast, eukaryotic organisms – animals, fungi and plants – have distinct cell cycle phases.
A cell's entire purpose can be reduced to one thing: Reproducing copies of itself so that the parent organism can grow, repair itself and ultimately reproduce offspring. The two main stages of cell division are called interphase, in which the cell is not actually dividing but rather gearing up for the next division, and mitosis, which is the division of the cell's genetic material into two daughter nuclei.
Description of Cell Cycle
A cell begins its life cycle by gradually enlarging and reproducing all of its own contents exclusive of that within its nucleus. Then, the genetic material within the nucleus also copies itself. At this point, the cell then reviews its own work to check for mistakes. Finally, the cell then divides in two from the inside out.
The first three sentences of the previous paragraph describe the three processes that occur during interphase, each of which will be described later. The last sentence describes mitosis, which itself includes five distinct steps. The entire cell then divides, beginning the cycle anew.
The rate at which cells move through the two top-level phases of division varies greatly between cell types and also within cells at different times. Usually, mitosis is a great deal shorter than interphase no matter what the absolute time frames are.
Stages of Cell Cycle: Interphase
A cell cycle diagram is ideal for helping keep track of the individual stages of both interphase and mitosis as well as the approximate time fraction of the total cell cycle each step consumes.
Interphase consists of the following individual steps:
G1 (first gap) phase: This phase and G2 both get their names from the fact that little appears to be happening in these phases, even under a microscope. The cell, however, is actually quite metabolically active in G1 as it is busy collecting the molecules it will need for DNA replication in the next stage of interphase, including proteins and adenosine triphosphate (ATP). ATP is the "energy currency" of all living cells.
S (synthesis) phase: Here, the single copies of the organism's chromosomes are replicated, or copied. This is made easier by the fact that chromosomes in interphase are very diffuse, or spread out and unwound; this unwinding exposes more of the DNA in the chromosomes to the enzymes and other factors that are needed for the accurate and complete copying of DNA molecules.
The result of this phase is a set of sister chromatids, which is just another name for a duplicated chromosome. These chromatids are joined along their length at a shared point called the centromere, which is not usually at the center of the chromosome.
G2 (second gap) phase: In this phase, the cell gathers the molecular resources it will need for mitosis, just as G1 sees the cell nucleus prepare for DNA replication. However, in G2, the cell also runs a check of its own work to this point in the cell cycle. The cell itself may enlarge in size generally, as it did in G1, and the nucleus begins to "borrow" proteins it will need for the mitotic spindle during mitosis.
Read more about what happens during interphase.
A Word on Chromosomes
Chromosomes are made of chromatin, which is deoxyribonucleic acid (DNA) packaged into a very tightly coiled shape along with proteins called histones. The histones allow the chromatin to be packed spectacularly tightly into the nucleus, which needs to happen because virtually every cell in the body contains a complete copy of the organism's DNA.
Humans have 46 chromosomes, 23 from each parent. These occur in pairs, meaning that you get one copy of chromosome 1 from each your mother and one from your father, and similarly for chromosomes 2 through 22. The 23rd pair of chromosomes is the sex chromosomes, a combination of X and X in females and X and Y in males. The paired numbered chromosomes are called homologous chromosomes.
Stages of Cell Cycle: M Phase
Mitosis is also known as the M phase, and it consists of five stages of its own. (Some sources omit prometaphase and sort the functions of this phase into either prophase or metaphase instead.)
Prophase: The duplicated chromosomes condense during prophase, creating their characteristic post-interphase appearance at this stage. Also, the mitotic spindle forms at the poles (i.e., opposite sides ) of the nucleus after the centrosome splits into two pieces, which move to the poles and begin generating spindle fibers. The mitotic spindle structure is made mainly of a protein called tubulin, which is also found in the cytoskeleton that supports the cell from within in the manner of girders and beams.
The nuclear envelope that forms the border between the outside of the nucleus and the cytoplasm dissolves during prophase, clearing the way for all of the remaining events of the M phase. Prophase usually takes up about half of mitosis, but this is still a small fraction of the overall cell cycle because of how short mitosis invariably is.
Prometaphase: The chromosomes begin to drift toward the center of the cell. Unlike the case in a meiotic cell division, homologous chromosomes do not physically associate with one another in mitosis; that is, how they ultimately become aligned during metaphase is entirely a matter of random chance. That means that your maternal copy of chromosome 9, for example, could wind up as far as possible from the copy of chromosome 9 you inherited from your father.
Metaphase: In this step, all 46 replicated chromosomes line up in a line that passes through their centromeres, one sister chromatid on each side. This line is called the metaphase plate.
Anaphase: This phase is when the duplicated chromosomes are pulled apart at their centromeres by the microtubules of the mitotic spindle, moving them toward opposite poles of the cell in a direction perpendicular to the metaphase plate.
Telophase: This phase is largely a reversal of prophase, in that a nuclear envelope forms around each new daughter nucleus, and the chromosomes begin to assume the diffuse physical format they spend most of the cell cycle in, and all of interphase in.
The M phase is followed directly by cytokinesis, or the cleavage of the entire cell into two daughter cells with identical DNA. The M phase and cytokinesis together are analogous to binary fission in prokaryotes, which do not have a nucleus or a cell cycle and usually have all of their DNA in a single ring-shaped chromosome in the cytoplasm.
Read more about cytokinesis.
About the Author
Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.