The cell is a complicated organism that required many hundreds of millions of years to develop after the earliest form of life, an organism similar to the present-day virus, first appeared on earth. Figure 1 shows the relative sizes of (1) the smallest known virus, (2) a large virus, (3) a Rickettsia, (4) a bacterium, and (5) a nucleated cell, demonstrating that the cell has a diameter about 1000 times that of the smallest virus and therefore a volume about 1 billion times that of the smallest virus. Correspondingly, the functions and anatomical organization of the cell are also far more complex than those of the virus.

Fig1.  Comparison of sizes of precellular organisms with that  of the average cell in the human body. 

The essential life-giving constituent of the small virus is a nucleic acid embedded in a coat of protein. This nucleic acid is composed of the same basic nucleic acid constituents (DNA or RNA) found in mammalian cells, and it is capable of reproducing itself under appropriate conditions. Thus, the virus propagates its lineage from generation to generation and is therefore a living structure in the same way that the cell and the human being are living structures.

As life evolved, other chemicals besides nucleic acid and simple proteins became integral parts of the organ ism, and specialized functions began to develop in different parts of the virus. A membrane formed around the virus, and inside the membrane, a fluid matrix appeared. Specialized chemicals then developed inside the fluid to perform special functions; many protein enzymes appeared that were capable of catalyzing chemical reactions, thus determining the organism’s activities.

In still later stages of life, particularly in the rickettsial and bacterial stages, organelles developed inside the organism, representing physical structures of chemical aggregates that perform functions in a more efficient manner than can be achieved by dispersed chemicals throughout the fluid matrix.

Finally, in the nucleated cell, still more complex organelles developed, the most important of which is the nucleus. The nucleus distinguishes this type of cell from all lower forms of life; the nucleus provides a control center for all cellular activities, and it provides for reproduction of new cells generation after generation, with each new cell having almost exactly the same structure as its progenitor.