Understanding the relative sizes of cells, bacteria, and viruses is a helpful way to understand the nature of disease.
It may seem reasonable to think that viruses are relatively large because of the severity of the diseases they cause. In fact, viruses—including the one that causes COVID-19—are so tiny that special care needs to be taken to avoid infection.
When compared to cells and bacteria, viruses—which consist only of proteins and nucleic acids—are the smallest. Bacteria, the next largest group, contain several types of organelles, but have no nucleus. Plant and animal cells contain many types of organelles, and their DNA is condensed into chromosomes that are within the cell’s nucleus. Plant cells tend to be larger than animal cells because of their very large central vacuole used in water regulation.
The human egg cell, by comparison, is very large, because it must contain all the cytoplasm, organelles, proteins, and other cellular components required for early embryonic development. The human sperm is designed to be highly mobile. It carries only what it needs to reach the egg and fertilize it: DNA, mitochondria, a centrosome, a few enzymes, and a tail-like flagellum.
DNA is extremely thin, but very long. To package DNA within the confines of a cell’s nucleus, it is tightly interwoven with proteins and coiled into chromosomes.