Viruses as aforementioned differ tremendously from other unicellular microorganisms in several ways and these shall be highlighted in this section.
- Viruses reproduce only in living cells. Thus, viruses are obligate intracellular parasites since they only reside and replicate within infected host cells.
- They lack cellular structure; and viruses generally have the ability to infect other forms of life including bacteria, Archaea, animals and plant cells. .
- Viruses lack functional organelles (e.g. ribosomes) for the synthesis of important cellular and metabolic molecules.
- Viruses have the ability to integrate their own genome (i.e. DNA or their RNA transcript) into the genome of their host cell.
- They have simple acellular organization that comprises mainly of a particular nucleic acid (DNA or RNA) and a protein coat.
- Viruses do not carry out binary fission or cell division the same way that the eukaryotic or prokaryotic cells do.
- Viruses lack a metabolic process or system of their own. Instead, they take advantage of the cellular and metabolic processes of their host cells to generate their own energy and thus carry out their metabolic functions.
- They are not inhibited or killed by antibiotics. But viruses can be affected by interferons. Interferons are proteinous substances produced by host cells especially in response to a viral invasion or infection; and they generally help to limit the spread of the virus in the host’s body.
- Viruses are per se the smallest forms of microorganisms and they usually range from 20 – 300 nm or 350 nm in size. And thus viruses cannot be seen with the light microscope (whose resolving power is about 300 nm) but only with the aid of the electron microscope. Due to their relatively small sizes; viruses or virions are measured in nanometers (nm). Parvoviruses are among the smallest viruses (about 20 nm in size) while the largest viral family have a size of about 300-350 nm (e.g. smallpox virus). The largest known virus is mimivirus (Figure 2).Figure 2: Transmission electron micrograph of mimivirus. Mimivirus is the largest known virus; and its shape is as large as that of some small bacteria (e.g. E. coli). This virion has a large genome and shape. Mimivirus mainly infect amoeba especially the Acanthamoeba species. However, it is also believed to be causative factor in pneumonia in humans.
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