Viruses as aforementioned differ tremendously from other unicellular microorganisms (such as bacteria, fungi, algae and protozoa) in several ways and these shall be highlighted in this section.
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- 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.
- Viruses generally have the ability to infect other forms of life including bacteria, Archaea, animals, humans and plant cells.
- Viruses lack functional organelles (for example, ribosomes) for the synthesis of important cellular and metabolic molecules such as proteins. Instead, viruses depend on the cellular and/or metabolic machinery of its infected host cell to synthesize its own proteins and other molecules required for the coupling and release of new virions from the already infected host cell.
- Viruses have the ability to integrate their own genome (i.e., DNA or their RNA transcript) into the genome of their host cell.
- Viruses are (per se) the smallest forms of microorganisms and they usually range from 20 – 300 nm or 350 nm in size. Viruses cannot be seen with the light microscope (whose resolving power is about 300-700 nm). They can only be seen with an electron microscope (with resolving power of about 0.1-10 or 100 nm), whose resolving power is about 1000 times better than the light microscopes. 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 1).
- Viruses 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. Antiviral agents are the class of antimicrobial agents used for the treatment and management of infections caused by pathogenic viruses in humans or animals.
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